3.2.0-rc.0

* Fix typos and formatting

* Add Solidity release dates: releases-stability

.babelrc

@@ -1,3 +0,0 @@
-{
-  "presets": ["es2015", "stage-2", "stage-3"]
-}

.circleci/config.yml

@@ -0,0 +1,84 @@
+version: 2
+# 2.1 does not yet support local run
+# unless with workaround. For simplicity just use it.
+# https://github.com/CircleCI-Public/circleci-cli/issues/79
+
+aliases:
+  - &defaults
+    docker:
+      - image: circleci/node:10
+
+  - &cache_key_node_modules
+    key: v1-node_modules-{{ checksum "package-lock.json" }}
+
+jobs:
+  dependencies:
+    <<: *defaults
+    steps:
+      - checkout
+      - restore_cache:
+          <<: *cache_key_node_modules
+      - run:
+          name: Install npm dependencies and prepare
+          command: |
+            if [ ! -d node_modules ]; then
+              npm ci
+            else
+              npm run prepare
+            fi
+      - persist_to_workspace:
+          root: .
+          paths:
+            - node_modules
+            - build
+      - save_cache:
+          paths:
+            - node_modules
+          <<: *cache_key_node_modules
+
+  lint:
+    <<: *defaults
+    steps:
+      - checkout
+      - attach_workspace:
+          at: .
+      - run:
+          name: Linter
+          command: npm run lint
+  test:
+    <<: *defaults
+    steps:
+      - checkout
+      - attach_workspace:
+          at: .
+      - run:
+          name: Unit tests
+          command: npm run test
+
+  coverage:
+    <<: *defaults
+    steps:
+      - checkout
+      - attach_workspace:
+          at: .
+      - run:
+          name: Unit tests with coverage report
+          command: npm run coverage
+
+      # TODO(xinbenlv, #1839): run SOLC_NIGHTLY to be run but allow it to fail.
+
+workflows:
+  version: 2
+  everything:
+    jobs:
+      - dependencies
+      - lint:
+          requires:
+            - dependencies
+      - test:
+          requires:
+            - dependencies
+      - coverage:
+          requires:
+            - dependencies
+

.codecov.yml

@@ -0,0 +1,3 @@
+comment: off
+coverage:
+  range: "100...100"

.editorconfig

@@ -6,7 +6,16 @@ root = true
 [*]
 charset = utf-8
 end_of_line = lf
-indent_size = 2
 indent_style = space
 insert_final_newline = true
-trim_trailing_whitespace = true
+trim_trailing_whitespace = false
+max_line_length = 120
+
+[*.sol]
+indent_size = 4
+
+[*.js]
+indent_size = 2
+
+[*.adoc]
+max_line_length = 0

.env.example

@@ -1,5 +0,0 @@
-# configure your infura api key (not technically required)
-INFURA_API_KEY=
-
-# change the mnemonic that your hd wallet is seeded with
-MNEMONIC=

.eslintrc

@@ -1,22 +1,23 @@
 {
   "extends" : [
     "standard",
-    "plugin:promise/recommended"
+    "plugin:promise/recommended",
   ],
   "plugins": [
-    "promise"
+    "mocha-no-only",
+    "promise",
   ],
   "env": {
     "browser" : true,
     "node"    : true,
     "mocha"   : true,
-    "jest"    : true
+    "jest"    : true,
   },
   "globals" : {
     "artifacts": false,
     "contract": false,
     "assert": false,
-    "web3": false
+    "web3": false,
   },
   "rules": {
 
@@ -24,14 +25,16 @@
     "strict": ["error", "global"],
 
     // Code style
+    "array-bracket-spacing": ["off"],
     "camelcase": ["error", {"properties": "always"}],
-    "comma-dangle": ["warn", "always-multiline"],
+    "comma-dangle": ["error", "always-multiline"],
     "comma-spacing": ["error", {"before": false, "after": true}],
     "dot-notation": ["error", {"allowKeywords": true, "allowPattern": ""}],
     "eol-last": ["error", "always"],
     "eqeqeq": ["error", "smart"],
     "generator-star-spacing": ["error", "before"],
     "indent": ["error", 2],
+    "linebreak-style": ["error", "unix"],
     "max-len": ["error", 120, 2],
     "no-debugger": "off",
     "no-dupe-args": "error",
@@ -48,7 +51,12 @@
     "semi": ["error", "always"],
     "space-before-function-paren": ["error", "always"],
 
+    "mocha-no-only/mocha-no-only": ["error"],
+
     "promise/always-return": "off",
     "promise/avoid-new": "off",
+  },
+  "parserOptions": {
+    "ecmaVersion": 2018
   }
 }

.github/ISSUE_TEMPLATE.md

@@ -1,34 +0,0 @@
-## 🎉 Description
-
-<!-- Briefly describe the issue you are experiencing (or the feature you want to see added to OpenZeppelin). Tell us what you were trying to do and what happened instead. **Remember, this is _not_ a place to ask for help debugging code; for that, we welcome you in the [OpenZeppelin Slack Channel](https://slack.openzeppelin.org/).** -->
-
-- [ ] 🐛 This is a bug report.
-- [ ] 📈 This is a feature request.
-
-<!-- Please check one of the above by placing an x in the box. -->
-
-## 💻 Environment
-
-Next, we need to know what your environment looks like.
-
-- Which version of OpenZeppelin are you using?
-- What network are you deploying to? Ganache? Ropsten?
-- How are you deploying your OpenZeppelin-backed contracts? truffle? Remix? Let us know!
-
-## 📝 Details
-
-Describe the problem you have been experiencing in more detail. Include as much information as you think is relevant. Keep in mind that transactions can fail for many reasons; context is key here.
-
-## 🔢 Code To Reproduce Issue [ Good To Have ]
-
-Please remember that with sample code it's easier to reproduce the bug and it's much faster to fix it.
-
-```
-insert short code snippets here
-```
-
-<!-- If your code is larger, consider linking us to a repo illustrating your issue. -->
-
-## 👍 Other Information
-
-<!-- List any other information that is relevant to your issue. Error logs, related issues, suggestions on how to fix, Stack Overflow links, forum links, etc. -->

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,21 @@
+---
+name: Bug report
+about: Report a bug in OpenZeppelin Contracts
+
+---
+
+<!-- Briefly describe the issue you're experiencing. Tell us what you were trying to do and what happened instead. -->
+
+<!-- Remember, this is not a place to ask for help debugging code. For that, we welcome you in the OpenZeppelin Community Forum: https://forum.openzeppelin.com/. -->
+
+**💻 Environment**
+
+<!-- Tell us what version of OpenZeppelin Contracts you're using, and how you're using it: Truffle, Remix, etc. -->
+
+**📝 Details**
+
+<!-- Describe the problem you have been experiencing in more detail. Include as much information as you think is relevant. Keep in mind that transactions can fail for many reasons; context is key here. -->
+
+**🔢 Code to reproduce bug**
+
+<!-- We will be able to better help if you provide a minimal example that triggers the bug. -->

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,4 @@
+contact_links:
+  - name: Support request
+    url: https://forum.openzeppelin.com/c/support/contracts/18
+    about: Ask the community in the Community Forum

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,14 @@
+---
+name: Feature request
+about: Suggest an idea for OpenZeppelin Contracts
+
+---
+
+**🧐 Motivation**
+<!-- Is your feature request related to a specific problem? Is it just a crazy idea? Tell us about it! -->
+
+**📝 Details**
+<!-- Please describe your feature request in detail. -->
+
+<!-- Make sure that you have reviewed the OpenZeppelin Contributor Guidelines. -->
+<!-- https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/CONTRIBUTING.md -->

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,17 +1,22 @@
 <!-- 0. 🎉 Thank you for submitting a PR! -->
 
-<!-- 1. **Does this close any open issues?** If so, list them here. If not, remove the `Fixes #` line. -->
+<!-- 1. Does this close any open issues? Please list them below. -->
+
+<!-- Keep in mind that new features have a better chance of being merged fast if
+they were first discussed and designed with the maintainers. If there is no
+corresponding issue, please consider opening one for discussion first! -->
 
 Fixes #
 
-# 🚀 Description
-
-<!-- 2. Describe the changes introduced in this pull request -->
+<!-- 2. Describe the changes introduced in this pull request. -->
 <!--    Include any context necessary for understanding the PR's purpose. -->
 
-<!-- 3. Before submitting, please review the following checklist: -->
-
-- [ ] 📘 I've reviewed the [OpenZeppelin Contributor Guidelines](../blob/master/CONTRIBUTING.md)
-- [ ] ✅ I've added tests where applicable to test my new functionality.
-- [ ] 📖 I've made sure that my contracts are well-documented.
-- [ ] 🎨 I've run the JS/Solidity linters and fixed any issues (`npm run lint:all:fix`).
+<!-- 3. Before submitting, please make sure that you have:
+  - reviewed the OpenZeppelin Contributor Guidelines
+    (https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/CONTRIBUTING.md),
+  - added tests where applicable to test new functionality,
+  - made sure that your contracts are well-documented,
+  - run the Solidity linter (`npm run lint:sol`) and fixed any issues,
+  - run the JS linter and fixed any issues (`npm run lint:fix`), and
+  - updated the changelog, if applicable.
+-->

.github/stale.yml

@@ -0,0 +1,67 @@
+# Configuration for probot-stale - https://github.com/probot/stale
+
+# Number of days of inactivity before an Issue or Pull Request becomes stale
+daysUntilStale: 15
+
+# Number of days of inactivity before an Issue or Pull Request with the stale label is closed.
+# Set to false to disable. If disabled, issues still need to be closed manually, but will remain marked as stale.
+daysUntilClose: 15
+
+# Issues or Pull Requests with these labels will never be considered stale. Set to `[]` to disable
+exemptLabels:
+  - on hold
+  - meta
+
+# Set to true to ignore issues in a project (defaults to false)
+exemptProjects: false
+
+# Set to true to ignore issues in a milestone (defaults to false)
+exemptMilestones: false
+
+# Set to true to ignore issues with an assignee (defaults to false)
+exemptAssignees: false
+
+# Label to use when marking as stale
+staleLabel: stale
+
+# Comment to post when marking as stale. Set to `false` to disable
+markComment: >
+  Hi all!
+
+  This Pull Request has not had any recent activity, is it still relevant? If so, what is blocking it?
+  Is there anything we can do to help move it forward?
+
+  Thanks!
+
+
+# Comment to post when removing the stale label.
+# unmarkComment: >
+#   Your comment here.
+
+# Comment to post when closing a stale Issue or Pull Request.
+closeComment: >
+  Hi folks!
+
+  This Pull Request is being closed as there was no response to the previous prompt.
+  However, please leave a comment whenever you're ready to resume, so it can be reopened.
+
+  Thanks again!
+
+
+# Limit the number of actions per hour, from 1-30. Default is 30
+limitPerRun: 30
+
+# Limit to only `issues` or `pulls`
+only: pulls
+
+# Optionally, specify configuration settings that are specific to just 'issues' or 'pulls':
+# pulls:
+#   daysUntilStale: 30
+#   markComment: >
+#     This pull request has been automatically marked as stale because it has not had
+#     recent activity. It will be closed if no further activity occurs. Thank you
+#     for your contributions.
+
+# issues:
+#   exemptLabels:
+#     - confirmed

.gitignore

@@ -32,8 +32,21 @@ npm-debug.log
 # truffle build directory
 build/
 
-# lol macs
-.DS_Store/
+# macOS
+.DS_Store
 
 # truffle
 .node-xmlhttprequest-*
+
+# IntelliJ IDE
+.idea
+
+# docs artifacts
+docs/modules/api
+
+# only used to package @openzeppelin/contracts
+contracts/build/
+contracts/README.md
+
+# temporary artifact from solidity-coverage
+allFiredEvents

.mocharc.js

@@ -0,0 +1,3 @@
+module.exports = {
+  timeout: 4000,
+};

.node-version

@@ -1 +0,0 @@
-v8.9.1

.solcover.js

@@ -1,9 +1,8 @@
 module.exports = {
     norpc: true,
-    testCommand: 'node --max-old-space-size=4096 ../node_modules/.bin/truffle test --network coverage',
-    compileCommand: 'node --max-old-space-size=4096 ../node_modules/.bin/truffle compile --network coverage',
+    testCommand: 'npm test',
+    compileCommand: 'npm run compile',
     skipFiles: [
-        'lifecycle/Migrations.sol',
-        'mocks'
+        'mocks',
     ]
 }

.solhint.json

@@ -0,0 +1,11 @@
+{
+  "extends": "solhint:recommended",
+  "rules": {
+    "func-order": "off",
+    "mark-callable-contracts": "off",
+    "no-empty-blocks": "off",
+    "compiler-version": ["error", "^0.6.0"],
+    "private-vars-leading-underscore": "error",
+    "reason-string": "off"
+  }
+}

.soliumignore

@@ -1 +0,0 @@
-node_modules

.soliumrc.json

@@ -1,17 +0,0 @@
-{
-  "extends": "solium:all",
-  "plugins": ["security"],
-  "rules": {
-    "error-reason": "off",
-    "indentation": ["error", 2],
-    "max-len": ["warning", 79],
-    "no-constant": ["error"],
-    "no-empty-blocks": "off",
-    "quotes": ["error", "double"],
-    "uppercase": "off",
-
-    "security/enforce-explicit-visibility": ["error"],
-    "security/no-block-members": ["warning"],
-    "security/no-inline-assembly": ["warning"]
-  }
-}

.travis.yml

@@ -1,31 +0,0 @@
-dist: trusty
-sudo: required
-group: beta
-language: node_js
-node_js:
-  - "8"
-cache:
-  directories:
-    - node_modules
-env:
-  -
-  - SOLIDITY_COVERAGE=true
-  - SOLC_NIGHTLY=true
-matrix:
-  fast_finish: true
-  allow_failures:
-    - env: SOLIDITY_COVERAGE=true
-    - env: SOLC_NIGHTLY=true
-before_script:
-  - truffle version
-script:
-  - npm run lint
-  - npm run lint:sol
-  - npm run test
-notifications:
-  slack:
-    rooms:
-      - secure: uEhwUkuwJp5pBNh+VTEytPHz3FDKsnPrKO+8MTAKv5hKi4PCRoVhLv6pklr82HUpL6pvSvJbUPA0HVebOXA+MMSxdny/BHZTh2mtw5Y78l2Ad0svDTWuV2Lus2pmhYigRhT0Wo00/SRX9+pxm0kg4EIFJSTS+uR9G76x0l9NljpEGXrqxlDxjxoHBgk8Ciru2LHaLzX/utE3jlABts4Sb1F3wc2BwFkjd6BDCRTGAPhVJwwFk41ZfnmLVbgSNUyk46Cb38oG5oXHb0FI3d3jV/k1OUhRyFfmA2fLXRk0wavibW8TG1gGJJWZ7xTCKzw/Cvup6mpehSAeQef8eekMdjpWEhF9hYRq1BvOs0384UU8NQ0O+BtdXU+X3Nyr84TMJN/iIfgN7gYX7AsvXH3jC0JfNUcIkWlJvyXdE6l2GV1hMmhL09GFEBbSpuSXRIWlOXTcYBlp5NbvE8xO8PUW+T6N5RG2XXjv1g8wCpr6Wwk1+LmRkX5trv8MFBZ2pM8p4H5da5++Ov8egLonNGK2jbx6aBLBX3tPf+g70LZEkiQ4eBfZw8VIgXIvKreisicppNuCD27gNmSEPNt0NkwiEBcTCJ9GSVAO0CU2g4ggvHDX2A+RW5XPET9bGkBXKLfFyV7Qe+MSQjXkCnW3bIRh7Wo1V31XiUiYOLuZPIiH3EQ=
-    on_success: change
-    on_failure: always
-    on_pull_requests: false

CHANGELOG.md

@@ -0,0 +1,220 @@
+# Changelog
+
+## 3.2.0 (unreleased)
+
+### New features
+ * Proxies: added the proxy contracts from OpenZeppelin SDK. ([#2335](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2335))
+
+### Improvements
+ * `Address.isContract`: switched from `extcodehash` to `extcodesize` for less gas usage. ([#2311](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2311))
+
+### Breaking changes
+ * `ERC20Snapshot`: switched to using `_beforeTokenTransfer` hook instead of overriding ERC20 operations. ([#2312](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2312))
+
+This small change in the way we implemented `ERC20Snapshot` may affect users who are combining this contract with
+other ERC20 flavors, since it no longer overrides `_transfer`, `_mint`, and `_burn`. This can result in having to remove Solidity `override(...)` specifiers in derived contracts for these functions, and to instead have to add it for `_beforeTokenTransfer`. See [Using Hooks](https://docs.openzeppelin.com/contracts/3.x/extending-contracts#using-hooks) in the documentation.
+
+## 3.1.0 (2020-06-23)
+
+### New features
+ * `SafeCast`: added functions to downcast signed integers (e.g. `toInt32`), improving usability of `SignedSafeMath`. ([#2243](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2243))
+ * `functionCall`: new helpers that replicate Solidity's function call semantics, reducing the need to rely on `call`. ([#2264](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2264))
+ * `ERC1155`: added support for a base implementation, non-standard extensions and a preset contract. ([#2014](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2014), [#2230](https://github.com/OpenZeppelin/openzeppelin-contracts/issues/2230))
+
+### Improvements
+ * `ReentrancyGuard`: reduced overhead of using the `nonReentrant` modifier. ([#2171](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2171))
+ * `AccessControl`: added a `RoleAdminChanged` event to `_setAdminRole`. ([#2214](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2214))
+ * Made all `public` functions in the token preset contracts `virtual`. ([#2257](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2257))
+
+### Deprecations
+ * `SafeERC20`: deprecated `safeApprove`. ([#2268](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2268))
+
+## 3.0.2 (2020-06-08)
+
+### Improvements
+ * Added SPX license identifier to all contracts. ([#2235](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2235))
+
+## 3.0.1 (2020-04-27)
+
+### Bugfixes
+ * `ERC777`: fixed the `_approve` internal function not validating some of their arguments for non-zero addresses. ([#2213](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2213))
+
+## 3.0.0 (2020-04-20)
+
+### New features
+ * `AccessControl`: new contract for managing permissions in a system, replacement for `Ownable` and `Roles`. ([#2112](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2112))
+ * `SafeCast`: new functions to convert to and from signed and unsigned values: `toUint256` and `toInt256`. ([#2123](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2123))
+ * `EnumerableMap`: a new data structure for key-value pairs (like `mapping`) that can be iterated over. ([#2160](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2160))
+
+### Breaking changes
+ * `ERC721`: `burn(owner, tokenId)` was removed, use `burn(tokenId)` instead. ([#2125](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2125))
+ * `ERC721`: `_checkOnERC721Received` was removed. ([#2125](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2125))
+ * `ERC721`: `_transferFrom` and `_safeTransferFrom` were renamed to `_transfer` and `_safeTransfer`. ([#2162](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2162))
+ * `Ownable`: removed `_transferOwnership`. ([#2162](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2162))
+ * `PullPayment`, `Escrow`: `withdrawWithGas` was removed. The old `withdraw` function now forwards all gas. ([#2125](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2125))
+ * `Roles` was removed, use `AccessControl` as a replacement. ([#2112](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2112))
+ * `ECDSA`: when receiving an invalid signature, `recover` now reverts instead of returning the zero address. ([#2114](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2114))
+ * `Create2`: added an `amount` argument to `deploy` for contracts with `payable` constructors. ([#2117](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2117))
+ * `Pausable`: moved to the `utils` directory. ([#2122](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2122))
+ * `Strings`: moved to the `utils` directory. ([#2122](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2122))
+ * `Counters`: moved to the `utils` directory. ([#2122](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2122))
+ * `SignedSafeMath`: moved to the `math` directory. ([#2122](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2122))
+ * `ERC20Snapshot`: moved to the `token/ERC20` directory. `snapshot` was changed into an `internal` function. ([#2122](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2122))
+ * `Ownable`: moved to the `access` directory. ([#2120](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2120))
+ * `Ownable`: removed `isOwner`. ([#2120](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2120))
+ * `Secondary`: removed from the library, use `Ownable` instead. ([#2120](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2120))
+ * `Escrow`, `ConditionalEscrow`, `RefundEscrow`: these now use `Ownable` instead of `Secondary`, their external API changed accordingly. ([#2120](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2120))
+ * `ERC20`: removed `_burnFrom`. ([#2119](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2119))
+ * `Address`: removed `toPayable`, use `payable(address)` instead. ([#2133](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2133))
+ * `ERC777`: `_send`, `_mint` and `_burn` now use the caller as the operator. ([#2134](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2134))
+ * `ERC777`: removed `_callsTokensToSend` and `_callTokensReceived`. ([#2134](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2134))
+ * `EnumerableSet`: renamed `get` to `at`. ([#2151](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2151))
+ * `ERC165Checker`: functions no longer have a leading underscore. ([#2150](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2150))
+ * `ERC721Metadata`, `ERC721Enumerable`: these contracts were removed, and their functionality merged into `ERC721`. ([#2160](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2160))
+ * `ERC721`: added a constructor for `name` and `symbol`. ([#2160](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2160))
+ * `ERC20Detailed`: this contract was removed and its functionality merged into `ERC20`. ([#2161](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2161))
+ * `ERC20`: added a constructor for `name` and `symbol`. `decimals` now defaults to 18. ([#2161](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2161))
+ * `Strings`: renamed `fromUint256` to `toString` ([#2188](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2188))
+
+## 2.5.1 (2020-04-24)
+
+### Bugfixes
+ * `ERC777`: fixed the `_send` and `_approve` internal functions not validating some of their arguments for non-zero addresses. ([#2212](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2212))
+
+## 2.5.0 (2020-02-04)
+
+### New features
+ * `SafeCast.toUintXX`: new library for integer downcasting, which allows for safe operation on smaller types (e.g. `uint32`) when combined with `SafeMath`. ([#1926](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1926))
+ * `ERC721Metadata`: added `baseURI`, which can be used for dramatic gas savings when all token URIs share a prefix (e.g. `http://api.myapp.com/tokens/<id>`). ([#1970](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1970))
+ * `EnumerableSet`: new library for storing enumerable sets of values. Only `AddressSet` is supported in this release. ([#2061](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/2061))
+ * `Create2`: simple library to make usage of the `CREATE2` opcode easier. ([#1744](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1744))
+
+### Improvements
+ * `ERC777`: `_burn` is now internal, providing more flexibility and making it easier to create tokens that deflate. ([#1908](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1908))
+ * `ReentrancyGuard`: greatly improved gas efficiency by using the net gas metering mechanism introduced in the Istanbul hardfork. ([#1992](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1992), [#1996](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1996))
+ * `ERC777`: improve extensibility by making `_send` and related functions `internal`. ([#2027](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2027))
+ * `ERC721`: improved revert reason when transferring tokens to a non-recipient contract. ([#2018](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2018))
+
+### Breaking changes
+ * `ERC165Checker` now requires a minimum Solidity compiler version of 0.5.10. ([#1829](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1829))
+
+## 2.4.0 (2019-10-29)
+
+### New features
+ * `Address.toPayable`: added a helper to convert between address types without having to resort to low-level casting. ([#1773](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1773))
+ * Facilities to make metatransaction-enabled contracts through the Gas Station Network. ([#1844](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1844))
+ * `Address.sendValue`: added a replacement to Solidity's `transfer`, removing the fixed gas stipend. ([#1962](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1962))
+ * Added replacement for functions that don't forward all gas (which have been deprecated): ([#1976](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1976))
+   * `PullPayment.withdrawPaymentsWithGas(address payable payee)`
+   * `Escrow.withdrawWithGas(address payable payee)`
+ * `SafeMath`: added support for custom error messages to `sub`, `div` and `mod` functions. ([#1828](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1828))
+
+### Improvements
+ * `Address.isContract`: switched from `extcodesize` to `extcodehash` for less gas usage. ([#1802](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1802))
+ * `ERC20` and `ERC777` updated to throw custom errors on subtraction overflows. ([#1828](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1828))
+
+### Deprecations
+ * Deprecated functions that don't forward all gas: ([#1976](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1976))
+   * `PullPayment.withdrawPayments(address payable payee)`
+   * `Escrow.withdraw(address payable payee)`
+
+### Breaking changes
+ * `Address` now requires a minimum Solidity compiler version of 0.5.5. ([#1802](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1802))
+ * `SignatureBouncer` has been removed from drafts, both to avoid confusions with the GSN and `GSNRecipientSignature` (previously called `GSNBouncerSignature`) and because the API was not very clear. ([#1879](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1879))
+
+### How to upgrade from 2.4.0-beta
+
+The final 2.4.0 release includes a refactor of the GSN contracts that will be a breaking change for 2.4.0-beta users.
+
+ * The default empty implementations of `_preRelayedCall` and `_postRelayedCall` were removed and must now be explicitly implemented always in custom recipients. If your custom recipient didn't include an implementation, you can provide an empty one.
+ * `GSNRecipient`, `GSNBouncerBase`, and `GSNContext` were all merged into `GSNRecipient`.
+ * `GSNBouncerSignature` and `GSNBouncerERC20Fee` were renamed to `GSNRecipientSignature` and `GSNRecipientERC20Fee`.
+ * It is no longer necessary to inherit from `GSNRecipient` when using `GSNRecipientSignature` and `GSNRecipientERC20Fee`.
+
+For example, a contract using `GSNBouncerSignature` would have to be changed in the following way.
+
+```diff
+-contract MyDapp is GSNRecipient, GSNBouncerSignature {
++contract MyDapp is GSNRecipientSignature {
+```
+
+Refer to the table below to adjust your inheritance list.
+
+| 2.4.0-beta                         | 2.4.0                        |
+| ---------------------------------- | ---------------------------- |
+| `GSNRecipient, GSNBouncerSignature`| `GSNRecipientSignature`      |
+| `GSNRecipient, GSNBouncerERC20Fee` | `GSNRecipientERC20Fee`       |
+| `GSNBouncerBase`                   | `GSNRecipient`               |
+
+## 2.3.0 (2019-05-27)
+
+### New features
+ * `ERC1820`: added support for interacting with the [ERC1820](https://eips.ethereum.org/EIPS/eip-1820) registry contract (`IERC1820Registry`), as well as base contracts that can be registered as implementers there. ([#1677](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1677))
+ * `ERC777`: support for the [ERC777 token](https://eips.ethereum.org/EIPS/eip-777), which has multiple improvements over `ERC20` (but is backwards compatible with it) such as built-in burning, a more  straightforward permission system, and optional sender and receiver hooks on transfer (mandatory for contracts!). ([#1684](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1684))
+ * All contracts now have revert reason strings, which give insight into error conditions, and help debug failing transactions. ([#1704](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1704))
+
+### Improvements
+ * Reverted the Solidity version bump done in v2.2.0, setting the minimum compiler version to v0.5.0, to prevent unexpected build breakage. Users are encouraged however to stay on top of new compiler releases, which usually include bugfixes. ([#1729](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1729))
+
+### Bugfixes
+ * `PostDeliveryCrowdsale`: some validations where skipped when paired with other crowdsale flavors, such as `AllowanceCrowdsale`, or `MintableCrowdsale` and `ERC20Capped`, which could cause buyers to not be able to claim their purchased tokens. ([#1721](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1721))
+ * `ERC20._transfer`: the `from` argument was allowed to be the zero address, so it was possible to internally trigger a transfer of 0 tokens from the zero address. This address is not a valid destinatary of transfers, nor can it give or receive allowance, so this behavior was inconsistent. It now reverts. ([#1752](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1752))
+
+## 2.2.0 (2019-03-14)
+
+### New features
+ * `ERC20Snapshot`: create snapshots on demand of the token balances and total supply, to later retrieve and e.g. calculate dividends at a past time. ([#1617](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1617))
+ * `SafeERC20`: `ERC20` contracts with no return value (i.e. that revert on failure) are now supported. ([#1655](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1655))
+ * `ERC20`: added internal `_approve(address owner, address spender, uint256 value)`, allowing derived contracts to set the allowance of arbitrary accounts. ([#1609](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1609))
+ * `ERC20Metadata`: added internal `_setTokenURI(string memory tokenURI)`. ([#1618](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1618))
+ * `TimedCrowdsale`: added internal `_extendTime(uint256 newClosingTime)` as well as `TimedCrowdsaleExtended(uint256 prevClosingTime, uint256 newClosingTime)` event allowing to extend the crowdsale, as long as it hasn't already closed.
+
+### Improvements
+ * Upgraded the minimum compiler version to v0.5.2: this removes many Solidity warnings that were false positives. ([#1606](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1606))
+ * `ECDSA`: `recover` no longer accepts malleable signatures (those using upper-range values for `s`, or 0/1 for `v`). ([#1622](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1622))
+ * ``ERC721``'s transfers are now more gas efficient due to removal of unnecessary `SafeMath` calls. ([#1610](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1610))
+ * Fixed variable shadowing issues. ([#1606](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1606))
+
+### Bugfixes
+ * (minor) `SafeERC20`: `safeApprove` wasn't properly checking for a zero allowance when attempting to set a non-zero allowance. ([#1647](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1647))
+
+### Breaking changes in drafts
+ * `TokenMetadata` has been renamed to `ERC20Metadata`. ([#1618](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1618))
+ * The library `Counter` has been renamed to `Counters` and its API has been improved. See an example in `ERC721`, lines [17](https://github.com/OpenZeppelin/openzeppelin-solidity/blob/3cb4a00fce1da76196ac0ac3a0ae9702b99642b5/contracts/token/ERC721/ERC721.sol#L17) and [204](https://github.com/OpenZeppelin/openzeppelin-solidity/blob/3cb4a00fce1da76196ac0ac3a0ae9702b99642b5/contracts/token/ERC721/ERC721.sol#L204). ([#1610](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1610))
+
+## 2.1.3 (2019-02-26)
+ * Backported `SafeERC20.safeApprove` bugfix. ([#1647](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1647))
+
+## 2.1.2 (2019-01-17)
+ * Removed most of the test suite from the npm package, except `PublicRole.behavior.js`, which may be useful to users testing their own `Roles`.
+
+## 2.1.1 (2019-01-04)
+ * Version bump to avoid conflict in the npm registry.
+
+## 2.1.0 (2019-01-04)
+
+### New features
+ * Now targeting the 0.5.x line of Solidity compilers. For 0.4.24 support, use version 2.0 of OpenZeppelin.
+ * `WhitelistCrowdsale`: a crowdsale where only whitelisted accounts (`WhitelistedRole`) can purchase tokens. Adding or removing accounts from the whitelist is done by whitelist admins (`WhitelistAdminRole`). Similar to the pre-2.0 `WhitelistedCrowdsale`. ([#1525](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1525), [#1589](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1589))
+ * `RefundablePostDeliveryCrowdsale`: replacement for `RefundableCrowdsale` (deprecated, see below) where tokens are only granted once the crowdsale ends (if it meets its goal). ([#1543](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1543))
+ * `PausableCrowdsale`: allows for pausers (`PauserRole`) to pause token purchases. Other crowdsale operations (e.g. withdrawals and refunds, if applicable) are not affected. ([#832](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/832))
+ * `ERC20`: `transferFrom` and `_burnFrom ` now emit `Approval` events, to represent the token's state comprehensively through events. ([#1524](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1524))
+ * `ERC721`: added `_burn(uint256 tokenId)`, replacing the similar deprecated function (see below). ([#1550](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1550))
+ * `ERC721`: added `_tokensOfOwner(address owner)`, allowing to internally retrieve the array of an account's owned tokens. ([#1522](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1522))
+ * Crowdsales: all constructors are now `public`, meaning it is not necessary to extend these contracts in order to deploy them. The exception is `FinalizableCrowdsale`, since it is meaningless unless extended. ([#1564](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1564))
+ * `SignedSafeMath`: added overflow-safe operations for signed integers (`int256`). ([#1559](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1559), [#1588](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1588))
+
+### Improvements
+ * The compiler version required by `Array` was behind the rest of the libray so it was updated to `v0.4.24`. ([#1553](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1553))
+ * Now conforming to a 4-space indentation code style. ([1508](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1508))
+ * `ERC20`: more gas efficient due to removed redundant `require`s. ([#1409](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1409))
+ * `ERC721`: fixed a bug that prevented internal data structures from being properly cleaned, missing potential gas refunds. ([#1539](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1539) and [#1549](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1549))
+ * `ERC721`: general gas savings on `transferFrom`, `_mint` and `_burn`, due to redudant `require`s and `SSTORE`s. ([#1549](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1549))
+
+### Bugfixes
+
+### Breaking changes
+
+### Deprecations
+ * `ERC721._burn(address owner, uint256 tokenId)`: due to the `owner` parameter being unnecessary. ([#1550](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1550))
+ * `RefundableCrowdsale`: due to trading abuse potential on crowdsales that miss their goal. ([#1543](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1543))

