How to recover btc address from txn witness data?

[saurabhburade]

• Old reference #1104
• I have an array of witness hex data like this https://mempool.space/tx/2bb85f4b004be6da54f766c17c1e855187327112c231ef2ff35ebad0ea67c69e
• I want to recover the address bc1pkqndh95dk*******uleyx4pqq7x4tl

[junderw]

It is impossible for p2tr key spend witnesses. (Since it’s only a signature) but every other type of segwit address can be recovered by its witness stack.

is that ok?

[junderw]

To be specific:

1. P2WPKH: The last item on the witness stack is the pubkey, HASH160 it and use bech32 with segwit version 0 to encode.
2. P2WSH: The last item on the witness stack is the witnessScript, SHA256 it and use bech32 with segwit version 0 to encode.
3. P2TR (script): Follow BIP341 script validation rules up until the point where it compares the final output to the witness program from the previous output. If the transaction is confirmed in a block we can assume this check passed, so therefore we can get the witness program by performing the validation rules and encoding the result using bech32m with segwit version 1.

[junderw]

If you give this a P2TR witness that you know is definitely a P2TR, then it will give you the address if it's a script spend.

Probably buggy though.

import * as bitcoin from 'bitcoinjs-lib';
import * as vint from 'varuint-bitcoin';
import * as ecc from 'tiny-secp256k1';

// Required
bitcoin.initEccLib(ecc);

function p2trScriptToAddress(witness: Buffer[]): string {
  if (!Array.isArray(witness) || !witness.every(b => Buffer.isBuffer(b))) {
    throw new Error('All witness elements must be Buffers');
  }

  const wlen = witness.length;
  if (wlen < 2) {
    throw new Error('Not a p2tr script spend witness');
  }

  let annexOffset = 0;
  if (witness[wlen - 1][0] === 0x50) {
    // has annex
    annexOffset = 1;
  }

  if (wlen < 2 + annexOffset) {
    throw new Error('Not a p2tr script spend witness');
  }

  const controlBlock = witness[wlen - annexOffset - 1];
  if (controlBlock.length < 33 || ((controlBlock.length - 33) / 32) % 1 !== 0) {
    throw new Error('Incorrect Control block length');
  }

  const script = witness[wlen - annexOffset - 2];
  const leafVersion = controlBlock[0] & 0xfe;
  const internalPubkey = controlBlock.subarray(1, 33);

  // Get preimage for tapleaf hash
  const preImageLeafHash = Buffer.allocUnsafe(
    script.length + vint.encodingLength(script.length) + 1,
  );
  preImageLeafHash.writeUInt8(leafVersion, 0);
  vint.encode(script.length, preImageLeafHash, 1);
  script.copy(preImageLeafHash, 1 + vint.encodingLength(script.length));
  // Get tapleaf hash
  let tapLeafHash = bitcoin.crypto.taggedHash('TapLeaf', preImageLeafHash);

  const loops = (controlBlock.length - 33) / 32;
  for (let j = 0; j < loops; j++) {
    const branch = controlBlock.subarray(33 + 32 * j, 65 + 32 * j);

    if (Buffer.compare(tapLeafHash, branch) < 0) {
      tapLeafHash = bitcoin.crypto.taggedHash(
        'TapBranch',
        Buffer.concat([tapLeafHash, branch]),
      );
    } else {
      tapLeafHash = bitcoin.crypto.taggedHash(
        'TapBranch',
        Buffer.concat([branch, tapLeafHash]),
      );
    }
  }

  const final = bitcoin.crypto.taggedHash(
    'TapTweak',
    Buffer.concat([internalPubkey, tapLeafHash]),
  );
  if (!ecc.isPrivate(final)) {
    throw new Error(
      'Rare error. Final taptree hash was higher than curve order',
    );
  }
  const result = ecc.xOnlyPointAddTweak(internalPubkey, final);
  if (result === null) {
    throw new Error('Error when tweaking');
  }

  return bitcoin.address.fromOutputScript(
    Buffer.concat([Buffer.from([0x51, 0x20]), Buffer.from(result.xOnlyPubkey)]),
  );
}

[saurabhburade]

@junderw Unable to recover address from https://mempool.space/tx/1bc6814dfab83f468e7ba4a24694974f1e33744e6b6ea0518ec6e99c80ff1016

[junderw]

If you're not going to read my replies, posts, and comments, then I am wasting my time here.

Good luck with whatever it is you're doing.

https://github.com/bitcoinjs/bitcoinjs-lib/issues/2088#issuecomment-2079221481 (Read this reply to your issue)