A Guide to creating TapRoot Scripts with bitcoinjs-lib
Tags: bitcoinjs, taproot, javascript
Published: 2023-02-19 21:31:40

Taproot and Schnorr are upgrades to the Bitcoin protocol designed to enhance the privacy, efficiency and flexibility of Bitcoin transactions.

Taproot introduces Taptrees, a feature that reduces the size of transaction data and ensures only necessary information is revealed on the blockchain, thereby preserving privacy. With Taproot, multisig transactions are also more private as unused spending conditions are hidden.

Schnorr signatures are 64 bytes long instead of the 72 bytes used by current ECDSA signature scheme. Taproot also use only x-values of public keys for signature creation which saves 1 byte. 

With the adoption of Taproot and Schnorr, Bitcoin transactions can be more efficient, flexible, and private.

We'll go over two examples. In our first example, we will create a pay-to-taproot(p2tr) address that will lock funds to a key and create a spend transaction for it. In our second example, we will jump straight into taproot script-spend transactions, we will create a `Taptree` consisting of two script-spend paths, a hash-lock script spend path and a pay-to-pubkey script spend path. We will create transactions that spend from both paths.

The full code for this article can be found on github at [taproot-with-bitcoinjs](https://github.com/Eunovo/taproot-with-bitcoinjs).

## Taproot Key-spend transaction
For illustration purposes, we'll use a random keypair
```typescript
const keypair = ECPair.makeRandom({ network });
```
We tweak this keypair with our pubkey.
```typescript
const tweakedSigner = tweakSigner(keypair, { network });
``` 
`bitcoinjs-lib` provides a `p2tr` function to generate p2tr outputs.
```typescript
const p2pktr = payments.p2tr({
  pubkey: toXOnly(tweakedSigner.publicKey),
  network
});
const p2pktr_addr = p2pktr.address ?? "";
console.log(p2pktr_addr);
```
The `toXOnly` function extracts the x-value of our public key.

You can use any testnet faucet that supports taproot addresses. I used [testnet-faucet.com/btc-testnet](https://testnet-faucet.com/btc-testnet/) while testing.

Creating a spend-transaction for this address with bitcoinjs-lib is straightforward.
```typescript
const psbt = new Psbt({ network });
psbt.addInput({
        hash: utxos[0].txid,
        index: utxos[0].vout,
        witnessUtxo: { value: utxos[0].value, script: p2pktr.output! },
        tapInternalKey: toXOnly(keypair.publicKey)
});

psbt.addOutput({
        address: "mohjSavDdQYHRYXcS3uS6ttaHP8amyvX78", // faucet address
        value: utxos[0].value - 150
});

psbt.signInput(0, tweakedSigner);
psbt.finalizeAllInputs();
```

Extract the transaction and broadcast the transaction hex.
```typescript
const tx = psbt.extractTransaction();
console.log(`Broadcasting Transaction Hex: ${tx.toHex()}`);
const txid = await broadcast(tx.toHex());
console.log(`Success! Txid is ${txid}`);
```

## Taproot Script-spend transaction
We'll create a Tap tree with two spend paths, a hash-lock spend path and a pay-to-pubkey spend path.

The hash-lock script-spend path will require the spender to include a preimage that will produce the hash specified in the script.

Let's make another random keypair for our hash-lock script
```typescript
const hash_lock_keypair = ECPair.makeRandom({ network });
```

Now, we will construct our hash-lock script
```typescript
const secret_bytes = Buffer.from('SECRET');
const hash = crypto.hash160(secret_bytes);
// Construct script to pay to hash_lock_keypair if the correct preimage/secret is provided
const hash_script_asm = `OP_HASH160 ${hash.toString('hex')} OP_EQUALVERIFY ${toXOnly(hash_lock_keypair.publicKey).toString('hex')} OP_CHECKSIG`;
const hash_lock_script = script.fromASM(hash_script_asm);
```
Notice that script still requires a signature to unlock funds.

The pay-to-pubkey spend path is much simpler
```typescript
const p2pk_script_asm = `${toXOnly(keypair.publicKey).toString('hex')} OP_CHECKSIG`;
const p2pk_script = script.fromASM(p2pk_script_asm);
```

We can now create our Taptree and p2tr address.
```typescript
const scriptTree: Taptree = [
        {
            output: hash_lock_script
        },
        {
            output: p2pk_script
        }
];
const script_p2tr = payments.p2tr({
        internalPubkey: toXOnly(keypair.publicKey),
        scriptTree,
        network
});
const script_addr = script_p2tr.address ?? '';
console.log(script_addr);
```
You can deposit some test btc into the address using a testnet faucet like [testnet-faucet.com/btc-testnet](https://testnet-faucet.com/btc-testnet/).

