ERC20Permit

The EIP-2612 standard, commonly referred to as ERC20Permit, is designed to support gasless token approvals. This is achieved with an off-chain signature following the SNIP12 standard, rather than with an on-chain transaction. The permit function verifies the signature and sets the spender’s allowance if the signature is valid. This approach improves user experience and reduces gas costs.

Differences from Solidity

Although this extension is mostly similar to the Solidity implementation of EIP-2612, there are some notable differences in the parameters of the permit function:

The deadline parameter is represented by u64 rather than u256.
The signature parameter is represented by a span of felts rather than v, r, and s values.

Unlike Solidity, there is no enforced format for signatures on Starknet. A signature is represented by an array or span of felts, and there is no universal method for validating signatures of unknown formats. Consequently, a signature provided to the permit function is validated through an external is_valid_signature call to the contract at the owner address.

Usage

The functionality is provided as an embeddable ERC20Permit trait of the ERC20Component.

#[abi(embed_v0)]
impl ERC20PermitImpl = ERC20Component::ERC20PermitImpl<ContractState>;

A contract must meet the following requirements to be able to use the ERC20Permit trait:

Implement ERC20Component.
Implement NoncesComponent.
Implement SNIP12Metadata trait (used in signature generation).

Typed message

To safeguard against replay attacks and ensure the uniqueness of each approval via permit, the data signed includes:

The address of the owner.
The parameters specified in the permit function (spender and amount)
The address of the token contract itself.
A nonce, which must be unique for each operation.
The chain_id, which protects against cross-chain replay attacks.

The format of the Permit structure in a signed permit message is as follows:

struct Permit {
    token: ContractAddress,
    spender: ContractAddress,
    amount: u256,
    nonce: felt252,
    deadline: u64,
}

The owner of the tokens is also part of the signed message. It is used as the signer parameter in the get_message_hash call.

Further details on preparing and signing a typed message can be found in the SNIP12 guide.