Use hardhat-exposed to reduce the need for mocks (#3666)

Co-authored-by: Francisco <fg@frang.io>

test/helpers/eip712.js

@@ -1,4 +1,5 @@
 const ethSigUtil = require('eth-sig-util');
+const keccak256 = require('keccak256');
 
 const EIP712Domain = [
   { name: 'name', type: 'string' },
@@ -15,16 +16,35 @@ const Permit = [
   { name: 'deadline', type: 'uint256' },
 ];
 
-async function domainSeparator (name, version, chainId, verifyingContract) {
-  return '0x' + ethSigUtil.TypedDataUtils.hashStruct(
-    'EIP712Domain',
-    { name, version, chainId, verifyingContract },
-    { EIP712Domain },
-  ).toString('hex');
+function bufferToHexString (buffer) {
+  return '0x' + buffer.toString('hex');
+}
+
+function hexStringToBuffer (hexstr) {
+  return Buffer.from(hexstr.replace(/^0x/, ''), 'hex');
+}
+
+async function domainSeparator ({ name, version, chainId, verifyingContract }) {
+  return bufferToHexString(
+    ethSigUtil.TypedDataUtils.hashStruct(
+      'EIP712Domain',
+      { name, version, chainId, verifyingContract },
+      { EIP712Domain },
+    ),
+  );
+}
+
+async function hashTypedData (domain, structHash) {
+  return domainSeparator(domain).then(separator => bufferToHexString(keccak256(Buffer.concat([
+    '0x1901',
+    separator,
+    structHash,
+  ].map(str => hexStringToBuffer(str))))));
 }
 
 module.exports = {
   EIP712Domain,
   Permit,
   domainSeparator,
+  hashTypedData,
 };