Add ERC20 Permit (EIP-2612)

contracts/cryptography/ECDSA.sol

@@ -43,6 +43,14 @@ library ECDSA {
             v := byte(0, mload(add(signature, 0x60)))
         }
 
+        return recover(hash, v, r, s);
+    }
+
+    /**
+     * @dev Overload of {ECDSA-recover-bytes32-bytes-} that receives the `v`,
+     * `r` and `s` signature fields separately.
+     */
+    function recover(bytes32 hash, uint8 v, bytes32 r, bytes32 s) internal pure returns (address) {
         // 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

contracts/drafts/ERC20Permit.sol

@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.5 <0.8.0;
+
+import "../token/ERC20/ERC20.sol";
+import "./IERC20Permit.sol";
+import "../cryptography/ECDSA.sol";
+import "../utils/Counters.sol";
+import "./EIP712.sol";
+
+/**
+ * @dev Implementation of the ERC20 Permit extension allowing approvals to be made via signatures, as defined in
+ * https://eips.ethereum.org/EIPS/eip-2612[EIP-2612].
+ *
+ * Adds the {permit} method, which can be used to change an account's ERC20 allowance (see {IERC20-allowance}) by
+ * presenting a message signed by the account. By not relying on `{IERC20-approve}`, the token holder account doesn't
+ * need to send a transaction, and thus is not required to hold Ether at all.
+ */
+abstract contract ERC20Permit is ERC20, IERC20Permit, EIP712 {
+    using Counters for Counters.Counter;
+
+    mapping (address => Counters.Counter) private _nonces;
+
+    // solhint-disable-next-line var-name-mixedcase
+    bytes32 private immutable _PERMIT_TYPEHASH = keccak256("Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)");
+
+    /**
+     * @dev Initializes the {EIP712} domain separator using the `name` parameter, and setting `version` to `"1"`.
+     *
+     * It's a good idea to use the same `name` that is defined as the ERC20 token name.
+     */
+    constructor(string memory name) internal EIP712(name, "1") {
+    }
+
+    /**
+     * @dev See {IERC20Permit-permit}.
+     */
+    function permit(address owner, address spender, uint256 amount, uint256 deadline, uint8 v, bytes32 r, bytes32 s) public virtual override {
+        // solhint-disable-next-line not-rely-on-time
+        require(block.timestamp <= deadline, "ERC20Permit: expired deadline");
+
+        bytes32 structHash = keccak256(
+            abi.encode(
+                _PERMIT_TYPEHASH,
+                owner,
+                spender,
+                amount,
+                _nonces[owner].current(),
+                deadline
+            )
+        );
+
+        bytes32 hash = _hashTypedDataV4(structHash);
+
+        address signer = ECDSA.recover(hash, v, r, s);
+        require(signer == owner, "ERC20Permit: invalid signature");
+
+        _nonces[owner].increment();
+        _approve(owner, spender, amount);
+    }
+
+    /**
+     * @dev See {IERC20Permit-nonces}.
+     */
+    function nonces(address owner) public view override returns (uint256) {
+        return _nonces[owner].current();
+    }
+
+    /**
+     * @dev See {IERC20Permit-DOMAIN_SEPARATOR}.
+     */
+    // solhint-disable-next-line func-name-mixedcase
+    function DOMAIN_SEPARATOR() external view override returns (bytes32) {
+        return _domainSeparatorV4();
+    }
+}