CONTRIBUTING.md

@@ -1,30 +1,27 @@
-Contributing to OpenZeppelin
+Contributing to OpenZeppelin Contracts
 =======
 
-We really appreciate and value contributions to OpenZeppelin. Please take 5' to review the items listed below to make sure that your contributions are merged as soon as possible. 
+We really appreciate and value contributions to OpenZeppelin Contracts. Please take 5' to review the items listed below to make sure that your contributions are merged as soon as possible.
 
 ## Contribution guidelines
 
-Smart contracts manage value and are highly vulnerable to errors and attacks. We have very strict guidelines, please make sure to review them: ["Contribution guidelines wiki entry"](https://github.com/OpenZeppelin/openzeppelin-solidity/wiki/Contribution-guidelines).
+Smart contracts manage value and are highly vulnerable to errors and attacks. We have very strict [guidelines], please make sure to review them!
 
 ## Creating Pull Requests (PRs)
 
 As a contributor, you are expected to fork this repository, work on your own fork and then submit pull requests. The pull requests will be reviewed and eventually merged into the main repo. See ["Fork-a-Repo"](https://help.github.com/articles/fork-a-repo/) for how this works.
 
-*IMPORTANT* 
-* Please see ["Git flow wiki entry"](https://github.com/OpenZeppelin/openzeppelin-solidity/wiki/Git-flow) for understanding how to use branches in this repository.
-
 ## A typical workflow
 
 1) Make sure your fork is up to date with the main repository:
 
 ```
-cd openzeppelin-solidity
-git remote add upstream https://github.com/OpenZeppelin/openzeppelin-solidity.git
+cd openzeppelin-contracts
+git remote add upstream https://github.com/OpenZeppelin/openzeppelin-contracts.git
 git fetch upstream
 git pull --rebase upstream master
 ```
-NOTE: The directory `openzeppelin-solidity` represents your fork's local copy.
+NOTE: The directory `openzeppelin-contracts` represents your fork's local copy.
 
 2) Branch out from `master` into `fix/some-bug-#123`:
 (Postfixing #123 will associate your PR with the issue #123 and make everyone's life easier =D)
@@ -32,7 +29,7 @@ NOTE: The directory `openzeppelin-solidity` represents your fork's local copy.
 git checkout -b fix/some-bug-#123
 ```
 
-3) Make your changes, add your files, commit and push to your fork.
+3) Make your changes, add your files, commit, and push to your fork.
 
 ```
 git add SomeFile.js
@@ -40,19 +37,35 @@ git commit "Fix some bug #123"
 git push origin fix/some-bug-#123
 ```
 
-4) Go to [github.com/OpenZeppelin/openzeppelin-solidity](https://github.com/OpenZeppelin/zeppelin-solidity) in your web browser and issue a new pull request.
+4) Run tests, linter, etc. This can be done by running local continuous integration and make sure it passes.
 
-*IMPORTANT* Read the PR template very carefully and make sure to follow all the instructions. These instructions 
-refer to some very important conditions that your PR must meet in order to be accepted, such as making sure that all tests pass, JS linting tests pass, solidity linting tests pass, etc.
+```bash
+npm test
+npm run lint
+```
 
-5) Maintainers will review your code and possibly ask for changes before your code is pulled in to the main repository. We'll check that all tests pass, review the coding style, and check for general code correctness. If everything is OK, we'll merge your pull request and your code will be part of OpenZeppelin.
+or you can simply run CircleCI locally
+```bash
+circleci local execute --job build
+circleci local execute --job test
+```
+*Note*: requires installing CircleCI and docker locally on your machine.
+
+5) Go to [github.com/OpenZeppelin/openzeppelin-contracts](https://github.com/OpenZeppelin/openzeppelin-contracts) in your web browser and issue a new pull request.
+
+*IMPORTANT* Read the PR template very carefully and make sure to follow all the instructions. These instructions
+refer to some very important conditions that your PR must meet in order to be accepted, such as making sure that all tests pass, JS linting tests pass, Solidity linting tests pass, etc.
+
+6) Maintainers will review your code and possibly ask for changes before your code is pulled in to the main repository. We'll check that all tests pass, review the coding style, and check for general code correctness. If everything is OK, we'll merge your pull request and your code will be part of OpenZeppelin.
 
 *IMPORTANT* Please pay attention to the maintainer's feedback, since its a necessary step to keep up with the standards OpenZeppelin attains to.
 
 ## All set!
 
-If you have any questions feel free to post them to github.com/OpenZeppelin/openzeppelin-solidity/issues.
+If you have any questions, feel free to post them to github.com/OpenZeppelin/openzeppelin-contracts/issues.
 
-Finally, if you're looking to collaborate and want to find easy tasks to start, look at the issues we marked as ["Good first issue"](https://github.com/OpenZeppelin/openzeppelin-solidity/labels/good%20first%20issue).
+Finally, if you're looking to collaborate and want to find easy tasks to start, look at the issues we marked as ["Good first issue"](https://github.com/OpenZeppelin/openzeppelin-contracts/labels/good%20first%20issue).
 
 Thanks for your time and code!
+
+[guidelines]: GUIDELINES.md

DOCUMENTATION.md

@@ -0,0 +1,16 @@
+Documentation is hosted at https://docs.openzeppelin.com/contracts.
+
+All of the content for the site is in this repository. The guides are in the
+[docs](/docs) directory, and the API Reference is extracted from comments in
+the source code. If you want to help improve the content, this is the
+repository you should be contributing to.
+
+[`solidity-docgen`](https://github.com/OpenZeppelin/solidity-docgen) is the
+program that extracts the API Reference from source code.
+
+The [`docs.openzeppelin.com`](https://github.com/OpenZeppelin/docs.openzeppelin.com)
+repository hosts the configuration for the entire site, which includes
+documentation for all of the OpenZeppelin projects.
+
+To run the docs locally you should run `npm run docs:watch` on this
+repository.

GUIDELINES.md

@@ -0,0 +1,105 @@
+Design Guidelines
+=======
+
+These are some global design goals in OpenZeppelin.
+
+#### D0 - Security in Depth
+We strive to provide secure, tested, audited code. To achieve this, we need to match intention with function. Thus, documentation, code clarity, community review and security discussions are fundamental.
+
+#### D1 - Simple and Modular
+Simpler code means easier audits, and better understanding of what each component does. We look for small files, small contracts, and small functions. If you can separate a contract into two independent functionalities you should probably do it.
+
+#### D2 - Naming Matters
+
+We take our time with picking names. Code is going to be written once, and read hundreds of times. Renaming for clarity is encouraged.
+
+#### D3 - Tests
+
+Write tests for all your code. We encourage Test Driven Development so we know when our code is right. Even though not all code in the repository is tested at the moment, we aim to test every line of code in the future.
+
+#### D4 - Check preconditions and post-conditions
+
+A very important way to prevent vulnerabilities is to catch a contract’s inconsistent state as early as possible. This is why we want functions to check pre- and post-conditions for executing its logic. When writing code, ask yourself what you are expecting to be true before and after the function runs, and express it in code.
+
+#### D5 - Code Consistency
+
+Consistency on the way classes are used is paramount to an easier understanding of the library. The codebase should be as unified as possible. Read existing code and get inspired before you write your own. Follow the style guidelines. Don’t hesitate to ask for help on how to best write a specific piece of code.
+
+#### D6 - Regular Audits
+Following good programming practices is a way to reduce the risk of vulnerabilities, but professional code audits are still needed. We will perform regular code audits on major releases, and hire security professionals to provide independent review.
+
+# Style Guidelines
+
+The design guidelines have quite a high abstraction level. These style guidelines are more concrete and easier to apply, and also more opinionated. We value clean code and consistency, and those are prerequisites for us to include new code in the repository. Before proposing a change, please read these guidelines and take some time to familiarize yourself with the style of the existing codebase.
+
+## Solidity code
+
+In order to be consistent with all the other Solidity projects, we follow the
+[official recommendations documented in the Solidity style guide](http://solidity.readthedocs.io/en/latest/style-guide.html).
+
+Any exception or additions specific to our project are documented below.
+
+* Try to avoid acronyms and abbreviations.
+
+* All state variables should be private.
+
+* Private state variables should have an underscore prefix.
+
+    ```
+    contract TestContract {
+      uint256 private _privateVar;
+      uint256 internal _internalVar;
+    }
+    ```
+
+* Parameters must not be prefixed with an underscore.
+
+    ```
+    function test(uint256 testParameter1, uint256 testParameter2) {
+    ...
+    }
+    ```
+
+* Internal and private functions should have an underscore prefix.
+
+    ```
+    function _testInternal() internal {
+      ...
+    }
+    ```
+
+    ```
+    function _testPrivate() private {
+      ...
+    }
+    ```
+
+* Events should be emitted immediately after the state change that they
+  represent, and consequently they should be named in past tense.
+
+    ```
+    function _burn(address who, uint256 value) internal {
+      super._burn(who, value);
+      emit TokensBurned(who, value);
+    }
+    ```
+
+  Some standards (e.g. ERC20) use present tense, and in those cases the
+  standard specification prevails.
+  
+* Interface names should have a capital I prefix.
+
+    ```
+    interface IERC777 {
+    ```
+
+
+## Tests
+
+* Tests Must be Written Elegantly
+
+    Tests are a good way to show how to use the library, and maintaining them is extremely necessary. Don't write long tests, write helper functions to make them be as short and concise as possible (they should take just a few lines each), and use good variable names.
+
+* Tests Must not be Random
+
+    Inputs for tests should not be generated randomly. Accounts used to create test contracts are an exception, those can be random. Also, the type and structure of outputs should be checked.

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2016 Smart Contract Solutions, Inc.
+Copyright (c) 2016-2020 zOS Global Limited
 
 Permission is hereby granted, free of charge, to any person obtaining
 a copy of this software and associated documentation files (the

README.md

@@ -1,131 +1,78 @@
-# OpenZeppelin Solidity
-[![NPM Package](https://img.shields.io/npm/v/openzeppelin-solidity.svg?style=flat-square)](https://www.npmjs.org/package/openzeppelin-solidity)
-[![Build Status](https://img.shields.io/travis/OpenZeppelin/openzeppelin-solidity.svg?branch=master&style=flat-square)](https://travis-ci.org/OpenZeppelin/openzeppelin-solidity)
-[![Coverage Status](https://img.shields.io/coveralls/github/OpenZeppelin/openzeppelin-solidity/master.svg?style=flat-square)](https://coveralls.io/github/OpenZeppelin/openzeppelin-solidity?branch=master)
+# <img src="logo.svg" alt="OpenZeppelin" height="40px">
 
-OpenZeppelin is a library for writing secure [Smart Contracts](https://en.wikipedia.org/wiki/Smart_contract) on Ethereum.
+[![Docs](https://img.shields.io/badge/docs-%F0%9F%93%84-blue)](https://docs.openzeppelin.com/contracts)
+[![NPM Package](https://img.shields.io/npm/v/@openzeppelin/contracts.svg)](https://www.npmjs.org/package/@openzeppelin/contracts)
+[![Build Status](https://circleci.com/gh/OpenZeppelin/openzeppelin-contracts.svg?style=shield)](https://circleci.com/gh/OpenZeppelin/openzeppelin-contracts)
+[![Coverage Status](https://codecov.io/gh/OpenZeppelin/openzeppelin-contracts/graph/badge.svg)](https://codecov.io/gh/OpenZeppelin/openzeppelin-contracts)
 
-With OpenZeppelin, you can build distributed applications, protocols and organizations:
-- using common contract security patterns (See [Onward with Ethereum Smart Contract Security](https://medium.com/bitcorps-blog/onward-with-ethereum-smart-contract-security-97a827e47702#.y3kvdetbz))
-- in the [Solidity language](https://solidity.readthedocs.io/en/develop/).
+**A library for secure smart contract development.** Build on a solid foundation of community-vetted code.
 
-> NOTE: New to smart contract development? Check our [introductory guide](https://medium.com/zeppelin-blog/the-hitchhikers-guide-to-smart-contracts-in-ethereum-848f08001f05#.cox40d2ut).
+ * Implementations of standards like [ERC20](https://docs.openzeppelin.com/contracts/erc20) and [ERC721](https://docs.openzeppelin.com/contracts/erc721).
+ * Flexible [role-based permissioning](https://docs.openzeppelin.com/contracts/access-control) scheme.
+ * Reusable [Solidity components](https://docs.openzeppelin.com/contracts/utilities) to build custom contracts and complex decentralized systems.
+ * First-class integration with the [Gas Station Network](https://docs.openzeppelin.com/contracts/gsn) for systems with no gas fees!
+ * Audited by leading security firms (_last full audit on v2.0.0_).
 
-## Getting Started
+## Overview
 
-OpenZeppelin integrates with [Truffle](https://github.com/ConsenSys/truffle) and [Embark](https://github.com/embark-framework/embark/).
+### Installation
 
-## truffle
-
-To use with Truffle, first install it and initialize your project with `truffle init`.
-
-```sh
-npm install -g truffle
-mkdir myproject && cd myproject
-truffle init
+```console
+$ npm install @openzeppelin/contracts
 ```
 
-## Embark
+OpenZeppelin Contracts features a [stable API](https://docs.openzeppelin.com/contracts/releases-stability#api-stability), which means your contracts won't break unexpectedly when upgrading to a newer minor version.
 
-To use with Embark, first install it and initialize your project with `embark new MyApp`.
+### Usage
 
-```sh
-npm install -g embark
-embark new MyApp
-cd MyApp
-```
-
-## Installing OpenZeppelin
-
-After installing either Framework, to install the OpenZeppelin library, run the following in your Solidity project root directory:
-
-```sh
-npm init -y
-npm install -E openzeppelin-solidity
-```
-
-**Note that OpenZeppelin does not currently follow semantic versioning.** You may encounter breaking changes upon a minor version bump. We recommend pinning the version of OpenZeppelin you use, as done by the `-E` (`--save-exact`) option.
-
-After that, you'll get all the library's contracts in the `node_modules/openzeppelin-solidity/contracts` folder. You can use the contracts in the library like so:
+Once installed, you can use the contracts in the library by importing them:
 
 ```solidity
-import 'openzeppelin-solidity/contracts/ownership/Ownable.sol';
+pragma solidity ^0.6.0;
 
-contract MyContract is Ownable {
-  ...
+import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
+
+contract MyCollectible is ERC721 {
+    constructor() ERC721("MyCollectible", "MCO") public {
+    }
 }
 ```
 
-If you are using Embark, you can also import directly from github:
+_If you're new to smart contract development, head to [Developing Smart Contracts](https://docs.openzeppelin.com/learn/developing-smart-contracts) to learn about creating a new project and compiling your contracts._
 
-```solidity
-import "github.com/OpenZeppelin/openzeppelin-solidity/contracts/ownership/Ownable.sol#v1.9.0";
+To keep your system secure, you should **always** use the installed code as-is, and neither copy-paste it from online sources, nor modify it yourself. The library is designed so that only the contracts and functions you use are deployed, so you don't need to worry about it needlessly increasing gas costs.
 
