04_1_P2WPKH_Spend_1_1.md

4.1: Spend a Native Segwit P2WPKH UTXO

To follow along this tutorial and enter the commands step-by-step

• Type node in a terminal after cd into ./code for a Javascript prompt
• 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 spend a native Segregated Witness P2WPKH output and create a new legacy P2PKH output.

Create a UTXO to spend from

Import libraries, test wallets and set the network

const bitcoin = require('bitcoinjs-lib')
const { alice, bob } = require('./wallets.json')
const network = bitcoin.networks.regtest

Send 1 BTC to Alice_0 native Segwit P2WPKH address in order to create a P2WPKH UTXO.

Check out 02_0: Generating and Importing Wallets and your wallets.json file in the code directory. Replace the address if necessary.

$ sendtoaddress bcrt1qlwyzpu67l7s9gwv4gzuv4psypkxa4fx4ggs05g 1

Get the output index so that we have the outpoint (txid / vout).

Find the output index (or vout) under details > vout.

$ gettransaction "txid"

Creating the transaction

Now let's spend the UTXO with BitcoinJS.

Create a bitcoinJS key pair object for Alice_0, the spender of our new UTXO, and the only one capable of spending it.

const keyPairAlice0 = bitcoin.ECPair.fromWIF(alice[0].wif, network)
const p2wpkhAlice0 = bitcoin.payments.p2wpkh({pubkey: keyPairAlice0.publicKey, network})

Create a key pair object and a P2PKH address for the recipient Bob_0.

const keyPairBob0 = bitcoin.ECPair.fromWIF(bob[0].wif, network)
const p2pkhBob0 = bitcoin.payments.p2pkh({pubkey: keyPairBob0.publicKey, network})

Create a BitcoinJS transaction builder object.

const txb = new bitcoin.TransactionBuilder(network)

Create the input by providing the outpoint but also the script of the previous transaction (vout > scriptPubKey > hex). Adding the prevTxScript is a specificity of P2WPKH spending.

The script is composed as follow: <version program 00> PUSHBYTES_14 The HASH160 of the public key must match the 20-bytes witness program.
$ bx bitcoin160 03745c9aceb84dcdeddf2c3cdc1edb0b0b5af2f9bf85612d73fa6394758eaee35d
fb8820f35effa054399540b8ca86040d8ddaa4d5
or with bitcoinJS bitcoin.crypto.hash160(Buffer.from('03745c9aceb84dcdeddf2c3cdc1edb0b0b5af2f9bf85612d73fa6394758eaee35d', 'hex')).toString('hex')

// txb.addInput(prevTx, input.vout, input.sequence, prevTxScript)
txb.addInput('TX_ID', TX_VOUT, null, p2wpkhAlice0.output) 

Pay 0.999 BTC to Bob_0 public key hash

txb.addOutput(p2pkhBob0.address, 999e5)

The miner fee is calculated by subtracting the outputs from the inputs. 100 000 000 - 99 900 000 = 100 000 100 000 satoshis equals 0,001 BTC, this is the miner fee.

No redeem script needed because we are spending a native segwit UTXO, not a P2SH-P2WPKH. But we need to sign the input value.

// txb.sign(index, keyPair, redeemScript, sign.hashType, value, witnessScript)
txb.sign(0, keyPairAlice0, null, null, 1e8)

Finally we can build the transaction and get the raw hex serialization.

const tx = txb.build()
console.log('tx.toHex()  ', tx.toHex())

Inspect the raw transaction with Bitcoin Core CLI, check that everything is correct.

$ decoderawtransaction "hexstring"

Broadcasting the transaction

It's time to broadcast the transaction via Bitcoin Core CLI.

$ sendrawtransaction "hexstring"

sendrawtransaction returns the transaction ID, with which you can inspect your transaction again.

Don't forget the second argument. If false, it returns the hex string, otherwise it returns a detailed json object.

$ getrawtransaction "txid" true

Observations

In the vin section we note that scriptSig is empty and that we have an additional txinwitness field which contains Alice signature and public key.

In the vout section we have one witness_v0_keyhash output, which is the code name for native Segwit.

What's Next?

Continue "PART TWO: PAY TO PUBLIC KEY HASH" with 4.2: Typical Transaction (1 input, 2 outputs) - Native Segwit P2WPKH.