@polkadot/util-crypto/hd/ledger/derivePrivate.js

// Copyright 2017-2023 @polkadot/util-crypto authors & contributors
// SPDX-License-Identifier: Apache-2.0

import { BN_EIGHT, bnToU8a, u8aConcat, u8aToBn } from '@polkadot/util';
import { BN_LE_32_OPTS, BN_LE_512_OPTS, BN_LE_OPTS } from "../../bn.js";
import { hmacShaAsU8a } from "../../hmac/index.js";

// performs hard-only derivation on the xprv
export function ledgerDerivePrivate(xprv, index) {
  const kl = xprv.subarray(0, 32);
  const kr = xprv.subarray(32, 64);
  const cc = xprv.subarray(64, 96);
  const data = u8aConcat([0], kl, kr, bnToU8a(index, BN_LE_32_OPTS));
  const z = hmacShaAsU8a(cc, data, 512);
  data[0] = 0x01;
  return u8aConcat(bnToU8a(u8aToBn(kl, BN_LE_OPTS).iadd(u8aToBn(z.subarray(0, 28), BN_LE_OPTS).imul(BN_EIGHT)), BN_LE_512_OPTS).subarray(0, 32), bnToU8a(u8aToBn(kr, BN_LE_OPTS).iadd(u8aToBn(z.subarray(32, 64), BN_LE_OPTS)), BN_LE_512_OPTS).subarray(0, 32), hmacShaAsU8a(cc, data, 512).subarray(32, 64));
}