-contract MyContract is Ownable {
-  ...
-}
-```
+## Learn More
 
-## Architecture
-The following provides visibility into how OpenZeppelin's contracts are organized:
+The guides in the [docs site](https://docs.openzeppelin.com/contracts) will teach about different concepts, and how to use the related contracts that OpenZeppelin Contracts provides:
 
-- **access** - Smart contracts that enable functionality that can be used for selective restrictions and basic authorization control functions. Includes address whitelisting and signature-based permissions management.
-	- **rbac** - A library used to manage addresses assigned to different user roles and an example Role-Based Access Control (RBAC) interface that demonstrates how to handle setters and getters for roles and addresses.
-- **crowdsale** - A collection of smart contracts used to manage token crowdsales that allow investors to purchase tokens with ETH. Includes a base contract which implements fundamental crowdsale functionality in its simplest form. The base contract can be extended in order to satisfy your crowdsale’s specific requirements.
-	- **distribution** - Includes extensions of the base crowdsale contract which can be used to customize the completion of a crowdsale.
-	- **emission** - Includes extensions of the base crowdsale contract which can be used to mint and manage how tokens are issued to purchasers.
-	- **price** - Includes extensions of the crowdsale contract that can be used to manage changes in token prices.
-	- **validation**  - Includes extensions of the crowdsale contract that can be used to enforce restraints and limit access to token purchases.
-- **examples** - A collection of simple smart contracts that demonstrate how to add new features to base contracts through multiple inheritance.
-- **introspection**  - An interface that can be used to make a contract comply with the ERC-165 standard as well as a contract that implements ERC-165 using a lookup table.
-- **lifecycle** - A collection of base contracts used to manage the existence and behavior of your contracts and their funds.
-- **math** - Libraries with safety checks on operations that throw on errors.
-- **mocks** - A collection of abstract contracts that are primarily used for unit testing. They also serve as good usage examples and demonstrate how to combine contracts with inheritence when developing your own custom applciations.
-- **ownership** - A collection of smart contracts that can be used to manage contract and token ownership
-- **payment** - A collection of smart contracts that can be used to manage payments through escrow arrangements, withdrawals, and claims. Includes support for both single payees and multiple payees.
-- **proposals** - A collection of smart contracts that reflect community Ethereum Improvement Proposals (EIPs). These contracts are under development and standardization. They are not recommended for production, but they are useful for experimentation with pending EIP standards. Go [here](https://github.com/OpenZeppelin/openzeppelin-solidity/wiki/ERC-Process) for more information.
+* [Access Control](https://docs.openzeppelin.com/contracts/access-control): decide who can perform each of the actions on your system.
+* [Tokens](https://docs.openzeppelin.com/contracts/tokens): create tradeable assets or collectives, and distribute them via [Crowdsales](https://docs.openzeppelin.com/contracts/crowdsales).
+* [Gas Station Network](https://docs.openzeppelin.com/contracts/gsn): let your users interact with your contracts without having to pay for gas themselves.
+* [Utilities](https://docs.openzeppelin.com/contracts/utilities): generic useful tools, including non-overflowing math, signature verification, and trustless paying systems.
 
-- **token** - A collection of approved ERC standard tokens -- their interfaces and implementations.
-	- **ERC20** - A standard interface for fungible tokens:
-		- *Interfaces* - Includes the ERC-20 token standard basic interface. I.e., what the contract’s ABI can represent.
-		- *Implementations* - Includes ERC-20 token implementations that include all required and some optional ERC-20 functionality.
-	- **ERC721** - A standard interface for non-fungible tokens 
-		- *Interfaces* - Includes the ERC-721 token standard basic interface. I.e., what the contract’s ABI can represent.
-		- *Implementations* - Includes ERC-721 token implementations that include all required and some optional ERC-721 functionality.
+The [full API](https://docs.openzeppelin.com/contracts/api/token/ERC20) is also thoroughly documented, and serves as a great reference when developing your smart contract application. You can also ask for help or follow Contracts's development in the [community forum](https://forum.openzeppelin.com).
 
-## Tests
-Unit test are critical to the OpenZeppelin framework. They help ensure code quality and mitigate against security vulnerabilities. The directory structure within the `/tests` directory corresponds to the `/contracts` directory. OpenZeppelin uses Mocha’s JavaScript testing framework and Chai’s assertion library. To learn more about how to tests are structured, please reference OpenZeppelin’s Testing Guide.
+Finally, you may want to take a look at the [guides on our blog](https://blog.openzeppelin.com/guides), which cover several common use cases and good practices.. The following articles provide great background reading, though please note, some of the referenced tools have changed as the tooling in the ecosystem continues to rapidly evolve.
 
-## How To Use And Modify OpenZeppelin Contracts
-When using OpenZeppelin to build your own distributed applications, for security reasons we encourage you NOT to modify the framework’s base contracts, libraries, and interfaces. In order to leverage and extend their functionality, we encourage you to inherit from them or compose them together with your own contracts.
-
-The Solidity programming language supports multiple inheritance. This is very powerful yet it can also be confusing: the more complexity you introduce to your distributed applications through multiple inheritance, the greater your application’s attack surface is.
-
-You’ll notice in the `/mocks` directory there are a collection of abstract contracts used primarily for unit testing purposes that can also be used as the foundation for your own custom implementations. These mock contracts demonstrate how OpenZeppelin’s secure base contracts can be used with multiple inheritance.
-
-To learn more about combining OpenZeppelin contracts with your own custom contracts using multiple inheritance we encourage you to read the following: [On crowdsales and multiple inheritance](https://blog.zeppelin.solutions/on-crowdsales-and-multiple-inheritance-af90c694e35f)
+* [The Hitchhiker’s Guide to Smart Contracts in Ethereum](https://blog.openzeppelin.com/the-hitchhikers-guide-to-smart-contracts-in-ethereum-848f08001f05) will help you get an overview of the various tools available for smart contract development, and help you set up your environment.
+* [A Gentle Introduction to Ethereum Programming, Part 1](https://blog.openzeppelin.com/a-gentle-introduction-to-ethereum-programming-part-1-783cc7796094) provides very useful information on an introductory level, including many basic concepts from the Ethereum platform.
+* For a more in-depth dive, you may read the guide [Designing the Architecture for Your Ethereum Application](https://blog.openzeppelin.com/designing-the-architecture-for-your-ethereum-application-9cec086f8317), which discusses how to better structure your application and its relationship to the real world.
 
 ## Security
-OpenZeppelin is meant to provide secure, tested and community-audited code, but please use common sense when doing anything that deals with real money! We take no responsibility for your implementation decisions and any security problem you might experience.
+
+This project is maintained by [OpenZeppelin](https://openzeppelin.com), and developed following our high standards for code quality and security. OpenZeppelin is meant to provide tested and community-audited code, but please use common sense when doing anything that deals with real money! We take no responsibility for your implementation decisions and any security problems you might experience.
 
 The core development principles and strategies that OpenZeppelin is based on include: security in depth, simple and modular code, clarity-driven naming conventions, comprehensive unit testing, pre-and-post-condition sanity checks, code consistency, and regular audits.
 
-If you find a security issue, please email [security@openzeppelin.org](mailto:security@openzeppelin.org).
+The latest audit was done on October 2018 on version 2.0.0.
 
-## Developer Resources
+Please report any security issues you find to security@openzeppelin.org.
 
-Building a distributed application, protocol or organization with OpenZeppelin?
+## Contribute
 
-- Read documentation: https://openzeppelin.org/api/docs/open-zeppelin.html
-
-- Ask for help and follow progress at: https://slack.openzeppelin.org/
-
-Interested in contributing to OpenZeppelin?
-
-- Framework proposal and roadmap: https://medium.com/zeppelin-blog/zeppelin-framework-proposal-and-development-roadmap-fdfa9a3a32ab#.iain47pak
-- Issue tracker: https://github.com/OpenZeppelin/openzeppelin-solidity/issues
-- Contribution guidelines: https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/CONTRIBUTING.md
-- Wiki: https://github.com/OpenZeppelin/openzeppelin-solidity/wiki
+OpenZeppelin exists thanks to its contributors. There are many ways you can participate and help build high quality software. Check out the [contribution guide](CONTRIBUTING.md)!
 
 ## License
-Code released under the [MIT License](https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/LICENSE).
+
+OpenZeppelin is released under the [MIT License](LICENSE).

RELEASING.md

@@ -0,0 +1,36 @@
+# Releasing
+
+> Visit the documentation for [details about release schedule].
+
+Start on an up-to-date `master` branch.
+
+Create the release branch with `npm run release start minor`.
+
+Publish a release candidate with `npm run release rc`.
+
+Publish the final release with `npm run release final`.
+
+Follow the general [OpenZeppelin release checklist].
+
+[details about release schedule]: https://docs.openzeppelin.com/contracts/releases-stability
+[OpenZeppelin release checklist]: https://github.com/OpenZeppelin/code-style/blob/master/RELEASE_CHECKLIST.md
+
+
+## Merging the release branch
+
+After the final release, the release branch should be merged back into `master`. This merge must not be squashed because it would lose the tagged release commit. Since the GitHub repo is set up to only allow squashed merges, the merge should be done locally and pushed.
+
+Make sure to have the latest changes from `upstream` in your local release branch.
+
+```
+git checkout release-vX.Y.Z
+git pull upstream
+```
+
+```
+git checkout master
+git merge --no-ff release-vX.Y.Z
+git push upstream master
+```
+
+The release branch can then be deleted on GitHub.

contracts/.npmignore

@@ -1,2 +0,0 @@
-mocks
-examples

contracts/AddressUtils.sol

@@ -1,29 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-/**
- * Utility library of inline functions on addresses
- */
-library AddressUtils {
-
-  /**
-   * Returns whether the target address is a contract
-   * @dev This function will return false if invoked during the constructor of a contract,
-   * as the code is not actually created until after the constructor finishes.
-   * @param _addr address to check
-   * @return whether the target address is a contract
-   */
-  function isContract(address _addr) internal view returns (bool) {
-    uint256 size;
-    // XXX Currently there is no better way to check if there is a contract in an address
-    // than to check the size of the code at that address.
-    // See https://ethereum.stackexchange.com/a/14016/36603
-    // for more details about how this works.
-    // TODO Check this again before the Serenity release, because all addresses will be
-    // contracts then.
-    // solium-disable-next-line security/no-inline-assembly
-    assembly { size := extcodesize(_addr) }
-    return size > 0;
-  }
-
-}

contracts/AutoIncrementing.sol

@@ -1,29 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-/**
- * @title AutoIncrementing
- * @author Matt Condon (@shrugs)
- * @dev Provides an auto-incrementing uint256 id acquired by the `Counter#nextId` getter.
- * Use this for issuing ERC721Token ids or keeping track of request ids, anything you want, really.
- *
- * Include with `using AutoIncrementing for AutoIncrementing.Counter;`
- * @notice Does not allow an Id of 0, which is popularly used to signify a null state in solidity.
- * Does not protect from overflows, but if you have 2^256 ids, you have other problems.
- * (But actually, it's generally impossible to increment a counter this many times, energy wise
- * so it's not something you have to worry about.)
- */
-library AutoIncrementing {
-
-  struct Counter {
-    uint256 prevId; // default: 0
-  }
-
-  function nextId(Counter storage _counter)
-    internal
-    returns (uint256)
-  {
-    _counter.prevId = _counter.prevId + 1;
-    return _counter.prevId;
-  }
-}

contracts/Bounty.sol

@@ -1,72 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-import "./payment/PullPayment.sol";
-import "./lifecycle/Destructible.sol";
-
-
-/**
- * @title Bounty
- * @dev This bounty will pay out to a researcher if they break invariant logic of the contract.
- */
-contract Bounty is PullPayment, Destructible {
-  bool public claimed;
-  mapping(address => address) public researchers;
-
-  event TargetCreated(address createdAddress);
-
-  /**
-   * @dev Fallback function allowing the contract to receive funds, if they haven't already been claimed.
-   */
-  function() external payable {
-    require(!claimed);
-  }
-
-  /**
-   * @dev Create and deploy the target contract (extension of Target contract), and sets the
-   * msg.sender as a researcher
-   * @return A target contract
-   */
-  function createTarget() public returns(Target) {
-    Target target = Target(deployContract());
-    researchers[target] = msg.sender;
-    emit TargetCreated(target);
-    return target;
-  }
-
-  /**
-   * @dev Transfers the contract funds to the researcher that proved the contract is broken.
-   * @param _target contract
-   */
-  function claim(Target _target) public {
-    address researcher = researchers[_target];
-    require(researcher != address(0));
-    // Check Target contract invariants
-    require(!_target.checkInvariant());
-    asyncTransfer(researcher, address(this).balance);
-    claimed = true;
-  }
-
-  /**
-   * @dev Internal function to deploy the target contract.
-   * @return A target contract address
-   */
-  function deployContract() internal returns(address);
-
-}
-
-
-/**
- * @title Target
- * @dev Your main contract should inherit from this class and implement the checkInvariant method.
- */
-contract Target {
-
-   /**
-    * @dev Checks all values a contract assumes to be true all the time. If this function returns
-    * false, the contract is broken in some way and is in an inconsistent state.
-    * In order to win the bounty, security researchers will try to cause this broken state.
-    * @return True if all invariant values are correct, false otherwise.
-    */
-  function checkInvariant() public returns(bool);
-}

contracts/ECRecovery.sol

@@ -1,72 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-/**
- * @title Elliptic curve signature operations
- * @dev Based on https://gist.github.com/axic/5b33912c6f61ae6fd96d6c4a47afde6d
- * TODO Remove this library once solidity supports passing a signature to ecrecover.
- * See https://github.com/ethereum/solidity/issues/864
- */
-
-library ECRecovery {
-
-  /**
-   * @dev Recover signer address from a message by using their signature
-   * @param _hash bytes32 message, the hash is the signed message. What is recovered is the signer address.
-   * @param _sig bytes signature, the signature is generated using web3.eth.sign()
-   */
-  function recover(bytes32 _hash, bytes _sig)
-    internal
-    pure
-    returns (address)
-  {
-    bytes32 r;
-    bytes32 s;
-    uint8 v;
-
-    // Check the signature length
-    if (_sig.length != 65) {
-      return (address(0));
-    }
-
-    // Divide the signature in r, s and v variables
-    // ecrecover takes the signature parameters, and the only way to get them
-    // currently is to use assembly.
-    // solium-disable-next-line security/no-inline-assembly
-    assembly {
-      r := mload(add(_sig, 32))
-      s := mload(add(_sig, 64))
-      v := byte(0, mload(add(_sig, 96)))
-    }
-
-    // Version of signature should be 27 or 28, but 0 and 1 are also possible versions
-    if (v < 27) {
-      v += 27;
-    }
-
-    // If the version is correct return the signer address
-    if (v != 27 && v != 28) {
-      return (address(0));
-    } else {
-      // solium-disable-next-line arg-overflow
-      return ecrecover(_hash, v, r, s);
-    }
-  }
-
-  /**
-   * toEthSignedMessageHash
-   * @dev prefix a bytes32 value with "\x19Ethereum Signed Message:"
-   * and hash the result
-   */
-  function toEthSignedMessageHash(bytes32 _hash)
-    internal
-    pure
-    returns (bytes32)
-  {
-    // 32 is the length in bytes of hash,
-    // enforced by the type signature above
-    return keccak256(
-      abi.encodePacked("\x19Ethereum Signed Message:\n32", _hash)
-    );
-  }
-}

contracts/GSN/Context.sol

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+/*
+ * @dev Provides information about the current execution context, including the
+ * sender of the transaction and its data. While these are generally available
+ * via msg.sender and msg.data, they should not be accessed in such a direct
+ * manner, since when dealing with GSN meta-transactions the account sending and
+ * paying for execution may not be the actual sender (as far as an application
+ * is concerned).
+ *
+ * This contract is only required for intermediate, library-like contracts.
+ */
+abstract contract Context {
+    function _msgSender() internal view virtual returns (address payable) {
+        return msg.sender;
+    }
+
+    function _msgData() internal view virtual returns (bytes memory) {
+        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
+        return msg.data;
+    }
+}

contracts/GSN/GSNRecipient.sol

@@ -0,0 +1,230 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "./IRelayRecipient.sol";
+import "./IRelayHub.sol";
+import "./Context.sol";
+
+/**
+ * @dev Base GSN recipient contract: includes the {IRelayRecipient} interface
+ * and enables GSN support on all contracts in the inheritance tree.
+ *
+ * TIP: This contract is abstract. The functions {IRelayRecipient-acceptRelayedCall},
+ *  {_preRelayedCall}, and {_postRelayedCall} are not implemented and must be
+ * provided by derived contracts. See the
+ * xref:ROOT:gsn-strategies.adoc#gsn-strategies[GSN strategies] for more
+ * information on how to use the pre-built {GSNRecipientSignature} and
+ * {GSNRecipientERC20Fee}, or how to write your own.
+ */
+abstract contract GSNRecipient is IRelayRecipient, Context {
+    // Default RelayHub address, deployed on mainnet and all testnets at the same address
+    address private _relayHub = 0xD216153c06E857cD7f72665E0aF1d7D82172F494;
+
+    uint256 constant private _RELAYED_CALL_ACCEPTED = 0;
+    uint256 constant private _RELAYED_CALL_REJECTED = 11;
+
+    // How much gas is forwarded to postRelayedCall
+    uint256 constant internal _POST_RELAYED_CALL_MAX_GAS = 100000;
+
+    /**
+     * @dev Emitted when a contract changes its {IRelayHub} contract to a new one.
+     */
+    event RelayHubChanged(address indexed oldRelayHub, address indexed newRelayHub);
+
+    /**
+     * @dev Returns the address of the {IRelayHub} contract for this recipient.
+     */
+    function getHubAddr() public view override returns (address) {
+        return _relayHub;
+    }
+
+    /**
+     * @dev Switches to a new {IRelayHub} instance. This method is added for future-proofing: there's no reason to not
+     * use the default instance.
+     *
+     * IMPORTANT: After upgrading, the {GSNRecipient} will no longer be able to receive relayed calls from the old
+     * {IRelayHub} instance. Additionally, all funds should be previously withdrawn via {_withdrawDeposits}.
+     */
+    function _upgradeRelayHub(address newRelayHub) internal virtual {
+        address currentRelayHub = _relayHub;
+        require(newRelayHub != address(0), "GSNRecipient: new RelayHub is the zero address");
+        require(newRelayHub != currentRelayHub, "GSNRecipient: new RelayHub is the current one");
+
+        emit RelayHubChanged(currentRelayHub, newRelayHub);
+
+        _relayHub = newRelayHub;
+    }
+
+    /**
+     * @dev Returns the version string of the {IRelayHub} for which this recipient implementation was built. If
+     * {_upgradeRelayHub} is used, the new {IRelayHub} instance should be compatible with this version.
+     */
+    // This function is view for future-proofing, it may require reading from
+    // storage in the future.
+    function relayHubVersion() public view returns (string memory) {
+        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
+        return "1.0.0";
+    }
+
+    /**
+     * @dev Withdraws the recipient's deposits in `RelayHub`.
+     *
+     * Derived contracts should expose this in an external interface with proper access control.
+     */
+    function _withdrawDeposits(uint256 amount, address payable payee) internal virtual {
+        IRelayHub(_relayHub).withdraw(amount, payee);
+    }
+
+    // Overrides for Context's functions: when called from RelayHub, sender and
+    // data require some pre-processing: the actual sender is stored at the end
+    // of the call data, which in turns means it needs to be removed from it
+    // when handling said data.
+
+    /**
+     * @dev Replacement for msg.sender. Returns the actual sender of a transaction: msg.sender for regular transactions,
+     * and the end-user for GSN relayed calls (where msg.sender is actually `RelayHub`).
+     *
+     * IMPORTANT: Contracts derived from {GSNRecipient} should never use `msg.sender`, and use {_msgSender} instead.
+     */
+    function _msgSender() internal view virtual override returns (address payable) {
+        if (msg.sender != _relayHub) {
+            return msg.sender;
+        } else {
+            return _getRelayedCallSender();
+        }
+    }
+
+    /**
+     * @dev Replacement for msg.data. Returns the actual calldata of a transaction: msg.data for regular transactions,
+     * and a reduced version for GSN relayed calls (where msg.data contains additional information).
+     *
+     * IMPORTANT: Contracts derived from {GSNRecipient} should never use `msg.data`, and use {_msgData} instead.
+     */
+    function _msgData() internal view virtual override returns (bytes memory) {
+        if (msg.sender != _relayHub) {
+            return msg.data;
+        } else {
+            return _getRelayedCallData();
+        }
+    }
+
+    // Base implementations for pre and post relayedCall: only RelayHub can invoke them, and data is forwarded to the
+    // internal hook.
+
+    /**
+     * @dev See `IRelayRecipient.preRelayedCall`.
+     *
+     * This function should not be overridden directly, use `_preRelayedCall` instead.
+     *
+     * * Requirements:
+     *
+     * - the caller must be the `RelayHub` contract.
+     */
+    function preRelayedCall(bytes memory context) public virtual override returns (bytes32) {
+        require(msg.sender == getHubAddr(), "GSNRecipient: caller is not RelayHub");
+        return _preRelayedCall(context);
+    }
+
+    /**
+     * @dev See `IRelayRecipient.preRelayedCall`.
+     *
+     * Called by `GSNRecipient.preRelayedCall`, which asserts the caller is the `RelayHub` contract. Derived contracts
+     * must implement this function with any relayed-call preprocessing they may wish to do.
+     *
+     */
+    function _preRelayedCall(bytes memory context) internal virtual returns (bytes32);
+
+    /**
+     * @dev See `IRelayRecipient.postRelayedCall`.
+     *
+     * This function should not be overridden directly, use `_postRelayedCall` instead.
+     *
+     * * Requirements:
+     *
+     * - the caller must be the `RelayHub` contract.
+     */
+    function postRelayedCall(bytes memory context, bool success, uint256 actualCharge, bytes32 preRetVal) public virtual override {
+        require(msg.sender == getHubAddr(), "GSNRecipient: caller is not RelayHub");
+        _postRelayedCall(context, success, actualCharge, preRetVal);
+    }
+
+    /**
+     * @dev See `IRelayRecipient.postRelayedCall`.
+     *
+     * Called by `GSNRecipient.postRelayedCall`, which asserts the caller is the `RelayHub` contract. Derived contracts
+     * must implement this function with any relayed-call postprocessing they may wish to do.
+     *
+     */
+    function _postRelayedCall(bytes memory context, bool success, uint256 actualCharge, bytes32 preRetVal) internal virtual;
+
+    /**
+     * @dev Return this in acceptRelayedCall to proceed with the execution of a relayed call. Note that this contract
+     * will be charged a fee by RelayHub
+     */
+    function _approveRelayedCall() internal pure returns (uint256, bytes memory) {
+        return _approveRelayedCall("");
+    }
+
+    /**
+     * @dev See `GSNRecipient._approveRelayedCall`.
+     *
+     * This overload forwards `context` to _preRelayedCall and _postRelayedCall.
+     */
+    function _approveRelayedCall(bytes memory context) internal pure returns (uint256, bytes memory) {
+        return (_RELAYED_CALL_ACCEPTED, context);
+    }
+
+    /**
+     * @dev Return this in acceptRelayedCall to impede execution of a relayed call. No fees will be charged.
+     */
+    function _rejectRelayedCall(uint256 errorCode) internal pure returns (uint256, bytes memory) {
+        return (_RELAYED_CALL_REJECTED + errorCode, "");
+    }
+
+    /*
+     * @dev Calculates how much RelayHub will charge a recipient for using `gas` at a `gasPrice`, given a relayer's
+     * `serviceFee`.
+     */
+    function _computeCharge(uint256 gas, uint256 gasPrice, uint256 serviceFee) internal pure returns (uint256) {
+        // The fee is expressed as a percentage. E.g. a value of 40 stands for a 40% fee, so the recipient will be
+        // charged for 1.4 times the spent amount.
+        return (gas * gasPrice * (100 + serviceFee)) / 100;
+    }
+
+    function _getRelayedCallSender() private pure returns (address payable result) {
+        // We need to read 20 bytes (an address) located at array index msg.data.length - 20. In memory, the array
+        // is prefixed with a 32-byte length value, so we first add 32 to get the memory read index. However, doing
+        // so would leave the address in the upper 20 bytes of the 32-byte word, which is inconvenient and would
+        // require bit shifting. We therefore subtract 12 from the read index so the address lands on the lower 20
+        // bytes. This can always be done due to the 32-byte prefix.
+
+        // The final memory read index is msg.data.length - 20 + 32 - 12 = msg.data.length. Using inline assembly is the
+        // easiest/most-efficient way to perform this operation.
+
+        // These fields are not accessible from assembly
+        bytes memory array = msg.data;
+        uint256 index = msg.data.length;
+
+        // solhint-disable-next-line no-inline-assembly
+        assembly {
+            // Load the 32 bytes word from memory with the address on the lower 20 bytes, and mask those.
+            result := and(mload(add(array, index)), 0xffffffffffffffffffffffffffffffffffffffff)
+        }
+        return result;
+    }
+
+    function _getRelayedCallData() private pure returns (bytes memory) {
+        // RelayHub appends the sender address at the end of the calldata, so in order to retrieve the actual msg.data,
+        // we must strip the last 20 bytes (length of an address type) from it.
+
+        uint256 actualDataLength = msg.data.length - 20;
+        bytes memory actualData = new bytes(actualDataLength);
+
+        for (uint256 i = 0; i < actualDataLength; ++i) {
+            actualData[i] = msg.data[i];
+        }
+
+        return actualData;
+    }
+}

