encodeByteHmac static method
Creates a hmac from a key that usually is the derived key, keylen,
challenge and hmacLength.
Implementation
static Uint8List encodeByteHmac(
    Uint8List key, int keylen, List<int> challenge,
    {hmacLength = 64}) {
  var mac = HMac(SHA256Digest(), hmacLength);
  mac.init(KeyParameter(key));
  mac.update(Uint8List.fromList(challenge), 0, challenge.length);
  var out = Uint8List(keylen);
  mac.doFinal(out, 0);
  return out;
}