Skip to main content

MetaTxRelayer Contract

Enables gasless deposits and withdrawals via EIP-712 signed meta-transactions. Users pay fees in ERC20 tokens instead of AVAX.

Overview​

The MetaTxRelayer eliminates the need for users to hold AVAX. A relay wallet submits transactions on behalf of users, and fees are paid in the deposited/withdrawn token.

User (no AVAX)                    Relay Wallet (has AVAX)
──────────────                    ──────────────────────
Sign EIP-712 message  ──────────→  Submit tx to MetaTxRelayer
                                   Pay gas in AVAX
Token fee deducted    ◄──────────  Receive token fee as compensation

Functions​

relayDeposit​

struct DepositRequest {
    address depositor;
    address pool;
    uint256 amount;
    uint256 commitment;
    uint256 fee;
    uint256 deadline;
    uint256 nonce;
    bytes signature;
}

function relayDeposit(DepositRequest calldata req) external

The contract:

  1. Verifies the EIP-712 signature matches the depositor
  2. Checks nonce and deadline
  3. Pulls (amount + fee) * amountScale tokens from the depositor
  4. Deposits amount into the ShieldedPool
  5. Sends fee to the relay wallet (msg.sender)
  6. Increments the depositor's nonce

relayWithdraw​

struct WithdrawRequest {
    address withdrawer;
    address pool;
    bytes proof;
    uint256 merkleRoot;
    uint256 nullifierHash;
    uint256 amount;
    uint256 changeCommitment;
    address recipient;
    bytes encryptedMemo;
    uint256 fee;
    uint256 deadline;
    uint256 nonce;
    bytes signature;
}

function relayWithdraw(WithdrawRequest calldata req) external

The contract:

  1. Verifies the EIP-712 signature
  2. Calls pool.withdraw(proof, ..., recipient=address(this)) — receives tokens itself
  3. Sends (amount - fee) to the actual recipient
  4. Sends fee to the relay wallet
info

The withdraw circuit's public inputs are [merkleRoot, nullifierHash, amount, changeCommitment] — recipient is NOT a public signal, so the proof doesn't bind to a specific recipient. This allows the relayer contract to receive tokens and redistribute them.

EIP-712 Domain​

name: "ShroudMetaTxRelayer"
version: "1"
chainId: <network chain ID>
verifyingContract: <MetaTxRelayer address>

Fee structure​

  • Default relay fee: 0.1% of amount (minimum 1 token unit)
  • Fee is deducted from the user's token balance (deposits) or withdrawal amount (withdrawals)
  • Relay wallet receives the fee as compensation for gas

Nonce management​

Each user has a sequential nonce tracked on-chain:

mapping(address => uint256) public nonces;

The nonce prevents replay attacks. After each successful relay operation, the nonce increments.

Token approval​

For gasless deposits, users must first approve the MetaTxRelayer to spend their tokens. This is a one-time operation that requires a small amount of AVAX for gas. After approval (set to MaxUint256), all subsequent deposits are truly gasless.