contracts/GSN/GSNRecipientERC20Fee.sol

@@ -0,0 +1,152 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "./GSNRecipient.sol";
+import "../math/SafeMath.sol";
+import "../access/Ownable.sol";
+import "../token/ERC20/SafeERC20.sol";
+import "../token/ERC20/ERC20.sol";
+
+/**
+ * @dev A xref:ROOT:gsn-strategies.adoc#gsn-strategies[GSN strategy] that charges transaction fees in a special purpose ERC20
+ * token, which we refer to as the gas payment token. The amount charged is exactly the amount of Ether charged to the
+ * recipient. This means that the token is essentially pegged to the value of Ether.
+ *
+ * The distribution strategy of the gas payment token to users is not defined by this contract. It's a mintable token
+ * whose only minter is the recipient, so the strategy must be implemented in a derived contract, making use of the
+ * internal {_mint} function.
+ */
+contract GSNRecipientERC20Fee is GSNRecipient {
+    using SafeERC20 for __unstable__ERC20Owned;
+    using SafeMath for uint256;
+
+    enum GSNRecipientERC20FeeErrorCodes {
+        INSUFFICIENT_BALANCE
+    }
+
+    __unstable__ERC20Owned private _token;
+
+    /**
+     * @dev The arguments to the constructor are the details that the gas payment token will have: `name` and `symbol`. `decimals` is hard-coded to 18.
+     */
+    constructor(string memory name, string memory symbol) public {
+        _token = new __unstable__ERC20Owned(name, symbol);
+    }
+
+    /**
+     * @dev Returns the gas payment token.
+     */
+    function token() public view returns (IERC20) {
+        return IERC20(_token);
+    }
+
+    /**
+     * @dev Internal function that mints the gas payment token. Derived contracts should expose this function in their public API, with proper access control mechanisms.
+     */
+    function _mint(address account, uint256 amount) internal virtual {
+        _token.mint(account, amount);
+    }
+
+    /**
+     * @dev Ensures that only users with enough gas payment token balance can have transactions relayed through the GSN.
+     */
+    function acceptRelayedCall(
+        address,
+        address from,
+        bytes memory,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256,
+        uint256,
+        bytes memory,
+        uint256 maxPossibleCharge
+    )
+        public
+        view
+        virtual
+        override
+        returns (uint256, bytes memory)
+    {
+        if (_token.balanceOf(from) < maxPossibleCharge) {
+            return _rejectRelayedCall(uint256(GSNRecipientERC20FeeErrorCodes.INSUFFICIENT_BALANCE));
+        }
+
+        return _approveRelayedCall(abi.encode(from, maxPossibleCharge, transactionFee, gasPrice));
+    }
+
+    /**
+     * @dev Implements the precharge to the user. The maximum possible charge (depending on gas limit, gas price, and
+     * fee) will be deducted from the user balance of gas payment token. Note that this is an overestimation of the
+     * actual charge, necessary because we cannot predict how much gas the execution will actually need. The remainder
+     * is returned to the user in {_postRelayedCall}.
+     */
+    function _preRelayedCall(bytes memory context) internal virtual override returns (bytes32) {
+        (address from, uint256 maxPossibleCharge) = abi.decode(context, (address, uint256));
+
+        // The maximum token charge is pre-charged from the user
+        _token.safeTransferFrom(from, address(this), maxPossibleCharge);
+    }
+
+    /**
+     * @dev Returns to the user the extra amount that was previously charged, once the actual execution cost is known.
+     */
+    function _postRelayedCall(bytes memory context, bool, uint256 actualCharge, bytes32) internal virtual override {
+        (address from, uint256 maxPossibleCharge, uint256 transactionFee, uint256 gasPrice) =
+            abi.decode(context, (address, uint256, uint256, uint256));
+
+        // actualCharge is an _estimated_ charge, which assumes postRelayedCall will use all available gas.
+        // This implementation's gas cost can be roughly estimated as 10k gas, for the two SSTORE operations in an
+        // ERC20 transfer.
+        uint256 overestimation = _computeCharge(_POST_RELAYED_CALL_MAX_GAS.sub(10000), gasPrice, transactionFee);
+        actualCharge = actualCharge.sub(overestimation);
+
+        // After the relayed call has been executed and the actual charge estimated, the excess pre-charge is returned
+        _token.safeTransfer(from, maxPossibleCharge.sub(actualCharge));
+    }
+}
+
+/**
+ * @title __unstable__ERC20Owned
+ * @dev An ERC20 token owned by another contract, which has minting permissions and can use transferFrom to receive
+ * anyone's tokens. This contract is an internal helper for GSNRecipientERC20Fee, and should not be used
+ * outside of this context.
+ */
+// solhint-disable-next-line contract-name-camelcase
+contract __unstable__ERC20Owned is ERC20, Ownable {
+    uint256 private constant _UINT256_MAX = 2**256 - 1;
+
+    constructor(string memory name, string memory symbol) public ERC20(name, symbol) { }
+
+    // The owner (GSNRecipientERC20Fee) can mint tokens
+    function mint(address account, uint256 amount) public onlyOwner {
+        _mint(account, amount);
+    }
+
+    // The owner has 'infinite' allowance for all token holders
+    function allowance(address tokenOwner, address spender) public view override returns (uint256) {
+        if (spender == owner()) {
+            return _UINT256_MAX;
+        } else {
+            return super.allowance(tokenOwner, spender);
+        }
+    }
+
+    // Allowance for the owner cannot be changed (it is always 'infinite')
+    function _approve(address tokenOwner, address spender, uint256 value) internal override {
+        if (spender == owner()) {
+            return;
+        } else {
+            super._approve(tokenOwner, spender, value);
+        }
+    }
+
+    function transferFrom(address sender, address recipient, uint256 amount) public override returns (bool) {
+        if (recipient == owner()) {
+            _transfer(sender, recipient, amount);
+            return true;
+        } else {
+            return super.transferFrom(sender, recipient, amount);
+        }
+    }
+}

contracts/GSN/GSNRecipientSignature.sol

@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "./GSNRecipient.sol";
+import "../cryptography/ECDSA.sol";
+
+/**
+ * @dev A xref:ROOT:gsn-strategies.adoc#gsn-strategies[GSN strategy] that allows relayed transactions through when they are
+ * accompanied by the signature of a trusted signer. The intent is for this signature to be generated by a server that
+ * performs validations off-chain. Note that nothing is charged to the user in this scheme. Thus, the server should make
+ * sure to account for this in their economic and threat model.
+ */
+contract GSNRecipientSignature is GSNRecipient {
+    using ECDSA for bytes32;
+
+    address private _trustedSigner;
+
+    enum GSNRecipientSignatureErrorCodes {
+        INVALID_SIGNER
+    }
+
+    /**
+     * @dev Sets the trusted signer that is going to be producing signatures to approve relayed calls.
+     */
+    constructor(address trustedSigner) public {
+        require(trustedSigner != address(0), "GSNRecipientSignature: trusted signer is the zero address");
+        _trustedSigner = trustedSigner;
+    }
+
+    /**
+     * @dev Ensures that only transactions with a trusted signature can be relayed through the GSN.
+     */
+    function acceptRelayedCall(
+        address relay,
+        address from,
+        bytes memory encodedFunction,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256 gasLimit,
+        uint256 nonce,
+        bytes memory approvalData,
+        uint256
+    )
+        public
+        view
+        virtual
+        override
+        returns (uint256, bytes memory)
+    {
+        bytes memory blob = abi.encodePacked(
+            relay,
+            from,
+            encodedFunction,
+            transactionFee,
+            gasPrice,
+            gasLimit,
+            nonce, // Prevents replays on RelayHub
+            getHubAddr(), // Prevents replays in multiple RelayHubs
+            address(this) // Prevents replays in multiple recipients
+        );
+        if (keccak256(blob).toEthSignedMessageHash().recover(approvalData) == _trustedSigner) {
+            return _approveRelayedCall();
+        } else {
+            return _rejectRelayedCall(uint256(GSNRecipientSignatureErrorCodes.INVALID_SIGNER));
+        }
+    }
+
+    function _preRelayedCall(bytes memory) internal virtual override returns (bytes32) { }
+
+    function _postRelayedCall(bytes memory, bool, uint256, bytes32) internal virtual override { }
+}

contracts/GSN/IRelayHub.sol

@@ -0,0 +1,269 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+/**
+ * @dev Interface for `RelayHub`, the core contract of the GSN. Users should not need to interact with this contract
+ * directly.
+ *
+ * See the https://github.com/OpenZeppelin/openzeppelin-gsn-helpers[OpenZeppelin GSN helpers] for more information on
+ * how to deploy an instance of `RelayHub` on your local test network.
+ */
+interface IRelayHub {
+    // Relay management
+
+    /**
+     * @dev Adds stake to a relay and sets its `unstakeDelay`. If the relay does not exist, it is created, and the caller
+     * of this function becomes its owner. If the relay already exists, only the owner can call this function. A relay
+     * cannot be its own owner.
+     *
+     * All Ether in this function call will be added to the relay's stake.
+     * Its unstake delay will be assigned to `unstakeDelay`, but the new value must be greater or equal to the current one.
+     *
+     * Emits a {Staked} event.
+     */
+    function stake(address relayaddr, uint256 unstakeDelay) external payable;
+
+    /**
+     * @dev Emitted when a relay's stake or unstakeDelay are increased
+     */
+    event Staked(address indexed relay, uint256 stake, uint256 unstakeDelay);
+
+    /**
+     * @dev Registers the caller as a relay.
+     * The relay must be staked for, and not be a contract (i.e. this function must be called directly from an EOA).
+     *
+     * This function can be called multiple times, emitting new {RelayAdded} events. Note that the received
+     * `transactionFee` is not enforced by {relayCall}.
+     *
+     * Emits a {RelayAdded} event.
+     */
+    function registerRelay(uint256 transactionFee, string calldata url) external;
+
+    /**
+     * @dev Emitted when a relay is registered or re-registered. Looking at these events (and filtering out
+     * {RelayRemoved} events) lets a client discover the list of available relays.
+     */
+    event RelayAdded(address indexed relay, address indexed owner, uint256 transactionFee, uint256 stake, uint256 unstakeDelay, string url);
+
+    /**
+     * @dev Removes (deregisters) a relay. Unregistered (but staked for) relays can also be removed.
+     *
+     * Can only be called by the owner of the relay. After the relay's `unstakeDelay` has elapsed, {unstake} will be
+     * callable.
+     *
+     * Emits a {RelayRemoved} event.
+     */
+    function removeRelayByOwner(address relay) external;
+
+    /**
+     * @dev Emitted when a relay is removed (deregistered). `unstakeTime` is the time when unstake will be callable.
+     */
+    event RelayRemoved(address indexed relay, uint256 unstakeTime);
+
+    /** Deletes the relay from the system, and gives back its stake to the owner.
+     *
+     * Can only be called by the relay owner, after `unstakeDelay` has elapsed since {removeRelayByOwner} was called.
+     *
+     * Emits an {Unstaked} event.
+     */
+    function unstake(address relay) external;
+
+    /**
+     * @dev Emitted when a relay is unstaked for, including the returned stake.
+     */
+    event Unstaked(address indexed relay, uint256 stake);
+
+    // States a relay can be in
+    enum RelayState {
+        Unknown, // The relay is unknown to the system: it has never been staked for
+        Staked, // The relay has been staked for, but it is not yet active
+        Registered, // The relay has registered itself, and is active (can relay calls)
+        Removed    // The relay has been removed by its owner and can no longer relay calls. It must wait for its unstakeDelay to elapse before it can unstake
+    }
+
+    /**
+     * @dev Returns a relay's status. Note that relays can be deleted when unstaked or penalized, causing this function
+     * to return an empty entry.
+     */
+    function getRelay(address relay) external view returns (uint256 totalStake, uint256 unstakeDelay, uint256 unstakeTime, address payable owner, RelayState state);
+
+    // Balance management
+
+    /**
+     * @dev Deposits Ether for a contract, so that it can receive (and pay for) relayed transactions.
+     *
+     * Unused balance can only be withdrawn by the contract itself, by calling {withdraw}.
+     *
+     * Emits a {Deposited} event.
+     */
+    function depositFor(address target) external payable;
+
+    /**
+     * @dev Emitted when {depositFor} is called, including the amount and account that was funded.
+     */
+    event Deposited(address indexed recipient, address indexed from, uint256 amount);
+
+    /**
+     * @dev Returns an account's deposits. These can be either a contract's funds, or a relay owner's revenue.
+     */
+    function balanceOf(address target) external view returns (uint256);
+
+    /**
+     * Withdraws from an account's balance, sending it back to it. Relay owners call this to retrieve their revenue, and
+     * contracts can use it to reduce their funding.
+     *
+     * Emits a {Withdrawn} event.
+     */
+    function withdraw(uint256 amount, address payable dest) external;
+
+    /**
+     * @dev Emitted when an account withdraws funds from `RelayHub`.
+     */
+    event Withdrawn(address indexed account, address indexed dest, uint256 amount);
+
+    // Relaying
+
+    /**
+     * @dev Checks if the `RelayHub` will accept a relayed operation.
+     * Multiple things must be true for this to happen:
+     *  - all arguments must be signed for by the sender (`from`)
+     *  - the sender's nonce must be the current one
+     *  - the recipient must accept this transaction (via {acceptRelayedCall})
+     *
+     * Returns a `PreconditionCheck` value (`OK` when the transaction can be relayed), or a recipient-specific error
+     * code if it returns one in {acceptRelayedCall}.
+     */
+    function canRelay(
+        address relay,
+        address from,
+        address to,
+        bytes calldata encodedFunction,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256 gasLimit,
+        uint256 nonce,
+        bytes calldata signature,
+        bytes calldata approvalData
+    ) external view returns (uint256 status, bytes memory recipientContext);
+
+    // Preconditions for relaying, checked by canRelay and returned as the corresponding numeric values.
+    enum PreconditionCheck {
+        OK,                         // All checks passed, the call can be relayed
+        WrongSignature,             // The transaction to relay is not signed by requested sender
+        WrongNonce,                 // The provided nonce has already been used by the sender
+        AcceptRelayedCallReverted,  // The recipient rejected this call via acceptRelayedCall
+        InvalidRecipientStatusCode  // The recipient returned an invalid (reserved) status code
+    }
+
+    /**
+     * @dev Relays a transaction.
+     *
+     * For this to succeed, multiple conditions must be met:
+     *  - {canRelay} must `return PreconditionCheck.OK`
+     *  - the sender must be a registered relay
+     *  - the transaction's gas price must be larger or equal to the one that was requested by the sender
+     *  - the transaction must have enough gas to not run out of gas if all internal transactions (calls to the
+     * recipient) use all gas available to them
+     *  - the recipient must have enough balance to pay the relay for the worst-case scenario (i.e. when all gas is
+     * spent)
+     *
+     * If all conditions are met, the call will be relayed and the recipient charged. {preRelayedCall}, the encoded
+     * function and {postRelayedCall} will be called in that order.
+     *
+     * Parameters:
+     *  - `from`: the client originating the request
+     *  - `to`: the target {IRelayRecipient} contract
+     *  - `encodedFunction`: the function call to relay, including data
+     *  - `transactionFee`: fee (%) the relay takes over actual gas cost
+     *  - `gasPrice`: gas price the client is willing to pay
+     *  - `gasLimit`: gas to forward when calling the encoded function
+     *  - `nonce`: client's nonce
+     *  - `signature`: client's signature over all previous params, plus the relay and RelayHub addresses
+     *  - `approvalData`: dapp-specific data forwarded to {acceptRelayedCall}. This value is *not* verified by the
+     * `RelayHub`, but it still can be used for e.g. a signature.
+     *
+     * Emits a {TransactionRelayed} event.
+     */
+    function relayCall(
+        address from,
+        address to,
+        bytes calldata encodedFunction,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256 gasLimit,
+        uint256 nonce,
+        bytes calldata signature,
+        bytes calldata approvalData
+    ) external;
+
+    /**
+     * @dev Emitted when an attempt to relay a call failed.
+     *
+     * This can happen due to incorrect {relayCall} arguments, or the recipient not accepting the relayed call. The
+     * actual relayed call was not executed, and the recipient not charged.
+     *
+     * The `reason` parameter contains an error code: values 1-10 correspond to `PreconditionCheck` entries, and values
+     * over 10 are custom recipient error codes returned from {acceptRelayedCall}.
+     */
+    event CanRelayFailed(address indexed relay, address indexed from, address indexed to, bytes4 selector, uint256 reason);
+
+    /**
+     * @dev Emitted when a transaction is relayed.
+     * Useful when monitoring a relay's operation and relayed calls to a contract
+     *
+     * Note that the actual encoded function might be reverted: this is indicated in the `status` parameter.
+     *
+     * `charge` is the Ether value deducted from the recipient's balance, paid to the relay's owner.
+     */
+    event TransactionRelayed(address indexed relay, address indexed from, address indexed to, bytes4 selector, RelayCallStatus status, uint256 charge);
+
+    // Reason error codes for the TransactionRelayed event
+    enum RelayCallStatus {
+        OK,                      // The transaction was successfully relayed and execution successful - never included in the event
+        RelayedCallFailed,       // The transaction was relayed, but the relayed call failed
+        PreRelayedFailed,        // The transaction was not relayed due to preRelatedCall reverting
+        PostRelayedFailed,       // The transaction was relayed and reverted due to postRelatedCall reverting
+        RecipientBalanceChanged  // The transaction was relayed and reverted due to the recipient's balance changing
+    }
+
+    /**
+     * @dev Returns how much gas should be forwarded to a call to {relayCall}, in order to relay a transaction that will
+     * spend up to `relayedCallStipend` gas.
+     */
+    function requiredGas(uint256 relayedCallStipend) external view returns (uint256);
+
+    /**
+     * @dev Returns the maximum recipient charge, given the amount of gas forwarded, gas price and relay fee.
+     */
+    function maxPossibleCharge(uint256 relayedCallStipend, uint256 gasPrice, uint256 transactionFee) external view returns (uint256);
+
+     // Relay penalization.
+     // Any account can penalize relays, removing them from the system immediately, and rewarding the
+    // reporter with half of the relay's stake. The other half is burned so that, even if the relay penalizes itself, it
+    // still loses half of its stake.
+
+    /**
+     * @dev Penalize a relay that signed two transactions using the same nonce (making only the first one valid) and
+     * different data (gas price, gas limit, etc. may be different).
+     *
+     * The (unsigned) transaction data and signature for both transactions must be provided.
+     */
+    function penalizeRepeatedNonce(bytes calldata unsignedTx1, bytes calldata signature1, bytes calldata unsignedTx2, bytes calldata signature2) external;
+
+    /**
+     * @dev Penalize a relay that sent a transaction that didn't target ``RelayHub``'s {registerRelay} or {relayCall}.
+     */
+    function penalizeIllegalTransaction(bytes calldata unsignedTx, bytes calldata signature) external;
+
+    /**
+     * @dev Emitted when a relay is penalized.
+     */
+    event Penalized(address indexed relay, address sender, uint256 amount);
+
+    /**
+     * @dev Returns an account's nonce in `RelayHub`.
+     */
+    function getNonce(address from) external view returns (uint256);
+}
+

contracts/GSN/IRelayRecipient.sol

@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+/**
+ * @dev Base interface for a contract that will be called via the GSN from {IRelayHub}.
+ *
+ * TIP: You don't need to write an implementation yourself! Inherit from {GSNRecipient} instead.
+ */
+interface IRelayRecipient {
+    /**
+     * @dev Returns the address of the {IRelayHub} instance this recipient interacts with.
+     */
+    function getHubAddr() external view returns (address);
+
+    /**
+     * @dev Called by {IRelayHub} to validate if this recipient accepts being charged for a relayed call. Note that the
+     * recipient will be charged regardless of the execution result of the relayed call (i.e. if it reverts or not).
+     *
+     * The relay request was originated by `from` and will be served by `relay`. `encodedFunction` is the relayed call
+     * calldata, so its first four bytes are the function selector. The relayed call will be forwarded `gasLimit` gas,
+     * and the transaction executed with a gas price of at least `gasPrice`. ``relay``'s fee is `transactionFee`, and the
+     * recipient will be charged at most `maxPossibleCharge` (in wei). `nonce` is the sender's (`from`) nonce for
+     * replay attack protection in {IRelayHub}, and `approvalData` is a optional parameter that can be used to hold a signature
+     * over all or some of the previous values.
+     *
+     * Returns a tuple, where the first value is used to indicate approval (0) or rejection (custom non-zero error code,
+     * values 1 to 10 are reserved) and the second one is data to be passed to the other {IRelayRecipient} functions.
+     *
+     * {acceptRelayedCall} is called with 50k gas: if it runs out during execution, the request will be considered
+     * rejected. A regular revert will also trigger a rejection.
+     */
+    function acceptRelayedCall(
+        address relay,
+        address from,
+        bytes calldata encodedFunction,
+        uint256 transactionFee,
+        uint256 gasPrice,
+        uint256 gasLimit,
+        uint256 nonce,
+        bytes calldata approvalData,
+        uint256 maxPossibleCharge
+    )
+        external
+        view
+        returns (uint256, bytes memory);
+
+    /**
+     * @dev Called by {IRelayHub} on approved relay call requests, before the relayed call is executed. This allows to e.g.
+     * pre-charge the sender of the transaction.
+     *
+     * `context` is the second value returned in the tuple by {acceptRelayedCall}.
+     *
+     * Returns a value to be passed to {postRelayedCall}.
+     *
+     * {preRelayedCall} is called with 100k gas: if it runs out during execution or otherwise reverts, the relayed call
+     * will not be executed, but the recipient will still be charged for the transaction's cost.
+     */
+    function preRelayedCall(bytes calldata context) external returns (bytes32);
+
+    /**
+     * @dev Called by {IRelayHub} on approved relay call requests, after the relayed call is executed. This allows to e.g.
+     * charge the user for the relayed call costs, return any overcharges from {preRelayedCall}, or perform
+     * contract-specific bookkeeping.
+     *
+     * `context` is the second value returned in the tuple by {acceptRelayedCall}. `success` is the execution status of
+     * the relayed call. `actualCharge` is an estimate of how much the recipient will be charged for the transaction,
+     * not including any gas used by {postRelayedCall} itself. `preRetVal` is {preRelayedCall}'s return value.
+     *
+     *
+     * {postRelayedCall} is called with 100k gas: if it runs out during execution or otherwise reverts, the relayed call
+     * and the call to {preRelayedCall} will be reverted retroactively, but the recipient will still be charged for the
+     * transaction's cost.
+     */
+    function postRelayedCall(bytes calldata context, bool success, uint256 actualCharge, bytes32 preRetVal) external;
+}

contracts/GSN/README.adoc

@@ -0,0 +1,31 @@
+= Gas Station Network (GSN)
+
+[.readme-notice]
+NOTE: This document is better viewed at https://docs.openzeppelin.com/contracts/api/gsn
+
+This set of contracts provide all the tools required to make a contract callable via the https://gsn.openzeppelin.com[Gas Station Network].
+
+TIP: If you're new to the GSN, head over to our xref:learn::sending-gasless-transactions.adoc[overview of the system] and basic guide to xref:ROOT:gsn.adoc[creating a GSN-capable contract].
+
+The core contract a recipient must inherit from is {GSNRecipient}: it includes all necessary interfaces, as well as some helper methods to make interacting with the GSN easier.
+
+Utilities to make writing xref:ROOT:gsn-strategies.adoc[GSN strategies] easy are available in {GSNRecipient}, or you can simply use one of our pre-made strategies:
+
+* {GSNRecipientERC20Fee} charges the end user for gas costs in an application-specific xref:ROOT:tokens.adoc#ERC20[ERC20 token]
+* {GSNRecipientSignature} accepts all relayed calls that have been signed by a trusted third party (e.g. a private key in a backend)
+
+You can also take a look at the two contract interfaces that make up the GSN protocol: {IRelayRecipient} and {IRelayHub}, but you won't need to use those directly.
+
+== Recipient
+
+{{GSNRecipient}}
+
+== Strategies
+
+{{GSNRecipientSignature}}
+{{GSNRecipientERC20Fee}}
+
+== Protocol
+
+{{IRelayRecipient}}
+{{IRelayHub}}

