Bitcoin: Help! About how to spend OP_checksigadd Multi-sig

Safely Managing Multi-Sig Transactions with Bitcoin.js

As a developer using Bitcoin.js, you’ve likely encountered the need to manage multi-sig transactions. A multi-signature (multiSig) wallet allows multiple addresses to authorize and validate transactions on behalf of a single address. In this article, we’ll explore how to create a secure and reliable multiSig setup in Bitcoin.js.

The Problem: OP_CHECKSIGADD

When you use OP_CHECKSIGADD, it adds the signature of one or more signatures to another public key. However, if there’s only one valid signature, the transaction is likely to be rejected by the network. To mitigate this risk, we’ll use a different approach using OP_CHECKSIG and OP_EQUAL with a 3-2 threshold.

Creating a Secure MultiSig Setup

To create a multiSig setup in Bitcoin.js, you need to follow these steps:

• Create a 3-2 threshold account: Define a public key that will be used for the primary account. This key should have at least three signatures, and at least two of them must be valid.

• Create a secondary account: Create another public key that will be used to authorize transactions on behalf of the primary account. This key should have less than 4 signatures (one signature is invalid).

• Generate a shared secret: Use OP_CHECKSIG to generate a shared secret between the two accounts. This shared secret will contain all necessary information for authorization.

• Verify the shared secret: Use OP_EQUAL with the shared secret and one of the secondary account’s signatures to verify its validity.

Sample Code

const Bitcoinjs = require('bitcoinjs-lib');


// Create a 3-2 threshold account

const primaryAccountPubkey = 'xprv...';

const primarySigPubkey1 = 'xprv...'; // 3 signatures

const primarySigPubkey2 = 'xprv...'; // 2 signatures

const primarySigPubkey3 = 'xprv...'; // invalid signature


// Create a secondary account

const secondaryAccountPubkey = 'xprv...';

const secondarySigPubkey1 = 'xprv...'; // less than 4 signatures (one is invalid)


// Generate a shared secret using OP_CHECKSIG and one of the primary accounts' signatures

async function generateSharedSecret() {

  const sharedSecretPubkey = await Bitcoinjs.Secp256k1.createKeyPair(primaryAccountPubkey);

  

  const signature1 = await Bitcoinjs.Secp256k1.sign(sharedSecretPubkey, primarySigPubkey1);

  const signature2 = await Bitcoinjs.Secp256k1.sign(sharedSecretPubkey, primarySigPubkey2);

  const signature3 = await Bitcoinjs.Secp256k1.sign(sharedSecretPubkey, primarySigPubkey3);

  

  return { sharedSecret: JSON.stringify({ secret: sharedSecretPubkey, signatures: [signature1, signature2, signature3] }) };

}


// Verify the shared secret using OP_EQUAL with one of the secondary account's signatures

async function verifySharedSecret(sharedSecret) {

  const { sharedSecret: { secret, signatures } } = JSON.parse(sharedSecret);

  

  const signature = await Bitcoinjs.Secp256k1.sign(secret, secondarySigPubkey1);

  return signature === signatures[0];

}


// Example usage:

generateSharedSecret().then(sharedSecret => {

  verifySharedSecret(sharedSecret).then(verified => console.log(verified));

});

Best Practices

• Use a secure and trusted seed for the primary account.

• Ensure that all secondary accounts have less than 4 signatures (one is invalid).

• Keep the shared secret safe to prevent unauthorized access.

• Consider using a more robust cryptographic library, such as Bitcoinjs-secp256k1, which provides better security features.

By following these steps and best practices, you can safely manage multiSig transactions in your Bitcoin.js application. Remember to always handle sensitive information with care and follow the guidelines provided by the Bitcoin community.

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