Digital Signatures: Principles and Applications in Smart Contract Whitelisting

Introduction to Digital Signatures

What is a Digital Signature?

Digital signatures are a cornerstone of blockchain technology, enabling address ownership verification without exposing private keys. Primarily used for signing transactions (or any arbitrary message), this guide focuses on their implementation within the Ethereum protocol.

Ethereum employs Elliptic Curve Digital Signature Algorithm (ECDSA)—a cryptographic method based on public-private key pairs that ensures:

• Authentication: Proof of private key ownership.
• Non-repudiation: Senders cannot deny transaction origination.
• Integrity: Protection against message tampering during transmission.

Understanding ECDSA Signatures

ECDSA (Elliptic Curve Digital Signature Algorithm) underpins the trust infrastructure of Bitcoin and Ethereum. It defines the process for signing/verifying messages and transactions.

Key Advantages:

1. Public keys cannot reverse-engineer private keys.
2. Private key ownership can be proven without revealing the key itself.

For smart contracts, understanding the workflow—rather than algorithmic intricacies—suffices for code implementation. Detailed ECDSA tutorials are available online.

Ethereum’s Signature Workflow

Signature Process

1. ECDSA Forward Algorithm:
   Message + Private Key + Random Number → Signature (r, s, v)

   • Inputs: Public message + Private key → Outputs signature components (r, s, v).

2. Verification Process:
   Message + Signature → Public Key

   • Derived public key is matched against known public keys.

Note: Ethereum’s ECDSA modifies the original algorithm by adding v for enhanced security.

Ethereum Transaction Signing Steps

1. Construct Raw Transaction:

   • Fields: nonce, gasPrice, gasLimit, to, value, data, chainId.
   • RLP Encoding: Ensures consistent serialization for Keccak256 hashing.

2. Signing via MetaMask or Ethers:

   // MetaMask Example
   const provider = new ethers.providers.Web3Provider(window.ethereum);
   const signer = provider.getSigner();
   const messageHash = ethers.utils.solidityKeccak256(["string"], ["HelloWorld"]);
   const signature = await signer.signMessage(ethers.utils.arrayify(messageHash));

3. Hardhat Implementation:

   // Signature Verification Contract
   function verify(address _signer, string memory _message, bytes32 r, bytes32 s, uint8 v) external pure returns (bool) {
       bytes32 messageDigest = keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", keccak256(abi.encodePacked(_message))));
       return ecrecover(messageDigest, v, r, s) == _signer;
   }

Practical Applications: NFT Whitelisting

Whitelist via Digital Signatures

1. Offline: Sign whitelisted addresses using a private key.
2. On-Chain: Store the signer’s public key in the contract for validation.

Contract Example:

contract Whitelist {
    address private SIGNER;
    constructor(address _signer) { SIGNER = _signer; }
    
    function verify(address user, uint8 _maxMint, bytes memory _signature) public view returns (bool) {
        bytes32 messageDigest = keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", keccak256(abi.encodePacked(user, _maxMint))));
        return recoverSigner(messageDigest, _signature) == SIGNER;
    }
}

Testing:

const [signer, addr1] = await ethers.getSigners();
const messageHash = ethers.utils.solidityKeccak256(["address", "uint8"], [addr1.address, 2]);
const signature = await signer.signMessage(ethers.utils.arrayify(messageHash));
const verified = await whitelistContract.verify(addr1.address, 2, signature); // Returns true

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FAQ

Q1: Why use ECDSA over Merkle Trees for whitelisting?
A1: ECDSA reduces gas costs and simplifies verification by eliminating on-chain storage of whitelist hashes.

Q2: How secure is MetaMask for signing transactions?
A2: MetaMask uses industry-standard encryption, ensuring secure private key management and transaction signing.

Q3: Can I verify signatures without deploying a contract?
A3: Yes, use tools like Etherscan’s Signature Verifier.

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