How to Structure Secure Identity Signatures

A secure identity signature is a cryptographic assertion that binds a user's identity to a specific action or claim, such as "I am Alice, and I authorize this transaction." The core structure relies on three components: a signing key (a private key), a verification key (a public key or address), and a payload (the data being signed). The signature itself is generated by applying a signing algorithm (like ECDSA with secp256k1 for Ethereum) to a hash of the structured payload. This creates a compact proof that can be verified by anyone with the public key, ensuring the data's integrity and origin.

The payload structure is critical for security and interoperability. A well-designed signature payload should include a domain separator, a nonce or timestamp to prevent replay attacks, and the core message or claim. The EIP-712 standard for typed structured data is the gold standard for Ethereum, as it provides a human-readable schema for the data being signed. This prevents ambiguity where the same raw data could be interpreted differently by different contracts. Structuring your payload with EIP-712 involves defining a domain (with chain ID, verifying contract, etc.) and a message type with named fields.

Here is a simplified example of an EIP-712 typed data structure for an identity attestation:

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const types = {
  Person: [
    { name: 'name', type: 'string' },
    { name: 'wallet', type: 'address' }
  ]
};
const message = {
  name: 'Alice',
  wallet: '0x...'
};

The user signs the hash of this structured data. When verifying, the contract reconstructs the hash using the same type definitions and checks the signature against the signer's address. This ensures the signer explicitly approved this specific, unambiguous claim.

Beyond basic structure, consider these security practices: always include a chain ID in your domain separator to prevent cross-chain replay attacks; use deadlines or nonces for time-bound or single-use authorizations; and never sign raw transactions or arbitrary hashes directly, as this can lead to signature malleability and phishing. For decentralized identity (DID) systems, standards like Verifiable Credentials (W3C VC) build upon these cryptographic signatures, packaging the signature, payload, and issuer's DID into a JSON-LD or JWT format for broader interoperability across systems.

Implementing these patterns correctly is non-negotiable for security. Test your signature flows extensively, using libraries like ethers.js's _signTypedData or viem's signTypedData. Audit the verification logic in your smart contracts to ensure it matches the off-chain signing process exactly. A properly structured identity signature is the foundation for secure login systems (like Sign-In with Ethereum), permissioned actions in DAOs, and verifiable off-chain attestations that can be trustlessly brought on-chain.

Secure identity signatures are the cornerstone of user authentication and authorization in decentralized systems. Unlike traditional web2 sessions, these signatures are cryptographically verifiable proofs that a user controls a specific blockchain address. The core prerequisite is understanding the Elliptic Curve Digital Signature Algorithm (ECDSA), specifically the secp256k1 curve used by Ethereum and many other chains. This algorithm allows a user to sign a message with their private key, producing a signature that anyone can verify using the corresponding public address, without revealing the private key itself. This forms the basis for signing login requests, transaction approvals, and verifiable credentials.

Before writing any code, you must set up a development environment capable of generating and managing cryptographic keys. For Ethereum-based development, the essential tools include Node.js (v18+), a package manager like npm or yarn, and the ethers.js v6 or viem libraries. These libraries provide secure, audited abstractions for key management and signing. Crucially, you should never hardcode private keys in your source code or commit them to version control. Instead, use environment variables with a .env file (using dotenv) or a dedicated secret management service. For testing, you can use the private keys from a local Hardhat or Anvil node.

The structure of the message you sign is as critical as the signature itself. A raw signature of arbitrary text is vulnerable to replay attacks and phishing. To prevent this, you must sign a structured, domain-bound message. The EIP-712 standard is the definitive specification for typed structured data hashing and signing. It allows you to define a domain (name, version, chainId, verifyingContract) and specific types for your message (e.g., LoginRequest with a walletAddress and nonce). Signing an EIP-712 payload produces a signature that is only valid for that specific context, preventing misuse across different applications or chains. Most modern wallets have built-in support for displaying EIP-712 requests clearly to users.

Here is a basic setup example using ethers.js to sign an EIP-712 structured message for a login request. First, define your domain separator and types:

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const domain = {
  name: 'MyDApp',
  version: '1',
  chainId: 1, // Mainnet
  verifyingContract: '0xCcCCccccCCCCcCCCCCCcCcCccCcCCCcCcccccccC'
};
const types = {
  LoginRequest: [
    { name: 'walletAddress', type: 'address' },
    { name: 'nonce', type: 'uint256' }
  ]
};
const value = {
  walletAddress: '0xAbC...',
  nonce: 12345 // Unique session nonce
};

Then, use a wallet signer to generate the signature: const signature = await signer._signTypedData(domain, types, value);.

Finally, you must implement secure verification on the backend. The verifying party (your API server or a smart contract) must reconstruct the signed message digest using the same domain, types, and value parameters, and then recover the signer's address from the signature using the ecrecover function (or its library equivalent). Always verify that the recovered address matches the expected user and that the nonce is fresh to prevent replay attacks. For smart contract verification, you can use OpenZeppelin's ECDSA library. This end-to-end flow—structured message generation, secure client-side signing, and robust server-side verification—forms the secure foundation for Web3 identity.

