Bitcoin multi-sig transaction part 2

Piotr Misiurek -

Welcome back bitcoin freaks. In the previous episode, we created a multi-sig transaction. Go back to that blog post if you don't know what multi-sig means.

Our code works. The transaction was made. The problem is that it handle only a theoretical use case when both private keys owners use the same machine to sign the transaction. We need to handle a real life situation when they use different machines. They have to be able to sign the transaction and send signatures to each other.

Send the signature

Let's assume we already received the UTXOs and the transaction is created. The only thing we have to cover is to sign the transaction. To achieve this goal we will use getSignatures method:

var firstSignatures = tx.getSignatures(privateKeys[0]);

We passed the first private key to this method. It means all inputs will be signed with this private key. As a result, we will receive an array of signatures. The number of array elements is equal to inputs that we use in the transaction. The input comes from UTXOs, so as many UTXOs will be used in the transaction as many signatures will be made. One for each input.

Now, the second person can apply signatures of the first person

for(var i = 0; i < firstSignatures.length; i++) {
  tx.applySignature(firstSignatures[i]);
}

and sign the transaction himself:

tx.sign(privateKeys[1]);

Send the signed transaction

The second approach is to sign the transaction, serialize it and sent to the second person.

The first person signs the transaction and serializes it:

tx.sign(privateKeys[0]);
var serializedTx = tx.toObject();

The second one creates a new transaction based on received serialized transaction and signs it as well:

tx = new bitcore.Transaction(serializedTx)
tx.sign(privateKeys[1]);

That’s all folks. The transaction is ready to be broadcasted to the bitcoin network.

Check out entire multi-sig transactions code from this and previous article.

In the next episode, we will talk about nLockTime option. See you soon :)

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