IERC20Permit

*Interface of the ERC-20 Permit extension allowing approvals to be made via signatures, as defined in https://eips.ethereum.org/EIPS/eip-2612[ERC-2612]. Adds the permit method, which can be used to change an account's ERC-20 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. ==== Security Considerations There are two important considerations concerning the use of permit. The first is that a valid permit signature expresses an allowance, and it should not be assumed to convey additional meaning. In particular, it should not be considered as an intention to spend the allowance in any specific way. The second is that because permits have built-in replay protection and can be submitted by anyone, they can be frontrun. A protocol that uses permits should take this into consideration and allow a permit call to fail. Combining these two aspects, a pattern that may be generally recommended is:

function doThingWithPermit(..., uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s) public {
try token.permit(msg.sender, address(this), value, deadline, v, r, s) {} catch {}
doThing(..., value);
}
function doThing(..., uint256 value) public {
token.safeTransferFrom(msg.sender, address(this), value);
...
}

Observe that: 1) msg.sender is used as the owner, leaving no ambiguity as to the signer intent, and 2) the use of try/catch allows the permit to fail and makes the code tolerant to frontrunning. (See also {SafeERC20-safeTransferFrom}). Additionally, note that smart contract wallets (such as Argent or Safe) are not able to produce permit signatures, so contracts should have entry points that don't rely on permit.*

Functions

permit

*Sets value 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]. CAUTION: See Security Considerations above.*

function permit(address owner, address spender, uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s)
    external;

nonces

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);

DOMAIN_SEPARATOR

Returns the domain separator used in the encoding of the signature for permit, as defined by {EIP712}.

function DOMAIN_SEPARATOR() external view returns (bytes32);