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When you retrieve an attestation from Circle’s Attestation Service, you can optionally verify the attestation signature before using it to mint USDC on the destination blockchain. This page explains how the verification process works and when you might want to use it.

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How verification works

The verification process uses cryptographic signature recovery to confirm that Circle’s Attestation Service signed the message. It involves the following steps:
1

Retrieve the public key

Fetch Circle’s current public key from the GET /v2/publicKeys endpoint.
2

Hash the message

Create a keccak256 hash of the message bytes.
3

Parse the attestation

Split the 65-byte attestation into its r, s, and v components (ECDSA signature format).
4

Recover the signer

Use the signature and message hash to recover the public key that signed the message.
5

Compare addresses

Convert both the recovered public key and Circle’s public key to Ethereum addresses and compare them.
If the addresses match, the attestation was signed by Circle’s Attestation Service and is valid.

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When to verify attestations

Attestation verification is optional because the CCTP contracts on the destination blockchain perform their own verification when you call receiveMessage. However, you might want to verify attestations before submitting the mint transaction if:
  • Your application requires an additional layer of security: Verifying before minting provides defense-in-depth by catching invalid attestations at the application layer.
  • You want to detect invalid attestations before paying gas fees: If an attestation is invalid, the mint transaction fails and you lose the gas fees. Pre-verification lets you catch this before submitting the transaction.
  • You’re building a relayer service that batches multiple attestations: Relayers can verify each attestation in a batch before submitting, preventing a single invalid attestation from affecting the entire batch.

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Verification code example

The following examples show how to verify an attestation signature using Viem or Ethers: 
import { keccak256, hexToBytes, recoverAddress, bytesToHex } from "viem";

interface PublicKey {
  publicKey: `0x${string}`;
  cctpVersion: number;
}

interface AttestationData {
  message: string;
  attestation: string;
}

function publicKeyToAddress(publicKey: `0x${string}`): `0x${string}` {
  // Remove '0x04' prefix (uncompressed public key marker)
  const publicKeyWithoutPrefix = `0x${publicKey.slice(4)}` as `0x${string}`;
  const hash = keccak256(hexToBytes(publicKeyWithoutPrefix));
  // Take last 20 bytes (40 hex chars) as address
  return `0x${hash.slice(-40)}`;
}

async function getPublicKeys() {
  const response = await fetch(
    "https://iris-api-sandbox.circle.com/v2/publicKeys",
  );
  const data = await response.json();
  return data.publicKeys
    .filter((key: PublicKey) => key.cctpVersion === 2)
    .map((key: PublicKey) => key.publicKey);
}

async function verifyAttestation(
  attestationData: AttestationData,
  publicKeys: `0x${string}`[],
) {
  try {
    const messageHash = keccak256(attestationData.message as `0x${string}`);
    const attestationBytes = hexToBytes(
      attestationData.attestation as `0x${string}`,
    );
    const signatureLength = 65;
    const numSignatures = attestationBytes.length / signatureLength;

    if (attestationBytes.length % signatureLength !== 0) {
      throw new Error(`Invalid attestation length: ${attestationBytes.length}`);
    }

    let validSignatures = 0;

    for (let i = 0; i < numSignatures; i++) {
      const start = i * signatureLength;
      const signature = attestationBytes.slice(start, start + signatureLength);

      const recoveredAddress = await recoverAddress({
        hash: messageHash,
        signature: bytesToHex(signature),
      });

      const isValid = publicKeys.some(
        (publicKey) =>
          publicKeyToAddress(publicKey).toLowerCase() ===
          recoveredAddress.toLowerCase(),
      );

      if (isValid) validSignatures++;
    }

    const threshold = Math.ceil(publicKeys.length / 2);
    console.log(
      `Valid signatures: ${validSignatures}/${numSignatures}, threshold: ${threshold}`,
    );

    return validSignatures >= threshold;
  } catch (error) {
    console.error(
      "Error verifying attestation:",
      error instanceof Error ? error.message : String(error),
    );
    return false;
  }
}

const attestationData: AttestationData = {
  message: "0x000000010000001a00000015...", // Full message hex from API
  attestation: "0x3c5951abd82a83369d603ebaf9...", // Full attestation hex from API
};

// Example usage
const publicKeys = await getPublicKeys();
const isValid = await verifyAttestation(attestationData, publicKeys);