A raw cryptographic signature, like an ECDSA signature on a hash, proves a signer approved a piece of data. However, for identity and authentication, signing a raw hash is insecure and opaque. A user signing 0x1234... has no context for what they're approving. The solution is structured data signing, which presents human-readable information to the signer before generating a secure cryptographic proof. This bridges the gap between user intent and on-chain verification.

The Ethereum community standardizes this with EIP-712: Typed Structured Data Hashing and Signing. Instead of a hash, you sign a structured JSON object with defined types. A EIP712Domain defines the contract's context (name, version, chainId, verifyingContract), preventing replay attacks across chains or contracts. The message itself is defined in a schema, like { name: string, timestamp: uint256 }. The wallet displays this readable data, and the resulting signature is bound to this specific, unambiguous structure.

For web3 logins, Sign-In with Ethereum (SIWE) builds on EIP-712. A SIWE message is a specific EIP-712 schema containing the domain, user's address, statement, URI, version, chain ID, nonce, issued-at timestamp, and expiration. This structure allows a service to request a signature that acts as a secure, self-custodied login credential. The user sees a clear statement like "I accept the Service Terms of Service," and the backend can verify the signature's validity and that the chainId and verifyingContract match expectations.

Implementing this requires careful setup. In your smart contract, you must replicate the hashing logic defined in EIP-712 to recover the signer from the signature. Off-chain, libraries like @metamask/eth-sig-util or ethers.js's Signer._signTypedData handle the formatting. The critical step is ensuring the EIP712Domain separator is identical across all signatures for a given contract. A mismatch here will cause signature verification to fail, as the computed hash will be different.

Structured signatures are essential for delegated authority (gasless transactions via ERC-20 permits), secure voting, and attribute attestations. They provide non-repudiation—a user cannot claim they signed something else. By moving beyond raw hashes to typed data, developers build safer, more transparent, and user-friendly applications that leverage cryptography for verifiable identity without sacrificing security or usability.

Traditional signature methods like personal_sign present signers with an opaque, hexadecimal string, making it impossible to verify the content before approval. This creates significant security risks, especially for complex transactions or identity attestations. EIP-712 solves this by defining a schema for typed structured data. Instead of signing a hash, users sign a JSON-like structure that their wallet can display in a clear, readable format. This allows users to see exactly what they are attesting to—such as a specific statement, timestamp, and verifier address—before confirming the signature.

The core of an EIP-712 signature is the type definition and the domain separator. You must define a EIP712Domain and your custom message types. The domain includes critical context like the contract's name, version, chainId, and verifyingContract address, which prevents signatures from being replayed on different chains or against different contracts. For an identity signature, your message type might include fields like identityStatement, timestamp, and verifier. This structured approach cryptographically binds the signature to this specific context and data schema.

Here is a basic JavaScript example using ethers.js to create a signer and prepare the typed data object for an identity attestation:

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const domain = {
  name: 'IdentityVerifier',
  version: '1',
  chainId: 1, // Mainnet
  verifyingContract: '0xCcCCccccCCCCcCCCCCCcCcCccCcCCCcCcccccccC'
};
const types = {
  IdentityAttestation: [
    { name: 'statement', type: 'string' },
    { name: 'timestamp', type: 'uint256' },
    { name: 'verifier', type: 'address' }
  ]
};
const value = {
  statement: 'I attest that I control this wallet.',
  timestamp: Math.floor(Date.now() / 1000),
  verifier: '0xAbc...'
};

The signer._signTypedData(domain, types, value) method then prompts the user with this readable data.

On the verification side, typically handled by a smart contract, you must reconstruct the same domain separator and type hash. The contract uses the ecrecover function to derive the signer's address from the signature and the hashed data. It's critical that the contract's verification logic uses identical type definitions and domain parameters. A mismatch in a single field, like chainId or the order of struct members, will cause the recovery to fail. This strict verification is what makes EIP-712 signatures cryptographically bound to their specific context, providing strong non-repudiation.

For production systems, pay close attention to the domain separator. Changing the name, version, or verifyingContract invalidates all previous signatures. Consider storing the hashed domain separator in your contract to save gas. Also, always include a chainId to prevent cross-chain replay attacks. When designing your message types, be specific—adding a nonce or deadline (validUntil) field can prevent signature reuse. The official EIP-712 specification is the definitive resource for understanding the hashing and encoding details.

Implementing EIP-712 correctly elevates your application's security and user experience. It transforms signatures from a blind trust mechanism into a transparent, auditable process. This is foundational for building reliable decentralized identity systems, secure login flows (like Sign-In with Ethereum), and complex permission schemas where users must understand the exact implications of their cryptographic consent.

EIP-1271 introduces a critical innovation: allowing smart contracts to validate signatures as if they were Externally Owned Accounts (EOAs). The standard specifies a single function, isValidSignature(bytes32 _hash, bytes memory _signature), which must return the magic value 0x1626ba7e for a valid signature. This enables contracts to sign messages, approve transactions, and interact with protocols that rely on ECDSA signatures, such as OpenSea for NFT listings or Gelato for meta-transactions. The contract itself defines the logic for what constitutes a valid signature, which can be based on multi-signature schemes, DAO votes, or custom authorization rules.

