signature-replay-analysis

# Signature & Replay Analysis

Detect vulnerabilities where **cryptographic signatures can be reused**, replayed across chains/contracts, or exploited through implementation flaws. Research shows 19.63% of Ethereum contracts using signatures contain replay vulnerabilities.

## When to Use

- Auditing contracts that verify signatures (`ecrecover`, ECDSA, EIP-712)
- Reviewing ERC-20 `permit()` / Uniswap Permit2 implementations
- Analyzing meta-transaction / gasless relay systems
- Verifying multi-sig signature aggregation
- Checking off-chain order books or signed message execution

## When NOT to Use

- Contracts without any signature verification
- Pure on-chain access control (use semantic-guard-analysis)
- Token standard compliance (use external-call-safety)

## Core Concept: The Signature Trust Model

A signature proves that a specific private key holder authorized a specific action. For this to be secure, the signature must be:

1. **Bound to context** — specific chain, contract, and version (domain separation)
2. **Used exactly once** — nonce prevents replay
3. **Time-limited** — deadline/expiry prevents late execution
4. **Correctly verified** — ecrecover edge cases handled

Any gap in this model creates a replay vulnerability.

## The Five Replay Types

### Type 1: Same-Chain Replay

The exact same signature is submitted multiple times to the same contract on the same chain.

```solidity
// VULNERABLE: No nonce — same signature works forever
function executeWithSig(address to, uint256 amount, bytes memory signature) external {
    bytes32 hash = keccak256(abi.encodePacked(to, amount));
    address signer = ECDSA.recover(hash, signature);
    require(signer == admin, "Invalid signer");
    token.transfer(to, amount);
    // Attacker can submit this same signature again and again!
}

// SAFE: Use nonce
mapping(address => uint256) public nonces;

function executeWithSig(address to, uint256 amount, uint256 nonce, bytes memory signature) external {
    require(nonce == nonces[admin], "Invalid nonce");
    bytes32 hash = keccak256(abi.encodePacked(to, amount, nonce));
    address signer = ECDSA.recover(hash, signature);
    require(signer == admin, "Invalid signer");
    nonces[admin]++;
    token.transfer(to, amount);
}
```

### Type 2: Cross-Chain Replay

A signature valid on one chain (e.g., Ethereum) is replayed on another chain (e.g., Polygon, Arbitrum) where the same contract is deployed.

```solidity
// VULNERABLE: No chainId in signed message
bytes32 hash = keccak256(abi.encodePacked(to, amount, nonce));
// This hash is identical on Ethereum, Polygon, Arbitrum, etc.

// SAFE: Include chainId (via EIP-712 domain separator)
bytes32 DOMAIN_SEPARATOR = keccak256(abi.encode(
    keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
    keccak256(bytes("MyContract")),
    keccak256(bytes("1")),
    block.chainid,
    address(this)
));
```

### Type 3: Cross-Contract Replay

A signature for Contract A is replayed on Contract B (same chain) if both accept the same message format without contract-specific binding.

```solidity
// VULNERABLE: No contract address in signed message
bytes32 hash = keccak256(abi.encodePacked(to, amount, nonce, block.chainid));
// Same hash for any contract on this chain

// SAFE: Include verifyingContract (via EIP-712)
// The domain separator includes address(this), binding to this specific contract
```

### Type 4: Nonce-Skip Replay

Nonce implementation allows gaps or out-of-order execution, enabling skipped nonces to be replayed later.

```solidity
// VULNERABLE: Bitmap nonce without invalidation
mapping(uint256 => bool) public usedNonces;

function execute(uint256 nonce, ...) external {
    require(!usedNonces[nonce], "Used");
    usedNonces[nonce] = true;
    // If nonces 1, 2, 3 are used but 4 is skipped,
    // nonce 4 can be used anytime in the future
    // This may be intentional OR a vulnerability depending on context
}

// SAFER for strict ordering: Sequential nonce
mapping(address => uint256) public nonces;

function execute(uint256 nonce, ...) external {
    require(nonce == nonces[signer], "Invalid nonce");
    nonces[signer]++;
}
```

### Type 5: Expired-Signature Replay

A signature without a deadline can be held and executed at an arbitrary future time when conditions have changed.

```solidity
// VULNERABLE: No deadline — signature valid forever
function permit(address owner, address spender, uint256 value, uint8 v, bytes32 r, bytes32 s) external {
    bytes32 hash = keccak256(abi.encodePacked(owner, spender, value, nonces[owner]++));
    require(ecrecover(hash, v, r, s) == owner, "Invalid");
    allowance[owner][spender] = value;
    // This permit can be executed weeks later when user doesn't expect it
}

// SAFE: Include deadline
function permit(address owner, address spender, uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s) external {
    require(block.timestamp <= deadline, "Expired");
    // ... rest of verification
}
```

## ecrecover Safety

### Edge Case 1: Returns address(0)

`ecrecover` returns `address(0)` for invalid signatures instead of reverting.

```solidity
// VULNERABLE: address(0) accepted as valid signer
address signer = ecrecover(hash, v, r, s);
require(signer == owner, "Invalid");
// If owner == address(0) AND signature is invalid → passes!

// SAFE: Explicit zero check
address signer = ecrecover(hash, v, r, s);
require(signer != address(0), "Invalid signature");
require(signer == owner, "Wrong signer");

// SAFEST: Use OpenZeppelin's ECDSA.recover() — reverts on address(0)
address signer = ECDSA.recover(hash, signature);
```

### Edge Case 2: Signature Malleability

For every valid ECDSA signature (r, s, v), there exists a second valid signature (r, s', v') for the same message. This allows anyone to create an alternate valid signature without the private key.

```solidity
// The Ethereum standard: s must be in the lower half of the curve
// s' = secp256k1n -