sign method

@override Future<Signature> sign(List<int> message, { required KeyPair keyPair, PublicKey? publicKey, })

Signs bytes.

Example

In this example, we use Ed25519:

import 'package:cryptography/cryptography.dart';

Future<void> main() async {
  final signedMessage = [1,2,3];

  final ed25519 = Ed25519();
  final keyPair = await ed25519.newKeyPair();
  final signature = await ed25519.sign(
    signedMessage,
    keyPair: keyPair,
  );

  // ...

  final isRealSignature = await ed25519.verify(
    signedMessage,
    signature: signature,
  );

  print('Signature verification result: $isRealSignature');
}

Implementation

@override
Future<Signature> sign(
  List<int> message, {
  required KeyPair keyPair,
  PublicKey? publicKey,
}) async {
  final keyPairData = (await keyPair.extract()) as SimpleKeyPairData;
  final privateKeyBytes = keyPairData.bytes;

  // Take SHA512 hash of the private key.
  final privateKeyHash = await _sha512.hash(privateKeyBytes);
  final privateKeyHashFixed = privateKeyHash.bytes.sublist(0, 32);
  _setPrivateKeyFixedBits(privateKeyHashFixed);

  // We get public key by multiplying the modified private key hash with G.
  final publicKeyBytes = _pointCompress(_pointMul(
    Register25519()..setBytes(Uint8List.fromList(privateKeyHashFixed)),
    Ed25519Point.base,
  ));
  final publicKey = SimplePublicKey(
    publicKeyBytes,
    type: KeyPairType.ed25519,
  );

  // Calculate hash of the input.
  // The second half of the seed hash is used as the salt.
  final mhSalt = privateKeyHash.bytes.sublist(32);
  final mhBytes = _join([mhSalt, message]);
  final mh = await _sha512.hash(mhBytes);
  final mhL = RegisterL()..readBytes(mh.bytes);

  // Calculate point R.
  final pointR = _pointMul(mhL.toRegister25519(), Ed25519Point.base);
  final pointRCompressed = _pointCompress(pointR);

  // Calculate s
  final shBytes = _join([
    pointRCompressed,
    publicKey.bytes,
    message,
  ]);
  final sh = await _sha512.hash(shBytes);
  final s = RegisterL()..readBytes(sh.bytes);
  s.mul(s, RegisterL()..readBytes(privateKeyHashFixed));
  s.add(s, mhL);
  final sBytes = s.toBytes();

  // The signature bytes are ready
  final result = Uint8List.fromList(<int>[
    ...pointRCompressed,
    ...sBytes,
  ]);

  return Signature(
    result,
    publicKey: publicKey,
  );
}