Implementation of ERC-6492 signature verification for acp.us using Viem

acp-signature-verification.js

import { createPublicClient, http } from "viem";
import { base, polygon } from "viem/chains";

// Signature should be on its own on the last line
const rawMessage = "…";
const messageSignerAddress = "0x…";

// Get message and signature separately
const [message, encodedSignature] = rawMessage.trimEnd().split(/\n+(.+)$/);

// Decode Base64
const normalized = encodedSignature.replace(/-/g, "+").replace(/_/g, "/");
const response = await fetch(`data:application/octet-stream;base64,${normalized}`);

// Decompress
const decompressed = response.body?.pipeThrough(new DecompressionStream("deflate-raw"));

// Convert to byte array
const signature = new Uint8Array(await new Response(decompressed).arrayBuffer());

let isSignatureValid = false;

for (const chain of [base, polygon]) {
  const client = createPublicClient({ chain, transport: http() });

  try {
    isSignatureValid = await client.verifyMessage({
      address: messageSignerAddress,
      message,
      signature,
    });

    if (isSignatureValid) break;
  } catch {}
}

console.log("Signature valid:", isSignatureValid);