contracts/LimitBalance.sol

@@ -1,31 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-/**
- * @title LimitBalance
- * @dev Simple contract to limit the balance of child contract.
- * Note this doesn't prevent other contracts to send funds by using selfdestruct(address);
- * See: https://github.com/ConsenSys/smart-contract-best-practices#remember-that-ether-can-be-forcibly-sent-to-an-account
- */
-contract LimitBalance {
-
-  uint256 public limit;
-
-  /**
-   * @dev Constructor that sets the passed value as a limit.
-   * @param _limit uint256 to represent the limit.
-   */
-  constructor(uint256 _limit) public {
-    limit = _limit;
-  }
-
-  /**
-   * @dev Checks if limit was reached. Case true, it throws.
-   */
-  modifier limitedPayable() {
-    require(address(this).balance <= limit);
-    _;
-
-  }
-
-}

contracts/MerkleProof.sol

@@ -1,43 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-/**
- * @title MerkleProof
- * @dev Merkle proof verification based on
- * https://github.com/ameensol/merkle-tree-solidity/blob/master/src/MerkleProof.sol
- */
-library MerkleProof {
-  /**
-   * @dev Verifies a Merkle proof proving the existence of a leaf in a Merkle tree. Assumes that each pair of leaves
-   * and each pair of pre-images are sorted.
-   * @param _proof Merkle proof containing sibling hashes on the branch from the leaf to the root of the Merkle tree
-   * @param _root Merkle root
-   * @param _leaf Leaf of Merkle tree
-   */
-  function verifyProof(
-    bytes32[] _proof,
-    bytes32 _root,
-    bytes32 _leaf
-  )
-    internal
-    pure
-    returns (bool)
-  {
-    bytes32 computedHash = _leaf;
-
-    for (uint256 i = 0; i < _proof.length; i++) {
-      bytes32 proofElement = _proof[i];
-
-      if (computedHash < proofElement) {
-        // Hash(current computed hash + current element of the proof)
-        computedHash = keccak256(abi.encodePacked(computedHash, proofElement));
-      } else {
-        // Hash(current element of the proof + current computed hash)
-        computedHash = keccak256(abi.encodePacked(proofElement, computedHash));
-      }
-    }
-
-    // Check if the computed hash (root) is equal to the provided root
-    return computedHash == _root;
-  }
-}

contracts/ReentrancyGuard.sol

@@ -1,39 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-/**
- * @title Helps contracts guard against reentrancy attacks.
- * @author Remco Bloemen <remco@2π.com>, Eenae <alexey@mixbytes.io>
- * @dev If you mark a function `nonReentrant`, you should also
- * mark it `external`.
- */
-contract ReentrancyGuard {
-
-  /// @dev Constant for unlocked guard state - non-zero to prevent extra gas costs.
-  /// See: https://github.com/OpenZeppelin/openzeppelin-solidity/issues/1056
-  uint private constant REENTRANCY_GUARD_FREE = 1;
-
-  /// @dev Constant for locked guard state
-  uint private constant REENTRANCY_GUARD_LOCKED = 2;
-
-  /**
-   * @dev We use a single lock for the whole contract.
-   */
-  uint private reentrancyLock = REENTRANCY_GUARD_FREE;
-
-  /**
-   * @dev Prevents a contract from calling itself, directly or indirectly.
-   * If you mark a function `nonReentrant`, you should also
-   * mark it `external`. Calling one `nonReentrant` function from
-   * another is not supported. Instead, you can implement a
-   * `private` function doing the actual work, and an `external`
-   * wrapper marked as `nonReentrant`.
-   */
-  modifier nonReentrant() {
-    require(reentrancyLock == REENTRANCY_GUARD_FREE);
-    reentrancyLock = REENTRANCY_GUARD_LOCKED;
-    _;
-    reentrancyLock = REENTRANCY_GUARD_FREE;
-  }
-
-}

contracts/access/AccessControl.sol

@@ -0,0 +1,217 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "../utils/EnumerableSet.sol";
+import "../utils/Address.sol";
+import "../GSN/Context.sol";
+
+/**
+ * @dev Contract module that allows children to implement role-based access
+ * control mechanisms.
+ *
+ * Roles are referred to by their `bytes32` identifier. These should be exposed
+ * in the external API and be unique. The best way to achieve this is by
+ * using `public constant` hash digests:
+ *
+ * ```
+ * bytes32 public constant MY_ROLE = keccak256("MY_ROLE");
+ * ```
+ *
+ * Roles can be used to represent a set of permissions. To restrict access to a
+ * function call, use {hasRole}:
+ *
+ * ```
+ * function foo() public {
+ *     require(hasRole(MY_ROLE, msg.sender));
+ *     ...
+ * }
+ * ```
+ *
+ * Roles can be granted and revoked dynamically via the {grantRole} and
+ * {revokeRole} functions. Each role has an associated admin role, and only
+ * accounts that have a role's admin role can call {grantRole} and {revokeRole}.
+ *
+ * By default, the admin role for all roles is `DEFAULT_ADMIN_ROLE`, which means
+ * that only accounts with this role will be able to grant or revoke other
+ * roles. More complex role relationships can be created by using
+ * {_setRoleAdmin}.
+ *
+ * WARNING: The `DEFAULT_ADMIN_ROLE` is also its own admin: it has permission to
+ * grant and revoke this role. Extra precautions should be taken to secure
+ * accounts that have been granted it.
+ */
+abstract contract AccessControl is Context {
+    using EnumerableSet for EnumerableSet.AddressSet;
+    using Address for address;
+
+    struct RoleData {
+        EnumerableSet.AddressSet members;
+        bytes32 adminRole;
+    }
+
+    mapping (bytes32 => RoleData) private _roles;
+
+    bytes32 public constant DEFAULT_ADMIN_ROLE = 0x00;
+
+    /**
+     * @dev Emitted when `newAdminRole` is set as ``role``'s admin role, replacing `previousAdminRole`
+     *
+     * `DEFAULT_ADMIN_ROLE` is the starting admin for all roles, despite
+     * {RoleAdminChanged} not being emitted signaling this.
+     *
+     * _Available since v3.1._
+     */
+    event RoleAdminChanged(bytes32 indexed role, bytes32 indexed previousAdminRole, bytes32 indexed newAdminRole);
+
+    /**
+     * @dev Emitted when `account` is granted `role`.
+     *
+     * `sender` is the account that originated the contract call, an admin role
+     * bearer except when using {_setupRole}.
+     */
+    event RoleGranted(bytes32 indexed role, address indexed account, address indexed sender);
+
+    /**
+     * @dev Emitted when `account` is revoked `role`.
+     *
+     * `sender` is the account that originated the contract call:
+     *   - if using `revokeRole`, it is the admin role bearer
+     *   - if using `renounceRole`, it is the role bearer (i.e. `account`)
+     */
+    event RoleRevoked(bytes32 indexed role, address indexed account, address indexed sender);
+
+    /**
+     * @dev Returns `true` if `account` has been granted `role`.
+     */
+    function hasRole(bytes32 role, address account) public view returns (bool) {
+        return _roles[role].members.contains(account);
+    }
+
+    /**
+     * @dev Returns the number of accounts that have `role`. Can be used
+     * together with {getRoleMember} to enumerate all bearers of a role.
+     */
+    function getRoleMemberCount(bytes32 role) public view returns (uint256) {
+        return _roles[role].members.length();
+    }
+
+    /**
+     * @dev Returns one of the accounts that have `role`. `index` must be a
+     * value between 0 and {getRoleMemberCount}, non-inclusive.
+     *
+     * Role bearers are not sorted in any particular way, and their ordering may
+     * change at any point.
+     *
+     * WARNING: When using {getRoleMember} and {getRoleMemberCount}, make sure
+     * you perform all queries on the same block. See the following
+     * https://forum.openzeppelin.com/t/iterating-over-elements-on-enumerableset-in-openzeppelin-contracts/2296[forum post]
+     * for more information.
+     */
+    function getRoleMember(bytes32 role, uint256 index) public view returns (address) {
+        return _roles[role].members.at(index);
+    }
+
+    /**
+     * @dev Returns the admin role that controls `role`. See {grantRole} and
+     * {revokeRole}.
+     *
+     * To change a role's admin, use {_setRoleAdmin}.
+     */
+    function getRoleAdmin(bytes32 role) public view returns (bytes32) {
+        return _roles[role].adminRole;
+    }
+
+    /**
+     * @dev Grants `role` to `account`.
+     *
+     * If `account` had not been already granted `role`, emits a {RoleGranted}
+     * event.
+     *
+     * Requirements:
+     *
+     * - the caller must have ``role``'s admin role.
+     */
+    function grantRole(bytes32 role, address account) public virtual {
+        require(hasRole(_roles[role].adminRole, _msgSender()), "AccessControl: sender must be an admin to grant");
+
+        _grantRole(role, account);
+    }
+
+    /**
+     * @dev Revokes `role` from `account`.
+     *
+     * If `account` had been granted `role`, emits a {RoleRevoked} event.
+     *
+     * Requirements:
+     *
+     * - the caller must have ``role``'s admin role.
+     */
+    function revokeRole(bytes32 role, address account) public virtual {
+        require(hasRole(_roles[role].adminRole, _msgSender()), "AccessControl: sender must be an admin to revoke");
+
+        _revokeRole(role, account);
+    }
+
+    /**
+     * @dev Revokes `role` from the calling account.
+     *
+     * Roles are often managed via {grantRole} and {revokeRole}: this function's
+     * purpose is to provide a mechanism for accounts to lose their privileges
+     * if they are compromised (such as when a trusted device is misplaced).
+     *
+     * If the calling account had been granted `role`, emits a {RoleRevoked}
+     * event.
+     *
+     * Requirements:
+     *
+     * - the caller must be `account`.
+     */
+    function renounceRole(bytes32 role, address account) public virtual {
+        require(account == _msgSender(), "AccessControl: can only renounce roles for self");
+
+        _revokeRole(role, account);
+    }
+
+    /**
+     * @dev Grants `role` to `account`.
+     *
+     * If `account` had not been already granted `role`, emits a {RoleGranted}
+     * event. Note that unlike {grantRole}, this function doesn't perform any
+     * checks on the calling account.
+     *
+     * [WARNING]
+     * ====
+     * This function should only be called from the constructor when setting
+     * up the initial roles for the system.
+     *
+     * Using this function in any other way is effectively circumventing the admin
+     * system imposed by {AccessControl}.
+     * ====
+     */
+    function _setupRole(bytes32 role, address account) internal virtual {
+        _grantRole(role, account);
+    }
+
+    /**
+     * @dev Sets `adminRole` as ``role``'s admin role.
+     *
+     * Emits a {RoleAdminChanged} event.
+     */
+    function _setRoleAdmin(bytes32 role, bytes32 adminRole) internal virtual {
+        emit RoleAdminChanged(role, _roles[role].adminRole, adminRole);
+        _roles[role].adminRole = adminRole;
+    }
+
+    function _grantRole(bytes32 role, address account) private {
+        if (_roles[role].members.add(account)) {
+            emit RoleGranted(role, account, _msgSender());
+        }
+    }
+
+    function _revokeRole(bytes32 role, address account) private {
+        if (_roles[role].members.remove(account)) {
+            emit RoleRevoked(role, account, _msgSender());
+        }
+    }
+}

contracts/access/Ownable.sol

@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "../GSN/Context.sol";
+/**
+ * @dev Contract module which provides a basic access control mechanism, where
+ * there is an account (an owner) that can be granted exclusive access to
+ * specific functions.
+ *
+ * By default, the owner account will be the one that deploys the contract. This
+ * can later be changed with {transferOwnership}.
+ *
+ * This module is used through inheritance. It will make available the modifier
+ * `onlyOwner`, which can be applied to your functions to restrict their use to
+ * the owner.
+ */
+contract Ownable is Context {
+    address private _owner;
+
+    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
+
+    /**
+     * @dev Initializes the contract setting the deployer as the initial owner.
+     */
+    constructor () internal {
+        address msgSender = _msgSender();
+        _owner = msgSender;
+        emit OwnershipTransferred(address(0), msgSender);
+    }
+
+    /**
+     * @dev Returns the address of the current owner.
+     */
+    function owner() public view returns (address) {
+        return _owner;
+    }
+
+    /**
+     * @dev Throws if called by any account other than the owner.
+     */
+    modifier onlyOwner() {
+        require(_owner == _msgSender(), "Ownable: caller is not the owner");
+        _;
+    }
+
+    /**
+     * @dev Leaves the contract without owner. It will not be possible to call
+     * `onlyOwner` functions anymore. Can only be called by the current owner.
+     *
+     * NOTE: Renouncing ownership will leave the contract without an owner,
+     * thereby removing any functionality that is only available to the owner.
+     */
+    function renounceOwnership() public virtual onlyOwner {
+        emit OwnershipTransferred(_owner, address(0));
+        _owner = address(0);
+    }
+
+    /**
+     * @dev Transfers ownership of the contract to a new account (`newOwner`).
+     * Can only be called by the current owner.
+     */
+    function transferOwnership(address newOwner) public virtual onlyOwner {
+        require(newOwner != address(0), "Ownable: new owner is the zero address");
+        emit OwnershipTransferred(_owner, newOwner);
+        _owner = newOwner;
+    }
+}

contracts/access/README.adoc

@@ -0,0 +1,12 @@
+= Access
+
+[.readme-notice]
+NOTE: This document is better viewed at https://docs.openzeppelin.com/contracts/api/access
+
+Contract modules for authorization and access control mechanisms.
+
+== Contracts
+
+{{Ownable}}
+
+{{AccessControl}}

contracts/access/SignatureBouncer.sol

@@ -1,159 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../ownership/Ownable.sol";
-import "../access/rbac/RBAC.sol";
-import "../ECRecovery.sol";
-
-
-/**
- * @title SignatureBouncer
- * @author PhABC, Shrugs and aflesher
- * @dev Bouncer allows users to submit a signature as a permission to do an action.
- * If the signature is from one of the authorized bouncer addresses, the signature
- * is valid. The owner of the contract adds/removes bouncers.
- * Bouncer addresses can be individual servers signing grants or different
- * users within a decentralized club that have permission to invite other members.
- * This technique is useful for whitelists and airdrops; instead of putting all
- * valid addresses on-chain, simply sign a grant of the form
- * keccak256(abi.encodePacked(`:contractAddress` + `:granteeAddress`)) using a valid bouncer address.
- * Then restrict access to your crowdsale/whitelist/airdrop using the
- * `onlyValidSignature` modifier (or implement your own using isValidSignature).
- * In addition to `onlyValidSignature`, `onlyValidSignatureAndMethod` and
- * `onlyValidSignatureAndData` can be used to restrict access to only a given method
- * or a given method with given parameters respectively.
- * See the tests Bouncer.test.js for specific usage examples.
- * @notice A method that uses the `onlyValidSignatureAndData` modifier must make the _sig
- * parameter the "last" parameter. You cannot sign a message that has its own
- * signature in it so the last 128 bytes of msg.data (which represents the
- * length of the _sig data and the _sig data itself) is ignored when validating.
- * Also non fixed sized parameters make constructing the data in the signature
- * much more complex. See https://ethereum.stackexchange.com/a/50616 for more details.
- */
-contract SignatureBouncer is Ownable, RBAC {
-  using ECRecovery for bytes32;
-
-  string public constant ROLE_BOUNCER = "bouncer";
-  uint constant METHOD_ID_SIZE = 4;
-  // signature size is 65 bytes (tightly packed v + r + s), but gets padded to 96 bytes
-  uint constant SIGNATURE_SIZE = 96;
-
-  /**
-   * @dev requires that a valid signature of a bouncer was provided
-   */
-  modifier onlyValidSignature(bytes _sig)
-  {
-    require(isValidSignature(msg.sender, _sig));
-    _;
-  }
-
-  /**
-   * @dev requires that a valid signature with a specifed method of a bouncer was provided
-   */
-  modifier onlyValidSignatureAndMethod(bytes _sig)
-  {
-    require(isValidSignatureAndMethod(msg.sender, _sig));
-    _;
-  }
-
-  /**
-   * @dev requires that a valid signature with a specifed method and params of a bouncer was provided
-   */
-  modifier onlyValidSignatureAndData(bytes _sig)
-  {
-    require(isValidSignatureAndData(msg.sender, _sig));
-    _;
-  }
-
-  /**
-   * @dev allows the owner to add additional bouncer addresses
-   */
-  function addBouncer(address _bouncer)
-    public
-    onlyOwner
-  {
-    require(_bouncer != address(0));
-    addRole(_bouncer, ROLE_BOUNCER);
-  }
-
-  /**
-   * @dev allows the owner to remove bouncer addresses
-   */
-  function removeBouncer(address _bouncer)
-    public
-    onlyOwner
-  {
-    require(_bouncer != address(0));
-    removeRole(_bouncer, ROLE_BOUNCER);
-  }
-
-  /**
-   * @dev is the signature of `this + sender` from a bouncer?
-   * @return bool
-   */
-  function isValidSignature(address _address, bytes _sig)
-    internal
-    view
-    returns (bool)
-  {
-    return isValidDataHash(
-      keccak256(abi.encodePacked(address(this), _address)),
-      _sig
-    );
-  }
-
-  /**
-   * @dev is the signature of `this + sender + methodId` from a bouncer?
-   * @return bool
-   */
-  function isValidSignatureAndMethod(address _address, bytes _sig)
-    internal
-    view
-    returns (bool)
-  {
-    bytes memory data = new bytes(METHOD_ID_SIZE);
-    for (uint i = 0; i < data.length; i++) {
-      data[i] = msg.data[i];
-    }
-    return isValidDataHash(
-      keccak256(abi.encodePacked(address(this), _address, data)),
-      _sig
-    );
-  }
-
-  /**
-    * @dev is the signature of `this + sender + methodId + params(s)` from a bouncer?
-    * @notice the _sig parameter of the method being validated must be the "last" parameter
-    * @return bool
-    */
-  function isValidSignatureAndData(address _address, bytes _sig)
-    internal
-    view
-    returns (bool)
-  {
-    require(msg.data.length > SIGNATURE_SIZE);
-    bytes memory data = new bytes(msg.data.length - SIGNATURE_SIZE);
-    for (uint i = 0; i < data.length; i++) {
-      data[i] = msg.data[i];
-    }
-    return isValidDataHash(
-      keccak256(abi.encodePacked(address(this), _address, data)),
-      _sig
-    );
-  }
-
-  /**
-   * @dev internal function to convert a hash to an eth signed message
-   * and then recover the signature and check it against the bouncer role
-   * @return bool
-   */
-  function isValidDataHash(bytes32 _hash, bytes _sig)
-    internal
-    view
-    returns (bool)
-  {
-    address signer = _hash
-      .toEthSignedMessageHash()
-      .recover(_sig);
-    return hasRole(signer, ROLE_BOUNCER);
-  }
-}

contracts/access/Whitelist.sol

@@ -1,91 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-import "../ownership/Ownable.sol";
-import "../access/rbac/RBAC.sol";
-
-
-/**
- * @title Whitelist
- * @dev The Whitelist contract has a whitelist of addresses, and provides basic authorization control functions.
- * This simplifies the implementation of "user permissions".
- */
-contract Whitelist is Ownable, RBAC {
-  string public constant ROLE_WHITELISTED = "whitelist";
-
-  /**
-   * @dev Throws if operator is not whitelisted.
-   * @param _operator address
-   */
-  modifier onlyIfWhitelisted(address _operator) {
-    checkRole(_operator, ROLE_WHITELISTED);
-    _;
-  }
-
-  /**
-   * @dev add an address to the whitelist
-   * @param _operator address
-   * @return true if the address was added to the whitelist, false if the address was already in the whitelist
-   */
-  function addAddressToWhitelist(address _operator)
-    public
-    onlyOwner
-  {
-    addRole(_operator, ROLE_WHITELISTED);
-  }
-
-  /**
-   * @dev getter to determine if address is in whitelist
-   */
-  function whitelist(address _operator)
-    public
-    view
-    returns (bool)
-  {
-    return hasRole(_operator, ROLE_WHITELISTED);
-  }
-
-  /**
-   * @dev add addresses to the whitelist
-   * @param _operators addresses
-   * @return true if at least one address was added to the whitelist,
-   * false if all addresses were already in the whitelist
-   */
-  function addAddressesToWhitelist(address[] _operators)
-    public
-    onlyOwner
-  {
-    for (uint256 i = 0; i < _operators.length; i++) {
-      addAddressToWhitelist(_operators[i]);
-    }
-  }
-
-  /**
-   * @dev remove an address from the whitelist
-   * @param _operator address
-   * @return true if the address was removed from the whitelist,
-   * false if the address wasn't in the whitelist in the first place
-   */
-  function removeAddressFromWhitelist(address _operator)
-    public
-    onlyOwner
-  {
-    removeRole(_operator, ROLE_WHITELISTED);
-  }
-
-  /**
-   * @dev remove addresses from the whitelist
-   * @param _operators addresses
-   * @return true if at least one address was removed from the whitelist,
-   * false if all addresses weren't in the whitelist in the first place
-   */
-  function removeAddressesFromWhitelist(address[] _operators)
-    public
-    onlyOwner
-  {
-    for (uint256 i = 0; i < _operators.length; i++) {
-      removeAddressFromWhitelist(_operators[i]);
-    }
-  }
-
-}

contracts/access/rbac/RBAC.sol

@@ -1,106 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "./Roles.sol";
-
-
-/**
- * @title RBAC (Role-Based Access Control)
- * @author Matt Condon (@Shrugs)
- * @dev Stores and provides setters and getters for roles and addresses.
- * Supports unlimited numbers of roles and addresses.
- * See //contracts/mocks/RBACMock.sol for an example of usage.
- * This RBAC method uses strings to key roles. It may be beneficial
- * for you to write your own implementation of this interface using Enums or similar.
- */
-contract RBAC {
-  using Roles for Roles.Role;
-
-  mapping (string => Roles.Role) private roles;
-
-  event RoleAdded(address indexed operator, string role);
-  event RoleRemoved(address indexed operator, string role);
-
-  /**
-   * @dev reverts if addr does not have role
-   * @param _operator address
-   * @param _role the name of the role
-   * // reverts
-   */
-  function checkRole(address _operator, string _role)
-    public
-    view
-  {
-    roles[_role].check(_operator);
-  }
-
-  /**
-   * @dev determine if addr has role
-   * @param _operator address
-   * @param _role the name of the role
-   * @return bool
-   */
-  function hasRole(address _operator, string _role)
-    public
-    view
-    returns (bool)
-  {
-    return roles[_role].has(_operator);
-  }
-
-  /**
-   * @dev add a role to an address
-   * @param _operator address
-   * @param _role the name of the role
-   */
-  function addRole(address _operator, string _role)
-    internal
-  {
-    roles[_role].add(_operator);
-    emit RoleAdded(_operator, _role);
-  }
-
-  /**
-   * @dev remove a role from an address
-   * @param _operator address
-   * @param _role the name of the role
-   */
-  function removeRole(address _operator, string _role)
-    internal
-  {
-    roles[_role].remove(_operator);
-    emit RoleRemoved(_operator, _role);
-  }
-
-  /**
-   * @dev modifier to scope access to a single role (uses msg.sender as addr)
-   * @param _role the name of the role
-   * // reverts
-   */
-  modifier onlyRole(string _role)
-  {
-    checkRole(msg.sender, _role);
-    _;
-  }
-
-  /**
-   * @dev modifier to scope access to a set of roles (uses msg.sender as addr)
-   * @param _roles the names of the roles to scope access to
-   * // reverts
-   *
-   * @TODO - when solidity supports dynamic arrays as arguments to modifiers, provide this
-   *  see: https://github.com/ethereum/solidity/issues/2467
-   */
-  // modifier onlyRoles(string[] _roles) {
-  //     bool hasAnyRole = false;
-  //     for (uint8 i = 0; i < _roles.length; i++) {
-  //         if (hasRole(msg.sender, _roles[i])) {
-  //             hasAnyRole = true;
-  //             break;
-  //         }
-  //     }
-
-  //     require(hasAnyRole);
-
-  //     _;
-  // }
-}

