authenticate method

Implementation

@override
Authenticate authenticate(Challenge challenge) {
  if (!challenge.extra.containsKey("challenge")) {
    throw ArgumentError("challenge string missing in extra");
  }
  var challengeHex = challenge.extra["challenge"];
  var signed = signCryptoSignChallenge(challengeHex, getPrivateKey());

  return Authenticate(signed + challengeHex, {});
}