hkdf static method

Uint8List hkdf(List<int> secret, List<int> info, { List<int> salt = const <int>[], int length = 32, })

HKDF-SHA256 of RFC 5869: length bytes made from secret, info and salt. An empty salt is 32 zero bytes, like the RFC says.

Implementation

static Uint8List hkdf(
  List<int> secret,
  List<int> info, {
  List<int> salt = const <int>[],
  int length = 32,
}) {
  if (length < 1 || length > 255 * 32) {
    throw ArgumentError.value(length, 'length', 'must be 1 to 8160');
  }
  // extract: PRK = HMAC(salt, secret)
  Uint8List prk = _hmac(salt.isEmpty ? Uint8List(32) : salt, secret);
  // expand: T(i) = HMAC(PRK, T(i-1) + info + i)
  Uint8List out = Uint8List(length);
  Uint8List block = Uint8List(0);
  int done = 0;
  for (int i = 1; done < length; i++) {
    block = _hmac(prk, <int>[...block, ...info, i]);
    int take = length - done < block.length ? length - done : block.length;
    out.setRange(done, done + take, block);
    done += take;
  }
  return out;
}