[doc] privateKeyTweakAdd and publicKeyTweakAdd for HD Wallets

[coolaj86]

I'm just documenting this for the sake of anyone else that's updating their HD Wallet libraries:
(see also #73)

The HD Wallet key functions are in the test files, no the main package.

• tweakUtils.privateAdd is known in other libraries as privateKeyTweakAdd
• tweakUtils.pointAddScalar is known in other libraries as publicKeyTweakAdd

  let tweakUtils = {
    /**
     * @param {Uint8Array} privateKey
     * @param {Uint8Array} tweak
     * @returns {Uint8Array} - a new (derivative) privateKey
     */
    privateAdd: function (privateKey, tweak) {
      const p = Secp256k1.utils._normalizePrivateKey(privateKey);
      const t = Secp256k1.utils._normalizePrivateKey(tweak);
      return Secp256k1.utils._bigintTo32Bytes(
        Secp256k1.utils.mod(p + t, Secp256k1.CURVE.n),
      );
    },

    /**
     * @param {Uint8Array} p
     * @param {Uint8Array} tweak
     * @param {Boolean} [isCompressed]
     * @returns {Uint8Array} - a new (derivative) publicKey
     */
    pointAddScalar: function (p, tweak, isCompressed) {
      const P = Secp256k1.Point.fromHex(p);
      const t = Secp256k1.utils._normalizePrivateKey(tweak);
      const Q = Secp256k1.Point.BASE.multiplyAndAddUnsafe(P, t, 1n);
      if (!Q) {
        throw new Error("Tweaked point at infinity");
      }
      return Q.toRawBytes(isCompressed);
    },
  };

Adapted from ./test/index.ts:

noble-secp256k1/test/index.ts

Lines 466 to 492 in e125abd

	const tweakUtils = {
	privateAdd: (privateKey: PrivKey, tweak: Hex): Uint8Array => {
	const p = normal(privateKey);
	const t = normal(tweak);
	return secp.utils._bigintTo32Bytes(secp.utils.mod(p + t, secp.CURVE.n));
	},
	privateNegate: (privateKey: PrivKey): Uint8Array => {
	const p = normal(privateKey);
	return secp.utils._bigintTo32Bytes(secp.CURVE.n - p);
	},
	pointAddScalar: (p: Hex, tweak: Hex, isCompressed?: boolean): Uint8Array => {
	const P = secp.Point.fromHex(p);
	const t = normal(tweak);
	const Q = secp.Point.BASE.multiplyAndAddUnsafe(P, t, 1n);
	if (!Q) throw new Error('Tweaked point at infinity');
	return Q.toRawBytes(isCompressed);
	},
	pointMultiply: (p: Hex, tweak: Hex, isCompressed?: boolean): Uint8Array => {
	const P = secp.Point.fromHex(p);
	const h = typeof tweak === 'string' ? tweak : secp.utils.bytesToHex(tweak);
	const t = BigInt(`0x${h}`);
	return P.multiply(t).toRawBytes(isCompressed);
	},
	};

This may need to be updated again for the upcoming major refactor.

[paulmillr]

should be updated for v2 now

[headfire94]

@paulmillr is this solved? I can't find these methods in lib

[paulmillr]

@headfire94 there are no plans to implement these methods because they look terrible. Users should not even think about "tweaks": instead, there are points and scalars. I consider noble api much more concise. If you want to use these methods, you can implement them for v2 in your own fork.

Also there is no need in re-implementing HD wallet over and over, there's audited http://github.com/paulmillr/scure-bip32 and that's it. You don't need another one. You may need a different lib for different curve.