contracts/drafts/IERC20Permit.sol

@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+/**
+ * @dev Interface of the ERC20 Permit extension allowing approvals to be made via signatures, as defined in
+ * https://eips.ethereum.org/EIPS/eip-2612[EIP-2612].
+ *
+ * Adds the {permit} method, which can be used to change an account's ERC20 allowance (see {IERC20-allowance}) by
+ * presenting a message signed by the account. By not relying on `{IERC20-approve}`, the token holder account doesn't
+ * need to send a transaction, and thus is not required to hold Ether at all.
+ */
+interface IERC20Permit {
+    /**
+     * @dev Sets `amount` as the allowance of `spender` over `owner`'s tokens,
+     * given `owner`'s signed approval.
+     *
+     * IMPORTANT: The same issues {IERC20-approve} has related to transaction
+     * ordering also apply here.
+     *
+     * Emits an {Approval} event.
+     *
+     * Requirements:
+     *
+     * - `spender` cannot be the zero address.
+     * - `deadline` must be a timestamp in the future.
+     * - `v`, `r` and `s` must be a valid `secp256k1` signature from `owner`
+     * over the EIP712-formatted function arguments.
+     * - the signature must use ``owner``'s current nonce (see {nonces}).
+     *
+     * For more information on the signature format, see the
+     * https://eips.ethereum.org/EIPS/eip-2612#specification[relevant EIP
+     * section].
+     */
+    function permit(address owner, address spender, uint256 amount, uint256 deadline, uint8 v, bytes32 r, bytes32 s) external;
+
+    /**
+     * @dev Returns the current nonce for `owner`. This value must be
+     * included whenever a signature is generated for {permit}.
+     *
+     * Every successful call to {permit} increases ``owner``'s nonce by one. This
+     * prevents a signature from being used multiple times.
+     */
+    function nonces(address owner) external view returns (uint256);
+
+    /**
+     * @dev Returns the domain separator used in the encoding of the signature for `permit`, as defined by {EIP712}.
+     */
+    // solhint-disable-next-line func-name-mixedcase
+    function DOMAIN_SEPARATOR() external view returns (bytes32);
+}

contracts/drafts/README.adoc

@@ -1,4 +1,4 @@
-= Draft EIPS
+= Draft EIPs
 
 This directory contains implementations of EIPs that are still in Draft status.
 
@@ -7,3 +7,9 @@ Due to their nature as drafts, the details of these contracts may change and we
 == Cryptography
 
 {{EIP712}}
+
+== ERC 20
+
+{{IERC20Permit}}
+
+{{ERC20Permit}}

contracts/mocks/ERC20PermitMock.sol

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+import "../drafts/ERC20Permit.sol";
+
+contract ERC20PermitMock is ERC20Permit {
+    constructor (
+        string memory name,
+        string memory symbol,
+        address initialAccount,
+        uint256 initialBalance
+    ) public payable ERC20(name, symbol) ERC20Permit(name) {
+        _mint(initialAccount, initialBalance);
+    }
+
+    function getChainId() external pure returns (uint256 chainId) {
+        // solhint-disable-next-line no-inline-assembly
+        assembly {
+            chainId := chainid()
+        }
+    }
+}

contracts/token/ERC20/README.adoc

@@ -15,6 +15,7 @@ There a few core contracts that implement the behavior specified in the EIP:
 Additionally there are multiple custom extensions, including:
 
 * designation of addresses that can pause token transfers for all users ({ERC20Pausable}).
+* gasless approval of tokens ({ERC20Permit}).
 * efficient storage of past token balances to be later queried at any point in time ({ERC20Snapshot}).
 * destruction of own tokens ({ERC20Burnable}).
 * enforcement of a cap to the total supply when minting tokens ({ERC20Capped}).
@@ -24,6 +25,11 @@ Finally, there are some utilities to interact with ERC20 contracts in various wa
 * {SafeERC20} is a wrapper around the interface that eliminates the need to handle boolean return values.
 * {TokenTimelock} can hold tokens for a beneficiary until a specified time.
 
+The following related EIPs are in draft status and can be found in the drafts directory.
+
+- {IERC20Permit}
+- {ERC20Permit}
+
 NOTE: This core set of contracts is designed to be unopinionated, allowing developers to access the internal functions in ERC20 (such as <<ERC20-_mint-address-uint256-,`_mint`>>) and expose them as external functions in the way they prefer. On the other hand, xref:ROOT:erc20.adoc#Presets[ERC20 Presets] (such as {ERC20PresetMinterPauser}) are designed using opinionated patterns to provide developers with ready to use, deployable contracts.
 
 == Core