0.1.1

v0.1.1

• Finally Taproot features are all supported!!!
• You can easily build Bitcoin Smart Contract with Tapscript!
• Convert value unit to satoshi to prevent wrong floating porint calculation
• Fix non-taproot transaction bug in v0.1.0(unpublished)

Taproot and Tapscript spend

import * as bitcoin from 'bitcoin-sdk-js';

// Let's send and spend HTLC in taproot and tapscript way!
/*
  Schnorr key is same with any bitcoin key pair, except it does not use public key prefix byte '02' or '03'.
  This key pair will be used as a master key to spend UTXO in taproot, after tweaked(will explain how to step by step).
*/
const schnorrPubkey = (await bitcoin.wallet.generateKeyPair()).publicKey.slice(2); // remove first byte (which is parity bit)
const schnorrPrivkey = (await bitcoin.wallet.generateKeyPair()).privateKey;

/*
  HTLC consists of conditional branch, in which OP_IF contains Time Lock Contract and OP_ELSE contains Hash Lock.
  We can construct tapscript tree where each leaf has its own contract(script), enable unlimited spending branches.
*/
// Originally OP_IF branch Time Lock Contract
const timeLockContract =
    (await bitcoin.script.generateTimeLockScript(2576085)) +
    (await bitcoin.data.pushData(schnorrPubkey1) + // must specify data to read(if not opcode)
    schnorrPubkey1 + // you must use schnorr key even in tapscript
    bitcoin.Opcode.OP_CHECKSIG;
// Originally OP_ELSE branch Hash Lock Contract
const hashLockContract =
    (await bitcoin.script.generateHashLockScript('abcdef')) +
    (await bitcoin.data.pushData(schnorrPubkey2) + // must specify data to read(if not opcode)
    schnorrPubkey2 + // you must use schnorr key even in tapscript
    bitcoin.Opcode.OP_CHECKSIG;

/*
  You can get tapbranch from a pair of script, and repeat over to reach to the root of tree(which is 'taproot') 
  Here we have only a single pair of script, so tapbranch of it is taproot
*/
const taproot = await bitcoin.tapscript.getTapBranch([
    await bitcoin.tapscript.getTapLeaf(timeLockContract),
    await bitcoin.tapscript.getTapLeaf(hashLockContract),
]);
// Then, get taptweak with schnorr key generated above
const tapTweak = await bitcoin.tapscript.getTapTweak(
    schnorrPubkey,
    taproot,
);
// Finally, you can tweak schnorr public key, which can be used as taproot key!
const tapTweakedPubkey = await bitcoin.tapscript.getTapTweakedPubkey(
    schnorrPubkey,
    tapTweak,
);
// generate taproot address with taproot key(and send to it!)
const toAddress = await bitcoin.address.generateAddress(tapTweakedPubkey.tweakedPubKey, 'taproot');



// Then, can be spent as an input!
// one thing to keep in mind, taproot or tapscript spend needs to provide all the script public key of input
await tx.addInput({
    txHash: txId,
    index: 0,
    value: value,
    script: await bitcoin.script.getScriptByAddress(toAddress),
});

await tx.setLocktime(2576085); // if transaction use timelock input, must set tx locktime bigger than input timelock

// taproot spend? just sign input with tweaked schnorr key(which is, taproot private key)
await tx.signInput(
    '', // we don't need to provide taproot public key when sign
    await bitcoin.tapscript.getTapTweakedPrivkey(schnorrPrivkey, tapTweak), // just provide private key
    0,
    'taproot',
);

// Or tapscript spend? Let's spend by hash lock script!
const sigStack = [
    await bitcoin.crypto.sign(
      await tx.getInputHashToSign(hashLockContract, 0, 'tapscript'),
      schnorrPrivkey2,
      'schnorr',
    ), // schnorr signature
    'abcdef', // hash to unlock
    hashLockContract, // spend script
    // you must specify path to find taproot with provided script(For more detail, check BIP341!)
    await bitcoin.tapscript.getTapControlBlock(
      schnorrPubkey,
      tapTweakedPubkey.parityBit,
      await bitcoin.tapscript.getTapLeaf(timeLockContract), // path to find taproot
    ),
];

await tx.signInputByScriptSig(sigStack, 0);

// You can broadcast signed tx here: https://blockstream.info/testnet/tx/push
const txToBroadcast: string = await tx.getSignedHex();

📜 License

This software is licensed under the MIT ©.