Lesson 11

EIP-2612 Permit: Gasless Approvals

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5 sections

approve()

text

The classic approve() + transferFrom() flow needs TWO transactions: first you approve, then the contract pulls your tokens. That means two gas fees, and — critically — you must already hold ETH to pay for the approve. EIP-2612 fixes this with permit(): you sign an allowance off-chain (free, no gas), and anyone can submit that signature on-chain, often bundled into the very same transaction as the action.

code

solidity

1// EIP-2612 adds permit() to an ERC-20:
2function permit(
3    address owner,
4    address spender,
5    uint256 value,
6    uint256 deadline,
7    uint8 v, bytes32 r, bytes32 s
8) external;
9
10// The owner signs this EIP-712 typed struct off-chain:
11// Permit(address owner,address spender,uint256 value,
12//        uint256 nonce,uint256 deadline)
13// nonces[owner] increments on each use -> no replay.

text

Here is the flow: the owner signs the EIP-712 typed data (owner, spender, value, the current nonce, and a deadline). The token's permit() recovers the signer with ecrecover, checks that block.timestamp <= deadline and that the nonce matches, then sets the allowance exactly as approve() would — but the gas is paid by whoever submits the signature, not necessarily the owner.

code

javascript

1// Sign off-chain with viem, then a router does permit + action atomically
2const signature = await walletClient.signTypedData({
3  domain: { name, version: "1", chainId, verifyingContract: token },
4  types: { Permit: [
5    { name: "owner", type: "address" },
6    { name: "spender", type: "address" },
7    { name: "value", type: "uint256" },
8    { name: "nonce", type: "uint256" },
9    { name: "deadline", type: "uint256" },
10  ]},
11  primaryType: "Permit",
12  message: { owner, spender, value, nonce, deadline },
13});
14// split signature -> v, r, s and pass to token.permit(...)

note

Key Insight

Security: a permit signature is a bearer authorization. If a phishing site tricks you into signing one, the attacker can submit it to grant themselves an allowance — just as dangerous as a malicious approve(). The deadline bounds how long it is valid, and the per-owner nonce makes each signature single-use, but always check what you are signing.

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