contracts/access/rbac/Roles.sol

@@ -1,55 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-/**
- * @title Roles
- * @author Francisco Giordano (@frangio)
- * @dev Library for managing addresses assigned to a Role.
- * See RBAC.sol for example usage.
- */
-library Roles {
-  struct Role {
-    mapping (address => bool) bearer;
-  }
-
-  /**
-   * @dev give an address access to this role
-   */
-  function add(Role storage _role, address _addr)
-    internal
-  {
-    _role.bearer[_addr] = true;
-  }
-
-  /**
-   * @dev remove an address' access to this role
-   */
-  function remove(Role storage _role, address _addr)
-    internal
-  {
-    _role.bearer[_addr] = false;
-  }
-
-  /**
-   * @dev check if an address has this role
-   * // reverts
-   */
-  function check(Role storage _role, address _addr)
-    internal
-    view
-  {
-    require(has(_role, _addr));
-  }
-
-  /**
-   * @dev check if an address has this role
-   * @return bool
-   */
-  function has(Role storage _role, address _addr)
-    internal
-    view
-    returns (bool)
-  {
-    return _role.bearer[_addr];
-  }
-}

contracts/crowdsale/Crowdsale.sol

@@ -1,203 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../token/ERC20/ERC20.sol";
-import "../math/SafeMath.sol";
-import "../token/ERC20/SafeERC20.sol";
-
-
-/**
- * @title Crowdsale
- * @dev Crowdsale is a base contract for managing a token crowdsale,
- * allowing investors to purchase tokens with ether. This contract implements
- * such functionality in its most fundamental form and can be extended to provide additional
- * functionality and/or custom behavior.
- * The external interface represents the basic interface for purchasing tokens, and conform
- * the base architecture for crowdsales. They are *not* intended to be modified / overridden.
- * The internal interface conforms the extensible and modifiable surface of crowdsales. Override
- * the methods to add functionality. Consider using 'super' where appropriate to concatenate
- * behavior.
- */
-contract Crowdsale {
-  using SafeMath for uint256;
-  using SafeERC20 for ERC20;
-
-  // The token being sold
-  ERC20 public token;
-
-  // Address where funds are collected
-  address public wallet;
-
-  // How many token units a buyer gets per wei.
-  // The rate is the conversion between wei and the smallest and indivisible token unit.
-  // So, if you are using a rate of 1 with a DetailedERC20 token with 3 decimals called TOK
-  // 1 wei will give you 1 unit, or 0.001 TOK.
-  uint256 public rate;
-
-  // Amount of wei raised
-  uint256 public weiRaised;
-
-  /**
-   * Event for token purchase logging
-   * @param purchaser who paid for the tokens
-   * @param beneficiary who got the tokens
-   * @param value weis paid for purchase
-   * @param amount amount of tokens purchased
-   */
-  event TokenPurchase(
-    address indexed purchaser,
-    address indexed beneficiary,
-    uint256 value,
-    uint256 amount
-  );
-
-  /**
-   * @param _rate Number of token units a buyer gets per wei
-   * @param _wallet Address where collected funds will be forwarded to
-   * @param _token Address of the token being sold
-   */
-  constructor(uint256 _rate, address _wallet, ERC20 _token) public {
-    require(_rate > 0);
-    require(_wallet != address(0));
-    require(_token != address(0));
-
-    rate = _rate;
-    wallet = _wallet;
-    token = _token;
-  }
-
-  // -----------------------------------------
-  // Crowdsale external interface
-  // -----------------------------------------
-
-  /**
-   * @dev fallback function ***DO NOT OVERRIDE***
-   */
-  function () external payable {
-    buyTokens(msg.sender);
-  }
-
-  /**
-   * @dev low level token purchase ***DO NOT OVERRIDE***
-   * @param _beneficiary Address performing the token purchase
-   */
-  function buyTokens(address _beneficiary) public payable {
-
-    uint256 weiAmount = msg.value;
-    _preValidatePurchase(_beneficiary, weiAmount);
-
-    // calculate token amount to be created
-    uint256 tokens = _getTokenAmount(weiAmount);
-
-    // update state
-    weiRaised = weiRaised.add(weiAmount);
-
-    _processPurchase(_beneficiary, tokens);
-    emit TokenPurchase(
-      msg.sender,
-      _beneficiary,
-      weiAmount,
-      tokens
-    );
-
-    _updatePurchasingState(_beneficiary, weiAmount);
-
-    _forwardFunds();
-    _postValidatePurchase(_beneficiary, weiAmount);
-  }
-
-  // -----------------------------------------
-  // Internal interface (extensible)
-  // -----------------------------------------
-
-  /**
-   * @dev Validation of an incoming purchase. Use require statements to revert state when conditions are not met. Use `super` in contracts that inherit from Crowdsale to extend their validations.
-   * Example from CappedCrowdsale.sol's _preValidatePurchase method: 
-   *   super._preValidatePurchase(_beneficiary, _weiAmount);
-   *   require(weiRaised.add(_weiAmount) <= cap);
-   * @param _beneficiary Address performing the token purchase
-   * @param _weiAmount Value in wei involved in the purchase
-   */
-  function _preValidatePurchase(
-    address _beneficiary,
-    uint256 _weiAmount
-  )
-    internal
-  {
-    require(_beneficiary != address(0));
-    require(_weiAmount != 0);
-  }
-
-  /**
-   * @dev Validation of an executed purchase. Observe state and use revert statements to undo rollback when valid conditions are not met.
-   * @param _beneficiary Address performing the token purchase
-   * @param _weiAmount Value in wei involved in the purchase
-   */
-  function _postValidatePurchase(
-    address _beneficiary,
-    uint256 _weiAmount
-  )
-    internal
-  {
-    // optional override
-  }
-
-  /**
-   * @dev Source of tokens. Override this method to modify the way in which the crowdsale ultimately gets and sends its tokens.
-   * @param _beneficiary Address performing the token purchase
-   * @param _tokenAmount Number of tokens to be emitted
-   */
-  function _deliverTokens(
-    address _beneficiary,
-    uint256 _tokenAmount
-  )
-    internal
-  {
-    token.safeTransfer(_beneficiary, _tokenAmount);
-  }
-
-  /**
-   * @dev Executed when a purchase has been validated and is ready to be executed. Not necessarily emits/sends tokens.
-   * @param _beneficiary Address receiving the tokens
-   * @param _tokenAmount Number of tokens to be purchased
-   */
-  function _processPurchase(
-    address _beneficiary,
-    uint256 _tokenAmount
-  )
-    internal
-  {
-    _deliverTokens(_beneficiary, _tokenAmount);
-  }
-
-  /**
-   * @dev Override for extensions that require an internal state to check for validity (current user contributions, etc.)
-   * @param _beneficiary Address receiving the tokens
-   * @param _weiAmount Value in wei involved in the purchase
-   */
-  function _updatePurchasingState(
-    address _beneficiary,
-    uint256 _weiAmount
-  )
-    internal
-  {
-    // optional override
-  }
-
-  /**
-   * @dev Override to extend the way in which ether is converted to tokens.
-   * @param _weiAmount Value in wei to be converted into tokens
-   * @return Number of tokens that can be purchased with the specified _weiAmount
-   */
-  function _getTokenAmount(uint256 _weiAmount)
-    internal view returns (uint256)
-  {
-    return _weiAmount.mul(rate);
-  }
-
-  /**
-   * @dev Determines how ETH is stored/forwarded on purchases.
-   */
-  function _forwardFunds() internal {
-    wallet.transfer(msg.value);
-  }
-}

contracts/crowdsale/distribution/FinalizableCrowdsale.sol

@@ -1,42 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../../math/SafeMath.sol";
-import "../../ownership/Ownable.sol";
-import "../validation/TimedCrowdsale.sol";
-
-
-/**
- * @title FinalizableCrowdsale
- * @dev Extension of Crowdsale where an owner can do extra work
- * after finishing.
- */
-contract FinalizableCrowdsale is Ownable, TimedCrowdsale {
-  using SafeMath for uint256;
-
-  bool public isFinalized = false;
-
-  event Finalized();
-
-  /**
-   * @dev Must be called after crowdsale ends, to do some extra finalization
-   * work. Calls the contract's finalization function.
-   */
-  function finalize() public onlyOwner {
-    require(!isFinalized);
-    require(hasClosed());
-
-    finalization();
-    emit Finalized();
-
-    isFinalized = true;
-  }
-
-  /**
-   * @dev Can be overridden to add finalization logic. The overriding function
-   * should call super.finalization() to ensure the chain of finalization is
-   * executed entirely.
-   */
-  function finalization() internal {
-  }
-
-}

contracts/crowdsale/distribution/PostDeliveryCrowdsale.sol

@@ -1,42 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../validation/TimedCrowdsale.sol";
-import "../../token/ERC20/ERC20.sol";
-import "../../math/SafeMath.sol";
-
-
-/**
- * @title PostDeliveryCrowdsale
- * @dev Crowdsale that locks tokens from withdrawal until it ends.
- */
-contract PostDeliveryCrowdsale is TimedCrowdsale {
-  using SafeMath for uint256;
-
-  mapping(address => uint256) public balances;
-
-  /**
-   * @dev Withdraw tokens only after crowdsale ends.
-   */
-  function withdrawTokens() public {
-    require(hasClosed());
-    uint256 amount = balances[msg.sender];
-    require(amount > 0);
-    balances[msg.sender] = 0;
-    _deliverTokens(msg.sender, amount);
-  }
-
-  /**
-   * @dev Overrides parent by storing balances instead of issuing tokens right away.
-   * @param _beneficiary Token purchaser
-   * @param _tokenAmount Amount of tokens purchased
-   */
-  function _processPurchase(
-    address _beneficiary,
-    uint256 _tokenAmount
-  )
-    internal
-  {
-    balances[_beneficiary] = balances[_beneficiary].add(_tokenAmount);
-  }
-
-}

contracts/crowdsale/distribution/RefundableCrowdsale.sol

@@ -1,72 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-import "../../math/SafeMath.sol";
-import "./FinalizableCrowdsale.sol";
-import "../../payment/RefundEscrow.sol";
-
-
-/**
- * @title RefundableCrowdsale
- * @dev Extension of Crowdsale contract that adds a funding goal, and
- * the possibility of users getting a refund if goal is not met.
- */
-contract RefundableCrowdsale is FinalizableCrowdsale {
-  using SafeMath for uint256;
-
-  // minimum amount of funds to be raised in weis
-  uint256 public goal;
-
-  // refund escrow used to hold funds while crowdsale is running
-  RefundEscrow private escrow;
-
-  /**
-   * @dev Constructor, creates RefundEscrow.
-   * @param _goal Funding goal
-   */
-  constructor(uint256 _goal) public {
-    require(_goal > 0);
-    escrow = new RefundEscrow(wallet);
-    goal = _goal;
-  }
-
-  /**
-   * @dev Investors can claim refunds here if crowdsale is unsuccessful
-   */
-  function claimRefund() public {
-    require(isFinalized);
-    require(!goalReached());
-
-    escrow.withdraw(msg.sender);
-  }
-
-  /**
-   * @dev Checks whether funding goal was reached.
-   * @return Whether funding goal was reached
-   */
-  function goalReached() public view returns (bool) {
-    return weiRaised >= goal;
-  }
-
-  /**
-   * @dev escrow finalization task, called when owner calls finalize()
-   */
-  function finalization() internal {
-    if (goalReached()) {
-      escrow.close();
-      escrow.beneficiaryWithdraw();
-    } else {
-      escrow.enableRefunds();
-    }
-
-    super.finalization();
-  }
-
-  /**
-   * @dev Overrides Crowdsale fund forwarding, sending funds to escrow.
-   */
-  function _forwardFunds() internal {
-    escrow.deposit.value(msg.value)(msg.sender);
-  }
-
-}

contracts/crowdsale/emission/AllowanceCrowdsale.sol

@@ -1,50 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../Crowdsale.sol";
-import "../../token/ERC20/ERC20.sol";
-import "../../token/ERC20/ERC20Basic.sol";
-import "../../token/ERC20/SafeERC20.sol";
-import "../../math/SafeMath.sol";
-
-
-/**
- * @title AllowanceCrowdsale
- * @dev Extension of Crowdsale where tokens are held by a wallet, which approves an allowance to the crowdsale.
- */
-contract AllowanceCrowdsale is Crowdsale {
-  using SafeMath for uint256;
-  using SafeERC20 for ERC20;
-
-  address public tokenWallet;
-
-  /**
-   * @dev Constructor, takes token wallet address.
-   * @param _tokenWallet Address holding the tokens, which has approved allowance to the crowdsale
-   */
-  constructor(address _tokenWallet) public {
-    require(_tokenWallet != address(0));
-    tokenWallet = _tokenWallet;
-  }
-
-  /**
-   * @dev Checks the amount of tokens left in the allowance.
-   * @return Amount of tokens left in the allowance
-   */
-  function remainingTokens() public view returns (uint256) {
-    return token.allowance(tokenWallet, this);
-  }
-
-  /**
-   * @dev Overrides parent behavior by transferring tokens from wallet.
-   * @param _beneficiary Token purchaser
-   * @param _tokenAmount Amount of tokens purchased
-   */
-  function _deliverTokens(
-    address _beneficiary,
-    uint256 _tokenAmount
-  )
-    internal
-  {
-    token.safeTransferFrom(tokenWallet, _beneficiary, _tokenAmount);
-  }
-}

contracts/crowdsale/emission/MintedCrowdsale.sol

@@ -1,28 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../Crowdsale.sol";
-import "../../token/ERC20/MintableToken.sol";
-
-
-/**
- * @title MintedCrowdsale
- * @dev Extension of Crowdsale contract whose tokens are minted in each purchase.
- * Token ownership should be transferred to MintedCrowdsale for minting.
- */
-contract MintedCrowdsale is Crowdsale {
-
-  /**
-   * @dev Overrides delivery by minting tokens upon purchase.
-   * @param _beneficiary Token purchaser
-   * @param _tokenAmount Number of tokens to be minted
-   */
-  function _deliverTokens(
-    address _beneficiary,
-    uint256 _tokenAmount
-  )
-    internal
-  {
-    // Potentially dangerous assumption about the type of the token.
-    require(MintableToken(address(token)).mint(_beneficiary, _tokenAmount));
-  }
-}

contracts/crowdsale/price/IncreasingPriceCrowdsale.sol

@@ -1,56 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../validation/TimedCrowdsale.sol";
-import "../../math/SafeMath.sol";
-
-
-/**
- * @title IncreasingPriceCrowdsale
- * @dev Extension of Crowdsale contract that increases the price of tokens linearly in time.
- * Note that what should be provided to the constructor is the initial and final _rates_, that is,
- * the amount of tokens per wei contributed. Thus, the initial rate must be greater than the final rate.
- */
-contract IncreasingPriceCrowdsale is TimedCrowdsale {
-  using SafeMath for uint256;
-
-  uint256 public initialRate;
-  uint256 public finalRate;
-
-  /**
-   * @dev Constructor, takes initial and final rates of tokens received per wei contributed.
-   * @param _initialRate Number of tokens a buyer gets per wei at the start of the crowdsale
-   * @param _finalRate Number of tokens a buyer gets per wei at the end of the crowdsale
-   */
-  constructor(uint256 _initialRate, uint256 _finalRate) public {
-    require(_initialRate >= _finalRate);
-    require(_finalRate > 0);
-    initialRate = _initialRate;
-    finalRate = _finalRate;
-  }
-
-  /**
-   * @dev Returns the rate of tokens per wei at the present time.
-   * Note that, as price _increases_ with time, the rate _decreases_.
-   * @return The number of tokens a buyer gets per wei at a given time
-   */
-  function getCurrentRate() public view returns (uint256) {
-    // solium-disable-next-line security/no-block-members
-    uint256 elapsedTime = block.timestamp.sub(openingTime);
-    uint256 timeRange = closingTime.sub(openingTime);
-    uint256 rateRange = initialRate.sub(finalRate);
-    return initialRate.sub(elapsedTime.mul(rateRange).div(timeRange));
-  }
-
-  /**
-   * @dev Overrides parent method taking into account variable rate.
-   * @param _weiAmount The value in wei to be converted into tokens
-   * @return The number of tokens _weiAmount wei will buy at present time
-   */
-  function _getTokenAmount(uint256 _weiAmount)
-    internal view returns (uint256)
-  {
-    uint256 currentRate = getCurrentRate();
-    return currentRate.mul(_weiAmount);
-  }
-
-}

contracts/crowdsale/validation/CappedCrowdsale.sol

@@ -1,48 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../../math/SafeMath.sol";
-import "../Crowdsale.sol";
-
-
-/**
- * @title CappedCrowdsale
- * @dev Crowdsale with a limit for total contributions.
- */
-contract CappedCrowdsale is Crowdsale {
-  using SafeMath for uint256;
-
-  uint256 public cap;
-
-  /**
-   * @dev Constructor, takes maximum amount of wei accepted in the crowdsale.
-   * @param _cap Max amount of wei to be contributed
-   */
-  constructor(uint256 _cap) public {
-    require(_cap > 0);
-    cap = _cap;
-  }
-
-  /**
-   * @dev Checks whether the cap has been reached.
-   * @return Whether the cap was reached
-   */
-  function capReached() public view returns (bool) {
-    return weiRaised >= cap;
-  }
-
-  /**
-   * @dev Extend parent behavior requiring purchase to respect the funding cap.
-   * @param _beneficiary Token purchaser
-   * @param _weiAmount Amount of wei contributed
-   */
-  function _preValidatePurchase(
-    address _beneficiary,
-    uint256 _weiAmount
-  )
-    internal
-  {
-    super._preValidatePurchase(_beneficiary, _weiAmount);
-    require(weiRaised.add(_weiAmount) <= cap);
-  }
-
-}

contracts/crowdsale/validation/IndividuallyCappedCrowdsale.sol

@@ -1,94 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../../math/SafeMath.sol";
-import "../Crowdsale.sol";
-import "../../ownership/Ownable.sol";
-
-
-/**
- * @title IndividuallyCappedCrowdsale
- * @dev Crowdsale with per-user caps.
- */
-contract IndividuallyCappedCrowdsale is Ownable, Crowdsale {
-  using SafeMath for uint256;
-
-  mapping(address => uint256) public contributions;
-  mapping(address => uint256) public caps;
-
-  /**
-   * @dev Sets a specific user's maximum contribution.
-   * @param _beneficiary Address to be capped
-   * @param _cap Wei limit for individual contribution
-   */
-  function setUserCap(address _beneficiary, uint256 _cap) external onlyOwner {
-    caps[_beneficiary] = _cap;
-  }
-
-  /**
-   * @dev Sets a group of users' maximum contribution.
-   * @param _beneficiaries List of addresses to be capped
-   * @param _cap Wei limit for individual contribution
-   */
-  function setGroupCap(
-    address[] _beneficiaries,
-    uint256 _cap
-  )
-    external
-    onlyOwner
-  {
-    for (uint256 i = 0; i < _beneficiaries.length; i++) {
-      caps[_beneficiaries[i]] = _cap;
-    }
-  }
-
-  /**
-   * @dev Returns the cap of a specific user.
-   * @param _beneficiary Address whose cap is to be checked
-   * @return Current cap for individual user
-   */
-  function getUserCap(address _beneficiary) public view returns (uint256) {
-    return caps[_beneficiary];
-  }
-
-  /**
-   * @dev Returns the amount contributed so far by a sepecific user.
-   * @param _beneficiary Address of contributor
-   * @return User contribution so far
-   */
-  function getUserContribution(address _beneficiary)
-    public view returns (uint256)
-  {
-    return contributions[_beneficiary];
-  }
-
-  /**
-   * @dev Extend parent behavior requiring purchase to respect the user's funding cap.
-   * @param _beneficiary Token purchaser
-   * @param _weiAmount Amount of wei contributed
-   */
-  function _preValidatePurchase(
-    address _beneficiary,
-    uint256 _weiAmount
-  )
-    internal
-  {
-    super._preValidatePurchase(_beneficiary, _weiAmount);
-    require(contributions[_beneficiary].add(_weiAmount) <= caps[_beneficiary]);
-  }
-
-  /**
-   * @dev Extend parent behavior to update user contributions
-   * @param _beneficiary Token purchaser
-   * @param _weiAmount Amount of wei contributed
-   */
-  function _updatePurchasingState(
-    address _beneficiary,
-    uint256 _weiAmount
-  )
-    internal
-  {
-    super._updatePurchasingState(_beneficiary, _weiAmount);
-    contributions[_beneficiary] = contributions[_beneficiary].add(_weiAmount);
-  }
-
-}

contracts/crowdsale/validation/TimedCrowdsale.sol

@@ -1,64 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../../math/SafeMath.sol";
-import "../Crowdsale.sol";
-
-
-/**
- * @title TimedCrowdsale
- * @dev Crowdsale accepting contributions only within a time frame.
- */
-contract TimedCrowdsale is Crowdsale {
-  using SafeMath for uint256;
-
-  uint256 public openingTime;
-  uint256 public closingTime;
-
-  /**
-   * @dev Reverts if not in crowdsale time range.
-   */
-  modifier onlyWhileOpen {
-    // solium-disable-next-line security/no-block-members
-    require(block.timestamp >= openingTime && block.timestamp <= closingTime);
-    _;
-  }
-
-  /**
-   * @dev Constructor, takes crowdsale opening and closing times.
-   * @param _openingTime Crowdsale opening time
-   * @param _closingTime Crowdsale closing time
-   */
-  constructor(uint256 _openingTime, uint256 _closingTime) public {
-    // solium-disable-next-line security/no-block-members
-    require(_openingTime >= block.timestamp);
-    require(_closingTime >= _openingTime);
-
-    openingTime = _openingTime;
-    closingTime = _closingTime;
-  }
-
-  /**
-   * @dev Checks whether the period in which the crowdsale is open has already elapsed.
-   * @return Whether crowdsale period has elapsed
-   */
-  function hasClosed() public view returns (bool) {
-    // solium-disable-next-line security/no-block-members
-    return block.timestamp > closingTime;
-  }
-
-  /**
-   * @dev Extend parent behavior requiring to be within contributing period
-   * @param _beneficiary Token purchaser
-   * @param _weiAmount Amount of wei contributed
-   */
-  function _preValidatePurchase(
-    address _beneficiary,
-    uint256 _weiAmount
-  )
-    internal
-    onlyWhileOpen
-  {
-    super._preValidatePurchase(_beneficiary, _weiAmount);
-  }
-
-}

contracts/crowdsale/validation/WhitelistedCrowdsale.sol

@@ -1,27 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../Crowdsale.sol";
-import "../../access/Whitelist.sol";
-
-
-/**
- * @title WhitelistedCrowdsale
- * @dev Crowdsale in which only whitelisted users can contribute.
- */
-contract WhitelistedCrowdsale is Whitelist, Crowdsale {
-  /**
-   * @dev Extend parent behavior requiring beneficiary to be in whitelist.
-   * @param _beneficiary Token beneficiary
-   * @param _weiAmount Amount of wei contributed
-   */
-  function _preValidatePurchase(
-    address _beneficiary,
-    uint256 _weiAmount
-  )
-    internal
-    onlyIfWhitelisted(_beneficiary)
-  {
-    super._preValidatePurchase(_beneficiary, _weiAmount);
-  }
-
-}

