Sign a Bitcoin Message

Produce a standard Bitcoin signed-message hash (the same format used by Bitcoin Core's signmessage) and sign it with an MPC key share. Useful for proof-of-ownership, off-chain authentication challenges, and OP_RETURN attestations.

Components used:

• COMPUTE_BITCOIN_SIGNATURE_HASH — produce the standard bitcoin-message hash from a plaintext string
• SIGN_WITH_KEY_SHARE — MPC sign the message hash

---

Code

import { WorkspaceClient, ComponentModule } from 'caller-sdk';

const workspace = new WorkspaceClient({ apiKey: process.env.WR_API_KEY! });

const KEY_ID = 'your-key-id';
const DERIV  = [2147532436, 2147483648, 2147483648, 0, 0]; // m/84'/0'/0'/0/0

async function signBitcoinMessage(message: string) {
  // 1. Hash the message in Bitcoin signed-message format
  //    Prepends "\x18Bitcoin Signed Message:\n" + varint length before double-SHA256
  const { messageHash } = await workspace
    .call(ComponentModule.COMPUTE_BITCOIN_SIGNATURE_HASH, {
      message,
    })
    .promise();

  console.log('Message hash:', messageHash); // 64-char hex

  // 2. MPC sign the hash
  const { signature } = await workspace
    .call(ComponentModule.SIGN_WITH_KEY_SHARE, {
      keyId: KEY_ID,
      derivationPath: DERIV,
      messageHash,
    })
    .promise();

  console.log('Signature:', signature);
  return { message, messageHash, signature };
}

const result = await signBitcoinMessage('I own this address — 2024-01-15');
console.log(result);

---

Raw SHA-256 variant

Use format: 'raw' to compute a plain double-SHA256 hash instead of the Bitcoin message prefix format. This is useful for signing raw data (e.g., commitment hashes stored in OP_RETURN outputs).

const { messageHash } = await workspace
  .call(ComponentModule.COMPUTE_BITCOIN_SIGNATURE_HASH, {
    message: 'raw-data-to-hash',
  })
  .promise();
// format is configured at the component level in the workflow canvas

---

In a workflow

[Trigger: message]
        │
        ▼
COMPUTE_BITCOIN_SIGNATURE_HASH ← format: 'bitcoin-message' (config)
        │ messageHash
        ▼
SIGN_WITH_KEY_SHARE ← keyId, derivationPath (constants)
        │ signature
        ▼
    [Output: { message, messageHash, signature }]

Verifying the signature

The resulting signature can be verified by any standard Bitcoin message verification tool (e.g., Bitcoin Core's verifymessage RPC, or libraries like bitcoinjs-message). Pass the address derived from the same key, the original message, and the signature.