To spend on any of the leaf scripts, you must present the leafVersion, script and controlBlock for that leaf script. The control block is data required to prove that the leaf script exists in the script tree(merkle proof).

bitcoinjs-lib will generate the control block for us.
```typescript
const hash_lock_redeem = {
    output: hash_lock_script,
    redeemVersion: 192,
};
const p2pk_redeem = {
    output: p2pk_script,
    redeemVersion: 192
}

const p2pk_p2tr = payments.p2tr({
    internalPubkey: toXOnly(keypair.publicKey),
    scriptTree,
    redeem: p2pk_redeem,
    network
});

const hash_lock_p2tr = payments.p2tr({
    internalPubkey: toXOnly(keypair.publicKey),
    scriptTree,
    redeem: hash_lock_redeem,
    network
});

console.log(`Waiting till UTXO is detected at this Address: ${script_addr}`);
let utxos = await waitUntilUTXO(script_addr)
console.log(`Trying the P2PK path with UTXO ${utxos[0].txid}:${utxos[0].vout}`);

const p2pk_psbt = new Psbt({ network });
p2pk_psbt.addInput({
    hash: utxos[0].txid,
    index: utxos[0].vout,
    witnessUtxo: { value: utxos[0].value, script: p2pk_p2tr.output! },
    tapLeafScript: [
        {
            leafVersion: p2pk_redeem.redeemVersion,
            script: p2pk_redeem.output,
            controlBlock: p2pk_p2tr.witness![p2pk_p2tr.witness!.length - 1] // extract control block from witness data
        }
    ]
});

p2pk_psbt.addOutput({
    address: "mohjSavDdQYHRYXcS3uS6ttaHP8amyvX78", // faucet address
    value: utxos[0].value - 150
});

p2pk_psbt.signInput(0, keypair);
p2pk_psbt.finalizeAllInputs();

let tx = p2pk_psbt.extractTransaction();
console.log(`Broadcasting Transaction Hex: ${tx.toHex()}`);
let txid = await broadcast(tx.toHex());
console.log(`Success! Txid is ${txid}`);
```

To spend using the hash-lock leaf script, we have to create a custom finalizer function. In our custom finalizer, we will create our witness stack of `signature, preimage, original hash-lock script and our control block`
```typescript
const tapLeafScript = {
    leafVersion: hash_lock_redeem.redeemVersion,
    script: hash_lock_redeem.output,
    controlBlock: hash_lock_p2tr.witness![hash_lock_p2tr.witness!.length - 1]
};

const psbt = new Psbt({ network });
psbt.addInput({
    hash: utxos[0].txid,
    index: utxos[0].vout,
    witnessUtxo: { value: utxos[0].value, script: hash_lock_p2tr.output! },
    tapLeafScript: [
        tapLeafScript
    ]
});

psbt.addOutput({
    address: "mohjSavDdQYHRYXcS3uS6ttaHP8amyvX78", // faucet address
    value: utxos[0].value - 150
});

psbt.signInput(0, hash_lock_keypair);

// We have to construct our witness script in a custom finalizer

const customFinalizer = (_inputIndex: number, input: any) => {
    const scriptSolution = [
        input.tapScriptSig[0].signature,
        secret_bytes
    ];
    const witness = scriptSolution
        .concat(tapLeafScript.script)
        .concat(tapLeafScript.controlBlock);

    return {
        finalScriptWitness: witnessStackToScriptWitness(witness)
    }
}

psbt.finalizeInput(0, customFinalizer);

tx = psbt.extractTransaction();
console.log(`Broadcasting Transaction Hex: ${tx.toHex()}`);
txid = await broadcast(tx.toHex());
console.log(`Success! Txid is ${txid}`);
```

## Conclusion
By reading this article, you should now have a better understanding of how to use bitcoinjs-lib to create and spend P2TR (Pay to Taproot) payments. With this knowledge, you are one step closer to leveraging the benefits of Taproot in your Bitcoin transactions, such as improved privacy, scalability, and the ability to create more complex smart contracts.
You can find more examples in [bitcoinjs-lib's repo](https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/test/integration/taproot.spec.ts). The full code for this article can be found on github at [taproot-with-bitcoinjs](https://github.com/Eunovo/taproot-with-bitcoinjs).
If you need some help understanding taproot, you can check this out [More on Taproot](https://dev.to/eunovo/more-on-taproot-41g8).