Implementing the interface correctly is paramount for security. Your contract's isValidSignature function must perform the verification logic and revert on failure; returning 0xffffffff or any other incorrect value is not sufficient. A common pattern is to store a mapping of authorized signers, often the contract owners or a governance module. When called, the function recovers the signer from the provided _signature and _hash using ecrecover, then checks it against the stored list. Always ensure the _hash is the exact Ethereum Signed Message hash (e.g., keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", messageHash))) that the user signed, to prevent signature malleability and replay attacks across different contexts.

For developers, integrating EIP-1271 verification into a dapp involves two steps. First, when you receive a signature, check if the signer is a contract by examining its bytecode length. If it is, call isValidSignature on that contract address. A robust off-chain verification function in JavaScript (using ethers.js or viem) might look like this:

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async function isValid1271Signature(address, hash, signature) {
  const code = await provider.getCode(address);
  if (code === '0x') {
    // EOA: use standard ecrecover
    return recoverSigner(hash, signature) === address;
  } else {
    // Contract: use EIP-1271
    const contract = new ethers.Contract(address, ['function isValidSignature(bytes32, bytes) view returns (bytes4)'], provider);
    try {
      const returnValue = await contract.isValidSignature(hash, signature);
      return returnValue === '0x1626ba7e';
    } catch {
      return false;
    }
  }
}

Key security considerations extend beyond basic implementation. Always verify the signer, not just the signature's validity, to ensure the right entity is authorizing the action. Be aware of the chain ID replay protection; include it in the signed message hash to prevent signatures from being valid on different networks. For contracts that hold significant value, consider implementing signature expiry timestamps to reduce the window for replay attacks. Furthermore, audit your signature verification logic thoroughly, as flaws here can lead to unauthorized asset transfers or permissions. The OpenZeppelin library provides a useful base contract, ERC1271Wallet, that demonstrates a secure, simple implementation for reference.

Delegation, or meta-transactions, allow a user to sign a message authorizing a third party (a relayer) to execute a transaction on their behalf. This pattern is fundamental for gasless transactions, where the relayer pays the gas fee. The core security mechanism is a signature verification function like EIP-712, which creates a structured, human-readable hash of the transaction data. The user signs this hash offline, and the smart contract recovers the signer's address using ecrecover or a library like OpenZeppelin's ECDSA. The contract must then check that the recovered address has the appropriate permissions.

A critical vulnerability in delegation is signature replay. A signed message could be reused on a different chain (cross-chain replay) or after a state change (replay on the same chain). To prevent this, signatures must include a nonce (a number that increments per user) and the chain ID. The EIP-712 standard elegantly encodes these values into the signed digest. Furthermore, contracts should implement a mapping to track used nonces and reject any signature with a nonce that has already been executed.

Batch transactions aggregate multiple operations into a single call, significantly reducing gas costs and improving UX. A user signs a single message containing an array of actions (e.g., [approve, swap, deposit]). The contract logic must iterate through the actions and execute them sequentially within the same transaction. Security here depends on validating that the entire batch is authorized by the signature and that the execution is atomic—if any action fails, the entire batch should revert to prevent partial state changes.

For maximum security, combine these patterns with deadlines. Include a validUntil timestamp in the signed data so the authorization expires. This prevents old, potentially compromised signatures from being executed later. Always use established libraries like OpenZeppelin's for signature recovery and nonce management, and thoroughly audit the logic that maps recovered signers to permissions. A flawed check can allow any address to spoof a delegation.

In practice, these patterns are used by major protocols. Uniswap's Permit2 uses signed approvals for token spending. Gas stations like Biconomy and Gelato Network act as relayers for meta-transactions. When implementing, test extensively: simulate signature replay across forks, check nonce enforcement, and verify the contract correctly validates the chain ID. The balance between user convenience and security is maintained by rigorous signature structuring and validation.

Implementing secure identity signatures requires a defense-in-depth approach. Your system should combine deterministic key derivation from a master secret, context-specific signing using standards like EIP-712 for structured data, and robust off-chain verification before submitting transactions. Always use audited libraries such as ethers.js or viem for cryptographic operations, and never roll your own crypto for production systems. The primary goal is to ensure the signature unequivocally proves user intent for a specific action.

For next steps, integrate these patterns into your application. Start by implementing a secure sign-in flow using Sign-In with Ethereum (SIWE), which provides a standardized message format for authentication. Then, design your protocol-specific actions—like voting, approving transactions, or updating profile data—using typed data signatures. Tools like the EIP-712 Developer Guide and the SIWE documentation are essential references. Test thoroughly on a testnet like Sepolia or Goerli before mainnet deployment.

To deepen your understanding, explore advanced topics. Study account abstraction (ERC-4337), which allows for signature schemes beyond ECDSA, such as multi-sig or quantum-resistant signatures. Investigate zero-knowledge proofs for privacy-preserving attestations, where a user can prove credential ownership without revealing the underlying data. Finally, regularly audit your signature verification logic and stay updated on emerging standards from groups like the Decentralized Identity Foundation to ensure long-term security and interoperability.