Multi-signature Native Segwit 2 of 4

To follow along this tutorial

  • Clone the Github repository

  • cd code

  • npm install or yarn install

  • Execute the transaction code by typing node tx_filename.js

  • Alternatively you can enter the commands step-by-step by cd into ./code then type node in a terminal to open the Node.js REPL

  • Open the Bitcoin Core GUI console or use bitcoin-cli for the Bitcoin Core commands

  • Use bx aka Libbitcoin-explorer as a handy complement

Let’s create, fund and spend a 2 of 4 multi-signature with a native Segwit P2WSH transaction.

Creating and Funding a P2WSH Multi-Signature UTXO

Import libraries, test wallets and set the network
const bitcoin = require('bitcoinjs-lib')
const {alice, bob, carol, dave} = require('./wallets.json')
const network = bitcoin.networks.regtest
Create the witness script with the special p2ms payment method.
const p2ms = bitcoin.payments.p2ms({
  m: 2, pubkeys: [
    Buffer.from(alice[1].pubKey, 'hex'),
    Buffer.from(bob[1].pubKey, 'hex'),
    Buffer.from(carol[1].pubKey, 'hex'),
    Buffer.from(dave[1].pubKey, 'hex'),
  ], network})

console.log('Witness script:')
console.log(p2ms.output.toString('hex'))
console.log()

console.log('Witness script SHA256:')
console.log(bitcoin.crypto.sha256(p2ms.output).toString('hex'))
console.log()
Check the locking script.
decodescript SCRIPT

The asm field should contain:

02 03745c9aceb84dcdeddf2c3cdc1edb0b0b5af2f9bf85612d73fa6394758eaee35d 027efbabf425077cdbceb73f6681c7ebe2ade74a65ea57ebcf0c42364d3822c590
023a11cfcedb993ff2e7523f92e359c4454072a66d42e8b74b4b27a8a1258abddd 02e9d617f38f8c3ab9a6bde36ce991bafb295d7adba457699f8620c8160ec9e87a 04
OP_CHECKMULTISIG
Feed the p2wsh method with the BitcoinJS p2ms object.
const p2wsh = bitcoin.payments.p2wsh({redeem: p2ms, network})
console.log('P2WSH address')
console.log(p2wsh.address) (1)
1 The p2wsh payment method generates an object that contains the P2WSH address.
Send 1 BTC to this P2WSH address.
sendtoaddress bcrt1qtvraes6lc2efmwqtupv7f9wg3adhvzwpu0vg3s2zgpnc7qpp0v7sj6dkmu 1
Get the output index so that we have the outpoint (txid / vout).
gettransaction TX_ID
Find the output index (or vout) under details  vout.

Preparing the spending transaction

Now let’s prepare the spending transaction by setting input and output and having two persons (private keys) to sign the transaction. In this tutorial, alice_1 and bob_1 will redeem the P2WSH multi-signature and send the funds to alice_2 P2WPKH address.

Create the PSBT by filling TX_ID and TX_OUT.
const psbt = new bitcoin.Psbt({network})
  .addInput({
    hash: 'TX_ID',
    index: TX_VOUT,
    witnessScript: p2wsh.redeem.output,
    witnessUtxo: {
      script: Buffer.from('0020' + bitcoin.crypto.sha256(p2ms.output).toString('hex'), 'hex'), (1)
      value: 1e8,
    }
  })
  .addOutput({
    address: alice[2].p2wpkh,
    value: 999e5,
  }) (2)
1 Locking script of the UTXO we are spending, which is a P2WSH witness program, <00> + PushByte 20 + SHA256(witness script)
2 Output locking the funds to alice_2 P2WPKH address, leaving 100 000 satoshis as mining fees
Prepare signers Alice_1 and Bob_1 keypairs.
const keyPairAlice1 = bitcoin.ECPair.fromWIF(alice[1].wif, network)
const keyPairBob1 = bitcoin.ECPair.fromWIF(bob[1].wif, network)
Alice_1 and bob_1 now sign the same input.
psbt
  .signInput(0, keyPairAlice1)
  .signInput(0, keyPairBob1)
Finalize the PSBT.
psbt.validateSignaturesOfInput(0, Buffer.from(alice[1].pubKey, 'hex'))
psbt.validateSignaturesOfInput(0, Buffer.from(bob[1].pubKey, 'hex'))

psbt.finalizeAllInputs()
Extract the transaction and get the raw hex serialization.
console.log('Transaction hexadecimal:')
console.log(psbt.extractTransaction().toHex())
Inspect the raw transaction with Bitcoin Core CLI, check that everything is correct.
decoderawtransaction TX_HEX

Broadcasting the transaction

It’s time to broadcast the transaction via Bitcoin Core CLI.
sendrawtransaction TX_HEX
Inspect the transaction.
getrawtransaction TX_ID true

Observations

We can see that the scriptSig unlocking script is empty. Instead all the data are located in the txinwitness field and contains:

  • An empty string that will convert to a dummy but mandatory 00 value due to a bug in OP_CHECKMULTISIG

  • Alice_1 and bob_1 signatures

  • The witness script

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