contracts/cryptography/ECDSA.sol

@@ -0,0 +1,83 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+/**
+ * @dev Elliptic Curve Digital Signature Algorithm (ECDSA) operations.
+ *
+ * These functions can be used to verify that a message was signed by the holder
+ * of the private keys of a given address.
+ */
+library ECDSA {
+    /**
+     * @dev Returns the address that signed a hashed message (`hash`) with
+     * `signature`. This address can then be used for verification purposes.
+     *
+     * The `ecrecover` EVM opcode allows for malleable (non-unique) signatures:
+     * this function rejects them by requiring the `s` value to be in the lower
+     * half order, and the `v` value to be either 27 or 28.
+     *
+     * IMPORTANT: `hash` _must_ be the result of a hash operation for the
+     * verification to be secure: it is possible to craft signatures that
+     * recover to arbitrary addresses for non-hashed data. A safe way to ensure
+     * this is by receiving a hash of the original message (which may otherwise
+     * be too long), and then calling {toEthSignedMessageHash} on it.
+     */
+    function recover(bytes32 hash, bytes memory signature) internal pure returns (address) {
+        // Check the signature length
+        if (signature.length != 65) {
+            revert("ECDSA: invalid signature length");
+        }
+
+        // Divide the signature in r, s and v variables
+        bytes32 r;
+        bytes32 s;
+        uint8 v;
+
+        // ecrecover takes the signature parameters, and the only way to get them
+        // currently is to use assembly.
+        // solhint-disable-next-line no-inline-assembly
+        assembly {
+            r := mload(add(signature, 0x20))
+            s := mload(add(signature, 0x40))
+            v := byte(0, mload(add(signature, 0x60)))
+        }
+
+        // EIP-2 still allows signature malleability for ecrecover(). Remove this possibility and make the signature
+        // unique. Appendix F in the Ethereum Yellow paper (https://ethereum.github.io/yellowpaper/paper.pdf), defines
+        // the valid range for s in (281): 0 < s < secp256k1n ÷ 2 + 1, and for v in (282): v ∈ {27, 28}. Most
+        // signatures from current libraries generate a unique signature with an s-value in the lower half order.
+        //
+        // If your library generates malleable signatures, such as s-values in the upper range, calculate a new s-value
+        // with 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141 - s1 and flip v from 27 to 28 or
+        // vice versa. If your library also generates signatures with 0/1 for v instead 27/28, add 27 to v to accept
+        // these malleable signatures as well.
+        if (uint256(s) > 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0) {
+            revert("ECDSA: invalid signature 's' value");
+        }
+
+        if (v != 27 && v != 28) {
+            revert("ECDSA: invalid signature 'v' value");
+        }
+
+        // If the signature is valid (and not malleable), return the signer address
+        address signer = ecrecover(hash, v, r, s);
+        require(signer != address(0), "ECDSA: invalid signature");
+
+        return signer;
+    }
+
+    /**
+     * @dev Returns an Ethereum Signed Message, created from a `hash`. This
+     * replicates the behavior of the
+     * https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_sign[`eth_sign`]
+     * JSON-RPC method.
+     *
+     * See {recover}.
+     */
+    function toEthSignedMessageHash(bytes32 hash) internal pure returns (bytes32) {
+        // 32 is the length in bytes of hash,
+        // enforced by the type signature above
+        return keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
+    }
+}

contracts/cryptography/MerkleProof.sol

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+/**
+ * @dev These functions deal with verification of Merkle trees (hash trees),
+ */
+library MerkleProof {
+    /**
+     * @dev Returns true if a `leaf` can be proved to be a part of a Merkle tree
+     * defined by `root`. For this, a `proof` must be provided, containing
+     * sibling hashes on the branch from the leaf to the root of the tree. Each
+     * pair of leaves and each pair of pre-images are assumed to be sorted.
+     */
+    function verify(bytes32[] memory proof, bytes32 root, bytes32 leaf) internal pure returns (bool) {
+        bytes32 computedHash = leaf;
+
+        for (uint256 i = 0; i < proof.length; i++) {
+            bytes32 proofElement = proof[i];
+
+            if (computedHash <= proofElement) {
+                // Hash(current computed hash + current element of the proof)
+                computedHash = keccak256(abi.encodePacked(computedHash, proofElement));
+            } else {
+                // Hash(current element of the proof + current computed hash)
+                computedHash = keccak256(abi.encodePacked(proofElement, computedHash));
+            }
+        }
+
+        // Check if the computed hash (root) is equal to the provided root
+        return computedHash == root;
+    }
+}

contracts/cryptography/README.adoc

@@ -0,0 +1,12 @@
+= Cryptography
+
+[.readme-notice]
+NOTE: This document is better viewed at https://docs.openzeppelin.com/contracts/api/cryptography
+
+This collection of libraries provides simple and safe ways to use different cryptographic primitives.
+
+== Libraries
+
+{{ECDSA}}
+
+{{MerkleProof}}

contracts/examples/RBACWithAdmin.sol

@@ -1,66 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../access/rbac/RBAC.sol";
-
-
-/**
- * @title RBACWithAdmin
- * @author Matt Condon (@Shrugs)
- * @dev It's recommended that you define constants in the contract,
- * like ROLE_ADMIN below, to avoid typos.
- * @notice RBACWithAdmin is probably too expansive and powerful for your
- * application; an admin is actually able to change any address to any role
- * which is a very large API surface. It's recommended that you follow a strategy
- * of strictly defining the abilities of your roles
- * and the API-surface of your contract.
- * This is just an example for example's sake.
- */
-contract RBACWithAdmin is RBAC {
-  /**
-   * A constant role name for indicating admins.
-   */
-  string public constant ROLE_ADMIN = "admin";
-
-  /**
-   * @dev modifier to scope access to admins
-   * // reverts
-   */
-  modifier onlyAdmin()
-  {
-    checkRole(msg.sender, ROLE_ADMIN);
-    _;
-  }
-
-  /**
-   * @dev constructor. Sets msg.sender as admin by default
-   */
-  constructor()
-    public
-  {
-    addRole(msg.sender, ROLE_ADMIN);
-  }
-
-  /**
-   * @dev add a role to an address
-   * @param _addr address
-   * @param _roleName the name of the role
-   */
-  function adminAddRole(address _addr, string _roleName)
-    public
-    onlyAdmin
-  {
-    addRole(_addr, _roleName);
-  }
-
-  /**
-   * @dev remove a role from an address
-   * @param _addr address
-   * @param _roleName the name of the role
-   */
-  function adminRemoveRole(address _addr, string _roleName)
-    public
-    onlyAdmin
-  {
-    removeRole(_addr, _roleName);
-  }
-}

contracts/examples/SampleCrowdsale.sol

@@ -1,60 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../crowdsale/validation/CappedCrowdsale.sol";
-import "../crowdsale/distribution/RefundableCrowdsale.sol";
-import "../crowdsale/emission/MintedCrowdsale.sol";
-import "../token/ERC20/MintableToken.sol";
-
-
-/**
- * @title SampleCrowdsaleToken
- * @dev Very simple ERC20 Token that can be minted.
- * It is meant to be used in a crowdsale contract.
- */
-contract SampleCrowdsaleToken is MintableToken {
-
-  string public constant name = "Sample Crowdsale Token";
-  string public constant symbol = "SCT";
-  uint8 public constant decimals = 18;
-
-}
-
-
-/**
- * @title SampleCrowdsale
- * @dev This is an example of a fully fledged crowdsale.
- * The way to add new features to a base crowdsale is by multiple inheritance.
- * In this example we are providing following extensions:
- * CappedCrowdsale - sets a max boundary for raised funds
- * RefundableCrowdsale - set a min goal to be reached and returns funds if it's not met
- *
- * After adding multiple features it's good practice to run integration tests
- * to ensure that subcontracts works together as intended.
- */
-// XXX There doesn't seem to be a way to split this line that keeps solium
-// happy. See:
-// https://github.com/duaraghav8/Solium/issues/205
-// --elopio - 2018-05-10
-// solium-disable-next-line max-len
-contract SampleCrowdsale is CappedCrowdsale, RefundableCrowdsale, MintedCrowdsale {
-
-  constructor(
-    uint256 _openingTime,
-    uint256 _closingTime,
-    uint256 _rate,
-    address _wallet,
-    uint256 _cap,
-    MintableToken _token,
-    uint256 _goal
-  )
-    public
-    Crowdsale(_rate, _wallet, _token)
-    CappedCrowdsale(_cap)
-    TimedCrowdsale(_openingTime, _closingTime)
-    RefundableCrowdsale(_goal)
-  {
-    //As goal needs to be met for a successful crowdsale
-    //the value needs to less or equal than a cap which is limit for accepted funds
-    require(_goal <= _cap);
-  }
-}

contracts/examples/SimpleSavingsWallet.sol

@@ -1,40 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../ownership/Heritable.sol";
-
-
-/**
- * @title SimpleSavingsWallet
- * @dev Simplest form of savings wallet whose ownership can be claimed by a heir
- * if owner dies.
- * In this example, we take a very simple savings wallet providing two operations
- * (to send and receive funds) and extend its capabilities by making it Heritable.
- * The account that creates the contract is set as owner, who has the authority to
- * choose an heir account. Heir account can reclaim the contract ownership in the
- * case that the owner dies.
- */
-contract SimpleSavingsWallet is Heritable {
-
-  event Sent(address indexed payee, uint256 amount, uint256 balance);
-  event Received(address indexed payer, uint256 amount, uint256 balance);
-
-
-  constructor(uint256 _heartbeatTimeout) Heritable(_heartbeatTimeout) public {}
-
-  /**
-   * @dev wallet can receive funds.
-   */
-  function () external payable {
-    emit Received(msg.sender, msg.value, address(this).balance);
-  }
-
-  /**
-   * @dev wallet can send funds
-   */
-  function sendTo(address _payee, uint256 _amount) public onlyOwner {
-    require(_payee != address(0) && _payee != address(this));
-    require(_amount > 0);
-    _payee.transfer(_amount);
-    emit Sent(_payee, _amount, address(this).balance);
-  }
-}

contracts/examples/SimpleToken.sol

@@ -1,30 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-import "../token/ERC20/StandardToken.sol";
-
-
-/**
- * @title SimpleToken
- * @dev Very simple ERC20 Token example, where all tokens are pre-assigned to the creator.
- * Note they can later distribute these tokens as they wish using `transfer` and other
- * `StandardToken` functions.
- */
-contract SimpleToken is StandardToken {
-
-  string public constant name = "SimpleToken";
-  string public constant symbol = "SIM";
-  uint8 public constant decimals = 18;
-
-  uint256 public constant INITIAL_SUPPLY = 10000 * (10 ** uint256(decimals));
-
-  /**
-   * @dev Constructor that gives msg.sender all of existing tokens.
-   */
-  constructor() public {
-    totalSupply_ = INITIAL_SUPPLY;
-    balances[msg.sender] = INITIAL_SUPPLY;
-    emit Transfer(address(0), msg.sender, INITIAL_SUPPLY);
-  }
-
-}

contracts/introspection/ERC165.sol

@@ -1,20 +1,54 @@
-pragma solidity ^0.4.24;
+// SPDX-License-Identifier: MIT
 
+pragma solidity ^0.6.0;
+
+import "./IERC165.sol";
 
 /**
- * @title ERC165
- * @dev https://github.com/ethereum/EIPs/blob/master/EIPS/eip-165.md
+ * @dev Implementation of the {IERC165} interface.
+ *
+ * Contracts may inherit from this and call {_registerInterface} to declare
+ * their support of an interface.
  */
-interface ERC165 {
+contract ERC165 is IERC165 {
+    /*
+     * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
+     */
+    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;
 
-  /**
-   * @notice Query if a contract implements an interface
-   * @param _interfaceId The interface identifier, as specified in ERC-165
-   * @dev Interface identification is specified in ERC-165. This function
-   * uses less than 30,000 gas.
-   */
-  function supportsInterface(bytes4 _interfaceId)
-    external
-    view
-    returns (bool);
+    /**
+     * @dev Mapping of interface ids to whether or not it's supported.
+     */
+    mapping(bytes4 => bool) private _supportedInterfaces;
+
+    constructor () internal {
+        // Derived contracts need only register support for their own interfaces,
+        // we register support for ERC165 itself here
+        _registerInterface(_INTERFACE_ID_ERC165);
+    }
+
+    /**
+     * @dev See {IERC165-supportsInterface}.
+     *
+     * Time complexity O(1), guaranteed to always use less than 30 000 gas.
+     */
+    function supportsInterface(bytes4 interfaceId) public view override returns (bool) {
+        return _supportedInterfaces[interfaceId];
+    }
+
+    /**
+     * @dev Registers the contract as an implementer of the interface defined by
+     * `interfaceId`. Support of the actual ERC165 interface is automatic and
+     * registering its interface id is not required.
+     *
+     * See {IERC165-supportsInterface}.
+     *
+     * Requirements:
+     *
+     * - `interfaceId` cannot be the ERC165 invalid interface (`0xffffffff`).
+     */
+    function _registerInterface(bytes4 interfaceId) internal virtual {
+        require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
+        _supportedInterfaces[interfaceId] = true;
+    }
 }

contracts/introspection/ERC165Checker.sol

@@ -0,0 +1,106 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.2;
+
+/**
+ * @dev Library used to query support of an interface declared via {IERC165}.
+ *
+ * Note that these functions return the actual result of the query: they do not
+ * `revert` if an interface is not supported. It is up to the caller to decide
+ * what to do in these cases.
+ */
+library ERC165Checker {
+    // As per the EIP-165 spec, no interface should ever match 0xffffffff
+    bytes4 private constant _INTERFACE_ID_INVALID = 0xffffffff;
+
+    /*
+     * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
+     */
+    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;
+
+    /**
+     * @dev Returns true if `account` supports the {IERC165} interface,
+     */
+    function supportsERC165(address account) internal view returns (bool) {
+        // Any contract that implements ERC165 must explicitly indicate support of
+        // InterfaceId_ERC165 and explicitly indicate non-support of InterfaceId_Invalid
+        return _supportsERC165Interface(account, _INTERFACE_ID_ERC165) &&
+            !_supportsERC165Interface(account, _INTERFACE_ID_INVALID);
+    }
+
+    /**
+     * @dev Returns true if `account` supports the interface defined by
+     * `interfaceId`. Support for {IERC165} itself is queried automatically.
+     *
+     * See {IERC165-supportsInterface}.
+     */
+    function supportsInterface(address account, bytes4 interfaceId) internal view returns (bool) {
+        // query support of both ERC165 as per the spec and support of _interfaceId
+        return supportsERC165(account) &&
+            _supportsERC165Interface(account, interfaceId);
+    }
+
+    /**
+     * @dev Returns true if `account` supports all the interfaces defined in
+     * `interfaceIds`. Support for {IERC165} itself is queried automatically.
+     *
+     * Batch-querying can lead to gas savings by skipping repeated checks for
+     * {IERC165} support.
+     *
+     * See {IERC165-supportsInterface}.
+     */
+    function supportsAllInterfaces(address account, bytes4[] memory interfaceIds) internal view returns (bool) {
+        // query support of ERC165 itself
+        if (!supportsERC165(account)) {
+            return false;
+        }
+
+        // query support of each interface in _interfaceIds
+        for (uint256 i = 0; i < interfaceIds.length; i++) {
+            if (!_supportsERC165Interface(account, interfaceIds[i])) {
+                return false;
+            }
+        }
+
+        // all interfaces supported
+        return true;
+    }
+
+    /**
+     * @notice Query if a contract implements an interface, does not check ERC165 support
+     * @param account The address of the contract to query for support of an interface
+     * @param interfaceId The interface identifier, as specified in ERC-165
+     * @return true if the contract at account indicates support of the interface with
+     * identifier interfaceId, false otherwise
+     * @dev Assumes that account contains a contract that supports ERC165, otherwise
+     * the behavior of this method is undefined. This precondition can be checked
+     * with {supportsERC165}.
+     * Interface identification is specified in ERC-165.
+     */
+    function _supportsERC165Interface(address account, bytes4 interfaceId) private view returns (bool) {
+        // success determines whether the staticcall succeeded and result determines
+        // whether the contract at account indicates support of _interfaceId
+        (bool success, bool result) = _callERC165SupportsInterface(account, interfaceId);
+
+        return (success && result);
+    }
+
+    /**
+     * @notice Calls the function with selector 0x01ffc9a7 (ERC165) and suppresses throw
+     * @param account The address of the contract to query for support of an interface
+     * @param interfaceId The interface identifier, as specified in ERC-165
+     * @return success true if the STATICCALL succeeded, false otherwise
+     * @return result true if the STATICCALL succeeded and the contract at account
+     * indicates support of the interface with identifier interfaceId, false otherwise
+     */
+    function _callERC165SupportsInterface(address account, bytes4 interfaceId)
+        private
+        view
+        returns (bool, bool)
+    {
+        bytes memory encodedParams = abi.encodeWithSelector(_INTERFACE_ID_ERC165, interfaceId);
+        (bool success, bytes memory result) = account.staticcall{ gas: 30000 }(encodedParams);
+        if (result.length < 32) return (false, false);
+        return (success, abi.decode(result, (bool)));
+    }
+}

contracts/introspection/ERC1820Implementer.sol

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "./IERC1820Implementer.sol";
+
+/**
+ * @dev Implementation of the {IERC1820Implementer} interface.
+ *
+ * Contracts may inherit from this and call {_registerInterfaceForAddress} to
+ * declare their willingness to be implementers.
+ * {IERC1820Registry-setInterfaceImplementer} should then be called for the
+ * registration to be complete.
+ */
+contract ERC1820Implementer is IERC1820Implementer {
+    bytes32 constant private _ERC1820_ACCEPT_MAGIC = keccak256(abi.encodePacked("ERC1820_ACCEPT_MAGIC"));
+
+    mapping(bytes32 => mapping(address => bool)) private _supportedInterfaces;
+
+    /**
+     * See {IERC1820Implementer-canImplementInterfaceForAddress}.
+     */
+    function canImplementInterfaceForAddress(bytes32 interfaceHash, address account) public view override returns (bytes32) {
+        return _supportedInterfaces[interfaceHash][account] ? _ERC1820_ACCEPT_MAGIC : bytes32(0x00);
+    }
+
+    /**
+     * @dev Declares the contract as willing to be an implementer of
+     * `interfaceHash` for `account`.
+     *
+     * See {IERC1820Registry-setInterfaceImplementer} and
+     * {IERC1820Registry-interfaceHash}.
+     */
+    function _registerInterfaceForAddress(bytes32 interfaceHash, address account) internal virtual {
+        _supportedInterfaces[interfaceHash][account] = true;
+    }
+}

contracts/introspection/IERC165.sol

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+/**
+ * @dev Interface of the ERC165 standard, as defined in the
+ * https://eips.ethereum.org/EIPS/eip-165[EIP].
+ *
+ * Implementers can declare support of contract interfaces, which can then be
+ * queried by others ({ERC165Checker}).
+ *
+ * For an implementation, see {ERC165}.
+ */
+interface IERC165 {
+    /**
+     * @dev Returns true if this contract implements the interface defined by
+     * `interfaceId`. See the corresponding
+     * https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified[EIP section]
+     * to learn more about how these ids are created.
+     *
+     * This function call must use less than 30 000 gas.
+     */
+    function supportsInterface(bytes4 interfaceId) external view returns (bool);
+}

contracts/introspection/IERC1820Implementer.sol

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+/**
+ * @dev Interface for an ERC1820 implementer, as defined in the
+ * https://eips.ethereum.org/EIPS/eip-1820#interface-implementation-erc1820implementerinterface[EIP].
+ * Used by contracts that will be registered as implementers in the
+ * {IERC1820Registry}.
+ */
+interface IERC1820Implementer {
+    /**
+     * @dev Returns a special value (`ERC1820_ACCEPT_MAGIC`) if this contract
+     * implements `interfaceHash` for `account`.
+     *
+     * See {IERC1820Registry-setInterfaceImplementer}.
+     */
+    function canImplementInterfaceForAddress(bytes32 interfaceHash, address account) external view returns (bytes32);
+}

contracts/introspection/IERC1820Registry.sol

@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+/**
+ * @dev Interface of the global ERC1820 Registry, as defined in the
+ * https://eips.ethereum.org/EIPS/eip-1820[EIP]. Accounts may register
+ * implementers for interfaces in this registry, as well as query support.
+ *
+ * Implementers may be shared by multiple accounts, and can also implement more
+ * than a single interface for each account. Contracts can implement interfaces
+ * for themselves, but externally-owned accounts (EOA) must delegate this to a
+ * contract.
+ *
+ * {IERC165} interfaces can also be queried via the registry.
+ *
+ * For an in-depth explanation and source code analysis, see the EIP text.
+ */
+interface IERC1820Registry {
+    /**
+     * @dev Sets `newManager` as the manager for `account`. A manager of an
+     * account is able to set interface implementers for it.
+     *
+     * By default, each account is its own manager. Passing a value of `0x0` in
+     * `newManager` will reset the manager to this initial state.
+     *
+     * Emits a {ManagerChanged} event.
+     *
+     * Requirements:
+     *
+     * - the caller must be the current manager for `account`.
+     */
+    function setManager(address account, address newManager) external;
+
+    /**
+     * @dev Returns the manager for `account`.
+     *
+     * See {setManager}.
+     */
+    function getManager(address account) external view returns (address);
+
+    /**
+     * @dev Sets the `implementer` contract as ``account``'s implementer for
+     * `interfaceHash`.
+     *
+     * `account` being the zero address is an alias for the caller's address.
+     * The zero address can also be used in `implementer` to remove an old one.
+     *
+     * See {interfaceHash} to learn how these are created.
+     *
+     * Emits an {InterfaceImplementerSet} event.
+     *
+     * Requirements:
+     *
+     * - the caller must be the current manager for `account`.
+     * - `interfaceHash` must not be an {IERC165} interface id (i.e. it must not
+     * end in 28 zeroes).
+     * - `implementer` must implement {IERC1820Implementer} and return true when
+     * queried for support, unless `implementer` is the caller. See
+     * {IERC1820Implementer-canImplementInterfaceForAddress}.
+     */
+    function setInterfaceImplementer(address account, bytes32 interfaceHash, address implementer) external;
+
+    /**
+     * @dev Returns the implementer of `interfaceHash` for `account`. If no such
+     * implementer is registered, returns the zero address.
+     *
+     * If `interfaceHash` is an {IERC165} interface id (i.e. it ends with 28
+     * zeroes), `account` will be queried for support of it.
+     *
+     * `account` being the zero address is an alias for the caller's address.
+     */
+    function getInterfaceImplementer(address account, bytes32 interfaceHash) external view returns (address);
+
+    /**
+     * @dev Returns the interface hash for an `interfaceName`, as defined in the
+     * corresponding
+     * https://eips.ethereum.org/EIPS/eip-1820#interface-name[section of the EIP].
+     */
+    function interfaceHash(string calldata interfaceName) external pure returns (bytes32);
+
+    /**
+     *  @notice Updates the cache with whether the contract implements an ERC165 interface or not.
+     *  @param account Address of the contract for which to update the cache.
+     *  @param interfaceId ERC165 interface for which to update the cache.
+     */
+    function updateERC165Cache(address account, bytes4 interfaceId) external;
+
+    /**
+     *  @notice Checks whether a contract implements an ERC165 interface or not.
+     *  If the result is not cached a direct lookup on the contract address is performed.
+     *  If the result is not cached or the cached value is out-of-date, the cache MUST be updated manually by calling
+     *  {updateERC165Cache} with the contract address.
+     *  @param account Address of the contract to check.
+     *  @param interfaceId ERC165 interface to check.
+     *  @return True if `account` implements `interfaceId`, false otherwise.
+     */
+    function implementsERC165Interface(address account, bytes4 interfaceId) external view returns (bool);
+
+    /**
+     *  @notice Checks whether a contract implements an ERC165 interface or not without using nor updating the cache.
+     *  @param account Address of the contract to check.
+     *  @param interfaceId ERC165 interface to check.
+     *  @return True if `account` implements `interfaceId`, false otherwise.
+     */
+    function implementsERC165InterfaceNoCache(address account, bytes4 interfaceId) external view returns (bool);
+
+    event InterfaceImplementerSet(address indexed account, bytes32 indexed interfaceHash, address indexed implementer);
+
+    event ManagerChanged(address indexed account, address indexed newManager);
+}

contracts/introspection/README.adoc

@@ -0,0 +1,31 @@
+= Introspection
+
+[.readme-notice]
+NOTE: This document is better viewed at https://docs.openzeppelin.com/contracts/api/introspection
+
+This set of interfaces and contracts deal with https://en.wikipedia.org/wiki/Type_introspection[type introspection] of contracts, that is, examining which functions can be called on them. This is usually referred to as a contract's _interface_.
+
+Ethereum contracts have no native concept of an interface, so applications must usually simply trust they are not making an incorrect call. For trusted setups this is a non-issue, but often unknown and untrusted third-party addresses need to be interacted with. There may even not be any direct calls to them! (e.g. `ERC20` tokens may be sent to a contract that lacks a way to transfer them out of it, locking them forever). In these cases, a contract _declaring_ its interface can be very helpful in preventing errors.
+
+There are two main ways to approach this.
+
+* Locally, where a contract implements `IERC165` and declares an interface, and a second one queries it directly via `ERC165Checker`.
+* Globally, where a global and unique registry (`IERC1820Registry`) is used to register implementers of a certain interface (`IERC1820Implementer`). It is then the registry that is queried, which allows for more complex setups, like contracts implementing interfaces for externally-owned accounts.
+
+Note that, in all cases, accounts simply _declare_ their interfaces, but they are not required to actually implement them. This mechanism can therefore be used to both prevent errors and allow for complex interactions (see `ERC777`), but it must not be relied on for security.
+
+== Local
+
+{{IERC165}}
+
+{{ERC165}}
+
+{{ERC165Checker}}
+
+== Global
+
+{{IERC1820Registry}}
+
+{{IERC1820Implementer}}
+
+{{ERC1820Implementer}}

