taproot.utils.ts

import * as ecc from 'tiny-secp256k1';
import ECPairFactory from 'ecpair';
import { toXOnly } from '../../src/psbt/bip371';
import * as bitcoin from '../..';

const ECPair = ECPairFactory(ecc);

// This logic will be extracted to ecpair
export function tweakSigner(
  signer: bitcoin.Signer,
  opts: any = {},
): bitcoin.Signer {
  // @ts-ignore
  let privateKey: Uint8Array | undefined = signer.privateKey!;
  if (!privateKey) {
    throw new Error('Private key is required for tweaking signer!');
  }
  if (signer.publicKey[0] === 3) {
    privateKey = ecc.privateNegate(privateKey);
  }

  const tweakedPrivateKey = ecc.privateAdd(
    privateKey,
    tapTweakHash(toXOnly(signer.publicKey), opts.tweakHash),
  );
  if (!tweakedPrivateKey) {
    throw new Error('Invalid tweaked private key!');
  }

  return ECPair.fromPrivateKey(Buffer.from(tweakedPrivateKey), {
    network: opts.network,
  });
}

function tapTweakHash(pubKey: Buffer, h: Buffer | undefined): Buffer {
  return bitcoin.crypto.taggedHash(
    'TapTweak',
    Buffer.concat(h ? [pubKey, h] : [pubKey]),
  );
}