contracts/introspection/SupportsInterfaceWithLookup.sol

@@ -1,54 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "./ERC165.sol";
-
-
-/**
- * @title SupportsInterfaceWithLookup
- * @author Matt Condon (@shrugs)
- * @dev Implements ERC165 using a lookup table.
- */
-contract SupportsInterfaceWithLookup is ERC165 {
-
-  bytes4 public constant InterfaceId_ERC165 = 0x01ffc9a7;
-  /**
-   * 0x01ffc9a7 ===
-   *   bytes4(keccak256('supportsInterface(bytes4)'))
-   */
-
-  /**
-   * @dev a mapping of interface id to whether or not it's supported
-   */
-  mapping(bytes4 => bool) internal supportedInterfaces;
-
-  /**
-   * @dev A contract implementing SupportsInterfaceWithLookup
-   * implement ERC165 itself
-   */
-  constructor()
-    public
-  {
-    _registerInterface(InterfaceId_ERC165);
-  }
-
-  /**
-   * @dev implement supportsInterface(bytes4) using a lookup table
-   */
-  function supportsInterface(bytes4 _interfaceId)
-    external
-    view
-    returns (bool)
-  {
-    return supportedInterfaces[_interfaceId];
-  }
-
-  /**
-   * @dev private method for registering an interface
-   */
-  function _registerInterface(bytes4 _interfaceId)
-    internal
-  {
-    require(_interfaceId != 0xffffffff);
-    supportedInterfaces[_interfaceId] = true;
-  }
-}

contracts/lifecycle/Destructible.sol

@@ -1,22 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-import "../ownership/Ownable.sol";
-
-
-/**
- * @title Destructible
- * @dev Base contract that can be destroyed by owner. All funds in contract will be sent to the owner.
- */
-contract Destructible is Ownable {
-  /**
-   * @dev Transfers the current balance to the owner and terminates the contract.
-   */
-  function destroy() public onlyOwner {
-    selfdestruct(owner);
-  }
-
-  function destroyAndSend(address _recipient) public onlyOwner {
-    selfdestruct(_recipient);
-  }
-}

contracts/lifecycle/Pausable.sol

@@ -1,49 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-import "../ownership/Ownable.sol";
-
-
-/**
- * @title Pausable
- * @dev Base contract which allows children to implement an emergency stop mechanism.
- */
-contract Pausable is Ownable {
-  event Pause();
-  event Unpause();
-
-  bool public paused = false;
-
-
-  /**
-   * @dev Modifier to make a function callable only when the contract is not paused.
-   */
-  modifier whenNotPaused() {
-    require(!paused);
-    _;
-  }
-
-  /**
-   * @dev Modifier to make a function callable only when the contract is paused.
-   */
-  modifier whenPaused() {
-    require(paused);
-    _;
-  }
-
-  /**
-   * @dev called by the owner to pause, triggers stopped state
-   */
-  function pause() public onlyOwner whenNotPaused {
-    paused = true;
-    emit Pause();
-  }
-
-  /**
-   * @dev called by the owner to unpause, returns to normal state
-   */
-  function unpause() public onlyOwner whenPaused {
-    paused = false;
-    emit Unpause();
-  }
-}

contracts/lifecycle/TokenDestructible.sol

@@ -1,36 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../ownership/Ownable.sol";
-import "../token/ERC20/ERC20Basic.sol";
-
-
-/**
- * @title TokenDestructible:
- * @author Remco Bloemen <remco@2π.com>
- * @dev Base contract that can be destroyed by owner. All funds in contract including
- * listed tokens will be sent to the owner.
- */
-contract TokenDestructible is Ownable {
-
-  constructor() public payable { }
-
-  /**
-   * @notice Terminate contract and refund to owner
-   * @param _tokens List of addresses of ERC20 or ERC20Basic token contracts to
-   refund.
-   * @notice The called token contracts could try to re-enter this contract. Only
-   supply token contracts you trust.
-   */
-  function destroy(address[] _tokens) public onlyOwner {
-
-    // Transfer tokens to owner
-    for (uint256 i = 0; i < _tokens.length; i++) {
-      ERC20Basic token = ERC20Basic(_tokens[i]);
-      uint256 balance = token.balanceOf(this);
-      token.transfer(owner, balance);
-    }
-
-    // Transfer Eth to owner and terminate contract
-    selfdestruct(owner);
-  }
-}

contracts/math/Math.sol

@@ -1,24 +1,31 @@
-pragma solidity ^0.4.24;
+// SPDX-License-Identifier: MIT
 
+pragma solidity ^0.6.0;
 
 /**
- * @title Math
- * @dev Assorted math operations
+ * @dev Standard math utilities missing in the Solidity language.
  */
 library Math {
-  function max64(uint64 _a, uint64 _b) internal pure returns (uint64) {
-    return _a >= _b ? _a : _b;
-  }
+    /**
+     * @dev Returns the largest of two numbers.
+     */
+    function max(uint256 a, uint256 b) internal pure returns (uint256) {
+        return a >= b ? a : b;
+    }
 
-  function min64(uint64 _a, uint64 _b) internal pure returns (uint64) {
-    return _a < _b ? _a : _b;
-  }
+    /**
+     * @dev Returns the smallest of two numbers.
+     */
+    function min(uint256 a, uint256 b) internal pure returns (uint256) {
+        return a < b ? a : b;
+    }
 
-  function max256(uint256 _a, uint256 _b) internal pure returns (uint256) {
-    return _a >= _b ? _a : _b;
-  }
-
-  function min256(uint256 _a, uint256 _b) internal pure returns (uint256) {
-    return _a < _b ? _a : _b;
-  }
+    /**
+     * @dev Returns the average of two numbers. The result is rounded towards
+     * zero.
+     */
+    function average(uint256 a, uint256 b) internal pure returns (uint256) {
+        // (a + b) / 2 can overflow, so we distribute
+        return (a / 2) + (b / 2) + ((a % 2 + b % 2) / 2);
+    }
 }

contracts/math/README.adoc

@@ -0,0 +1,14 @@
+= Math
+
+[.readme-notice]
+NOTE: This document is better viewed at https://docs.openzeppelin.com/contracts/api/math
+
+These are math-related utilities.
+
+== Libraries
+
+{{SafeMath}}
+
+{{SignedSafeMath}}
+
+{{Math}}

contracts/math/SafeMath.sol

@@ -1,52 +1,159 @@
-pragma solidity ^0.4.24;
+// SPDX-License-Identifier: MIT
 
+pragma solidity ^0.6.0;
 
 /**
- * @title SafeMath
- * @dev Math operations with safety checks that throw on error
+ * @dev Wrappers over Solidity's arithmetic operations with added overflow
+ * checks.
+ *
+ * Arithmetic operations in Solidity wrap on overflow. This can easily result
+ * in bugs, because programmers usually assume that an overflow raises an
+ * error, which is the standard behavior in high level programming languages.
+ * `SafeMath` restores this intuition by reverting the transaction when an
+ * operation overflows.
+ *
+ * Using this library instead of the unchecked operations eliminates an entire
+ * class of bugs, so it's recommended to use it always.
  */
 library SafeMath {
+    /**
+     * @dev Returns the addition of two unsigned integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `+` operator.
+     *
+     * Requirements:
+     *
+     * - Addition cannot overflow.
+     */
+    function add(uint256 a, uint256 b) internal pure returns (uint256) {
+        uint256 c = a + b;
+        require(c >= a, "SafeMath: addition overflow");
 
-  /**
-  * @dev Multiplies two numbers, throws on overflow.
-  */
-  function mul(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
-    // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
-    // benefit is lost if 'b' is also tested.
-    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
-    if (_a == 0) {
-      return 0;
+        return c;
     }
 
-    c = _a * _b;
-    assert(c / _a == _b);
-    return c;
-  }
+    /**
+     * @dev Returns the subtraction of two unsigned integers, reverting on
+     * overflow (when the result is negative).
+     *
+     * Counterpart to Solidity's `-` operator.
+     *
+     * Requirements:
+     *
+     * - Subtraction cannot overflow.
+     */
+    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
+        return sub(a, b, "SafeMath: subtraction overflow");
+    }
 
-  /**
-  * @dev Integer division of two numbers, truncating the quotient.
-  */
-  function div(uint256 _a, uint256 _b) internal pure returns (uint256) {
-    // assert(_b > 0); // Solidity automatically throws when dividing by 0
-    // uint256 c = _a / _b;
-    // assert(_a == _b * c + _a % _b); // There is no case in which this doesn't hold
-    return _a / _b;
-  }
+    /**
+     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
+     * overflow (when the result is negative).
+     *
+     * Counterpart to Solidity's `-` operator.
+     *
+     * Requirements:
+     *
+     * - Subtraction cannot overflow.
+     */
+    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
+        require(b <= a, errorMessage);
+        uint256 c = a - b;
 
-  /**
-  * @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
-  */
-  function sub(uint256 _a, uint256 _b) internal pure returns (uint256) {
-    assert(_b <= _a);
-    return _a - _b;
-  }
+        return c;
+    }
 
-  /**
-  * @dev Adds two numbers, throws on overflow.
-  */
-  function add(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
-    c = _a + _b;
-    assert(c >= _a);
-    return c;
-  }
+    /**
+     * @dev Returns the multiplication of two unsigned integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `*` operator.
+     *
+     * Requirements:
+     *
+     * - Multiplication cannot overflow.
+     */
+    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
+        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
+        // benefit is lost if 'b' is also tested.
+        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
+        if (a == 0) {
+            return 0;
+        }
+
+        uint256 c = a * b;
+        require(c / a == b, "SafeMath: multiplication overflow");
+
+        return c;
+    }
+
+    /**
+     * @dev Returns the integer division of two unsigned integers. Reverts on
+     * division by zero. The result is rounded towards zero.
+     *
+     * Counterpart to Solidity's `/` operator. Note: this function uses a
+     * `revert` opcode (which leaves remaining gas untouched) while Solidity
+     * uses an invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     *
+     * - The divisor cannot be zero.
+     */
+    function div(uint256 a, uint256 b) internal pure returns (uint256) {
+        return div(a, b, "SafeMath: division by zero");
+    }
+
+    /**
+     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
+     * division by zero. The result is rounded towards zero.
+     *
+     * Counterpart to Solidity's `/` operator. Note: this function uses a
+     * `revert` opcode (which leaves remaining gas untouched) while Solidity
+     * uses an invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     *
+     * - The divisor cannot be zero.
+     */
+    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
+        require(b > 0, errorMessage);
+        uint256 c = a / b;
+        // assert(a == b * c + a % b); // There is no case in which this doesn't hold
+
+        return c;
+    }
+
+    /**
+     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
+     * Reverts when dividing by zero.
+     *
+     * Counterpart to Solidity's `%` operator. This function uses a `revert`
+     * opcode (which leaves remaining gas untouched) while Solidity uses an
+     * invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     *
+     * - The divisor cannot be zero.
+     */
+    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
+        return mod(a, b, "SafeMath: modulo by zero");
+    }
+
+    /**
+     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
+     * Reverts with custom message when dividing by zero.
+     *
+     * Counterpart to Solidity's `%` operator. This function uses a `revert`
+     * opcode (which leaves remaining gas untouched) while Solidity uses an
+     * invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     *
+     * - The divisor cannot be zero.
+     */
+    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
+        require(b != 0, errorMessage);
+        return a % b;
+    }
 }

contracts/math/SignedSafeMath.sol

@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+/**
+ * @title SignedSafeMath
+ * @dev Signed math operations with safety checks that revert on error.
+ */
+library SignedSafeMath {
+    int256 constant private _INT256_MIN = -2**255;
+
+    /**
+     * @dev Returns the multiplication of two signed integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `*` operator.
+     *
+     * Requirements:
+     *
+     * - Multiplication cannot overflow.
+     */
+    function mul(int256 a, int256 b) internal pure returns (int256) {
+        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
+        // benefit is lost if 'b' is also tested.
+        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
+        if (a == 0) {
+            return 0;
+        }
+
+        require(!(a == -1 && b == _INT256_MIN), "SignedSafeMath: multiplication overflow");
+
+        int256 c = a * b;
+        require(c / a == b, "SignedSafeMath: multiplication overflow");
+
+        return c;
+    }
+
+    /**
+     * @dev Returns the integer division of two signed integers. Reverts on
+     * division by zero. The result is rounded towards zero.
+     *
+     * Counterpart to Solidity's `/` operator. Note: this function uses a
+     * `revert` opcode (which leaves remaining gas untouched) while Solidity
+     * uses an invalid opcode to revert (consuming all remaining gas).
+     *
+     * Requirements:
+     *
+     * - The divisor cannot be zero.
+     */
+    function div(int256 a, int256 b) internal pure returns (int256) {
+        require(b != 0, "SignedSafeMath: division by zero");
+        require(!(b == -1 && a == _INT256_MIN), "SignedSafeMath: division overflow");
+
+        int256 c = a / b;
+
+        return c;
+    }
+
+    /**
+     * @dev Returns the subtraction of two signed integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `-` operator.
+     *
+     * Requirements:
+     *
+     * - Subtraction cannot overflow.
+     */
+    function sub(int256 a, int256 b) internal pure returns (int256) {
+        int256 c = a - b;
+        require((b >= 0 && c <= a) || (b < 0 && c > a), "SignedSafeMath: subtraction overflow");
+
+        return c;
+    }
+
+    /**
+     * @dev Returns the addition of two signed integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `+` operator.
+     *
+     * Requirements:
+     *
+     * - Addition cannot overflow.
+     */
+    function add(int256 a, int256 b) internal pure returns (int256) {
+        int256 c = a + b;
+        require((b >= 0 && c >= a) || (b < 0 && c < a), "SignedSafeMath: addition overflow");
+
+        return c;
+    }
+}

contracts/mocks/AccessControlMock.sol

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "../access/AccessControl.sol";
+
+contract AccessControlMock is AccessControl {
+    constructor() public {
+        _setupRole(DEFAULT_ADMIN_ROLE, _msgSender());
+    }
+
+    function setRoleAdmin(bytes32 roleId, bytes32 adminRoleId) public {
+        _setRoleAdmin(roleId, adminRoleId);
+    }
+}

contracts/mocks/AddressImpl.sol

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "../utils/Address.sol";
+
+contract AddressImpl {
+    event CallReturnValue(string data);
+
+    function isContract(address account) external view returns (bool) {
+        return Address.isContract(account);
+    }
+
+    function sendValue(address payable receiver, uint256 amount) external {
+        Address.sendValue(receiver, amount);
+    }
+
+    function functionCall(address target, bytes calldata data) external {
+        bytes memory returnData = Address.functionCall(target, data);
+
+        emit CallReturnValue(abi.decode(returnData, (string)));
+    }
+
+    function functionCallWithValue(address target, bytes calldata data, uint256 value) external payable {
+        bytes memory returnData = Address.functionCallWithValue(target, data, value);
+
+        emit CallReturnValue(abi.decode(returnData, (string)));
+    }
+
+    // sendValue's tests require the contract to hold Ether
+    receive () external payable { }
+}

contracts/mocks/AllowanceCrowdsaleImpl.sol

@@ -1,21 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../token/ERC20/ERC20.sol";
-import "../crowdsale/emission/AllowanceCrowdsale.sol";
-
-
-contract AllowanceCrowdsaleImpl is AllowanceCrowdsale {
-
-  constructor (
-    uint256 _rate,
-    address _wallet,
-    ERC20 _token,
-    address _tokenWallet
-  )
-    public
-    Crowdsale(_rate, _wallet, _token)
-    AllowanceCrowdsale(_tokenWallet)
-  {
-  }
-
-}

contracts/mocks/ArraysImpl.sol

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "../utils/Arrays.sol";
+
+contract ArraysImpl {
+    using Arrays for uint256[];
+
+    uint256[] private _array;
+
+    constructor (uint256[] memory array) public {
+        _array = array;
+    }
+
+    function findUpperBound(uint256 element) external view returns (uint256) {
+        return _array.findUpperBound(element);
+    }
+}

contracts/mocks/AutoIncrementingImpl.sol

@@ -1,21 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../AutoIncrementing.sol";
-
-
-contract AutoIncrementingImpl {
-  using AutoIncrementing for AutoIncrementing.Counter;
-
-  uint256 public theId;
-
-  // use whatever key you want to track your counters
-  mapping(string => AutoIncrementing.Counter) private counters;
-
-  function doThing(string _key)
-    public
-    returns (uint256)
-  {
-    theId = counters[_key].nextId();
-    return theId;
-  }
-}

contracts/mocks/BasicTokenMock.sol

@@ -1,15 +0,0 @@
-pragma solidity ^0.4.24;
-
-
-import "../token/ERC20/BasicToken.sol";
-
-
-// mock class using BasicToken
-contract BasicTokenMock is BasicToken {
-
-  constructor(address _initialAccount, uint256 _initialBalance) public {
-    balances[_initialAccount] = _initialBalance;
-    totalSupply_ = _initialBalance;
-  }
-
-}

contracts/mocks/BouncerMock.sol

@@ -1,66 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../access/SignatureBouncer.sol";
-
-
-contract SignatureBouncerMock is SignatureBouncer {
-  function checkValidSignature(address _address, bytes _sig)
-    public
-    view
-    returns (bool)
-  {
-    return isValidSignature(_address, _sig);
-  }
-
-  function onlyWithValidSignature(bytes _sig)
-    public
-    onlyValidSignature(_sig)
-    view
-  {
-
-  }
-
-  function checkValidSignatureAndMethod(address _address, bytes _sig)
-    public
-    view
-    returns (bool)
-  {
-    return isValidSignatureAndMethod(_address, _sig);
-  }
-
-  function onlyWithValidSignatureAndMethod(bytes _sig)
-    public
-    onlyValidSignatureAndMethod(_sig)
-    view
-  {
-
-  }
-
-  function checkValidSignatureAndData(
-    address _address,
-    bytes,
-    uint,
-    bytes _sig
-  )
-    public
-    view
-    returns (bool)
-  {
-    return isValidSignatureAndData(_address, _sig);
-  }
-
-  function onlyWithValidSignatureAndData(uint, bytes _sig)
-    public
-    onlyValidSignatureAndData(_sig)
-    view
-  {
-
-  }
-
-  function theWrongMethod(bytes)
-    public
-    pure
-  {
-
-  }
-}

contracts/mocks/BurnableTokenMock.sol

@@ -1,13 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../token/ERC20/BurnableToken.sol";
-
-
-contract BurnableTokenMock is BurnableToken {
-
-  constructor(address _initialAccount, uint _initialBalance) public {
-    balances[_initialAccount] = _initialBalance;
-    totalSupply_ = _initialBalance;
-  }
-
-}

contracts/mocks/CallReceiverMock.sol

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+contract CallReceiverMock {
+
+    event MockFunctionCalled();
+
+    uint256[] private _array;
+
+    function mockFunction() public payable returns (string memory) {
+        emit MockFunctionCalled();
+
+        return "0x1234";
+    }
+
+    function mockFunctionNonPayable() public returns (string memory) {
+        emit MockFunctionCalled();
+
+        return "0x1234";
+    }
+
+    function mockFunctionRevertsNoReason() public payable {
+        revert();
+    }
+
+    function mockFunctionRevertsReason() public payable {
+        revert("CallReceiverMock: reverting");
+    }
+
+    function mockFunctionThrows() public payable {
+        assert(false);
+    }
+
+    function mockFunctionOutOfGas() public payable {
+        for (uint256 i = 0; ; ++i) {
+            _array.push(i);
+        }
+    }
+}

contracts/mocks/CappedCrowdsaleImpl.sol

@@ -1,21 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../token/ERC20/ERC20.sol";
-import "../crowdsale/validation/CappedCrowdsale.sol";
-
-
-contract CappedCrowdsaleImpl is CappedCrowdsale {
-
-  constructor (
-    uint256 _rate,
-    address _wallet,
-    ERC20 _token,
-    uint256 _cap
-  )
-    public
-    Crowdsale(_rate, _wallet, _token)
-    CappedCrowdsale(_cap)
-  {
-  }
-
-}

contracts/mocks/ClashingImplementation.sol

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+
+/**
+ * @dev Implementation contract with an admin() function made to clash with
+ * @dev TransparentUpgradeableProxy's to test correct functioning of the
+ * @dev Transparent Proxy feature.
+ */
+contract ClashingImplementation {
+
+  function admin() external pure returns (address) {
+    return 0x0000000000000000000000000000000011111142;
+  }
+
+  function delegatedFunction() external pure returns (bool) {
+    return true;
+  }
+}

contracts/mocks/ConditionalEscrowMock.sol

@@ -1,18 +1,18 @@
-pragma solidity ^0.4.24;
+// SPDX-License-Identifier: MIT
 
+pragma solidity ^0.6.0;
 
-import "../payment/ConditionalEscrow.sol";
-
+import "../payment/escrow/ConditionalEscrow.sol";
 
 // mock class using ConditionalEscrow
 contract ConditionalEscrowMock is ConditionalEscrow {
-  mapping(address => bool) public allowed;
+    mapping(address => bool) private _allowed;
 
-  function setAllowed(address _payee, bool _allowed) public {
-    allowed[_payee] = _allowed;
-  }
+    function setAllowed(address payee, bool allowed) public {
+        _allowed[payee] = allowed;
+    }
 
-  function withdrawalAllowed(address _payee) public view returns (bool) {
-    return allowed[_payee];
-  }
+    function withdrawalAllowed(address payee) public view override returns (bool) {
+        return _allowed[payee];
+    }
 }

contracts/mocks/ContextMock.sol

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "../GSN/Context.sol";
+
+contract ContextMock is Context {
+    event Sender(address sender);
+
+    function msgSender() public {
+        emit Sender(_msgSender());
+    }
+
+    event Data(bytes data, uint256 integerValue, string stringValue);
+
+    function msgData(uint256 integerValue, string memory stringValue) public {
+        emit Data(_msgData(), integerValue, stringValue);
+    }
+}
+
+contract ContextMockCaller {
+    function callSender(ContextMock context) public {
+        context.msgSender();
+    }
+
+    function callData(ContextMock context, uint256 integerValue, string memory stringValue) public {
+        context.msgData(integerValue, stringValue);
+    }
+}

contracts/mocks/CountersImpl.sol

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "../utils/Counters.sol";
+
+contract CountersImpl {
+    using Counters for Counters.Counter;
+
+    Counters.Counter private _counter;
+
+    function current() public view returns (uint256) {
+        return _counter.current();
+    }
+
+    function increment() public {
+        _counter.increment();
+    }
+
+    function decrement() public {
+        _counter.decrement();
+    }
+}

contracts/mocks/Create2Impl.sol

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+import "../utils/Create2.sol";
+import "../introspection/ERC1820Implementer.sol";
+
+contract Create2Impl {
+    function deploy(uint256 value, bytes32 salt, bytes memory code) public {
+        Create2.deploy(value, salt, code);
+    }
+
+    function deployERC1820Implementer(uint256 value, bytes32 salt) public {
+        // solhint-disable-next-line indent
+        Create2.deploy(value, salt, type(ERC1820Implementer).creationCode);
+    }
+
+    function computeAddress(bytes32 salt, bytes32 codeHash) public view returns (address) {
+        return Create2.computeAddress(salt, codeHash);
+    }
+
+    function computeAddressWithDeployer(bytes32 salt, bytes32 codeHash, address deployer) public pure returns (address) {
+        return Create2.computeAddress(salt, codeHash, deployer);
+    }
+
+    receive() payable external {}
+}

contracts/mocks/DestructibleMock.sol

@@ -1,8 +0,0 @@
-pragma solidity ^0.4.24;
-
-import "../lifecycle/Destructible.sol";
-
-
-contract DestructibleMock is Destructible {
-  function() public payable {}
-}