sign function

MsgSignature sign(Uint8List messageHash, Uint8List privateKey)

Signs the hashed data in messageHash using the given private key.

Implementation

MsgSignature sign(Uint8List messageHash, Uint8List privateKey) {
  final digest = SHA256Digest();
  final signer = ECDSASigner(null, HMac(digest, 64));
  final key = ECPrivateKey(bytesToUnsignedInt(privateKey), params);

  signer.init(true, PrivateKeyParameter(key));
  var sig = signer.generateSignature(messageHash) as ECSignature;

  /*
	This is necessary because if a message can be signed by (r, s), it can also
	be signed by (r, -s (mod N)) which N being the order of the elliptic function
	used. In order to ensure transactions can't be tampered with (even though it
	would be harmless), Ethereum only accepts the signature with the lower value
	of s to make the signature for the message unique.
	More details at
	https://github.com/web3j/web3j/blob/master/crypto/src/main/java/org/web3j/crypto/ECDSASignature.java#L27
	 */
  if (sig.s.compareTo(_halfCurveOrder) > 0) {
    final canonicalisedS = params.n - sig.s;
    sig = ECSignature(sig.r, canonicalisedS);
  }

  final publicKey = bytesToUnsignedInt(privateKeyBytesToPublic(privateKey));

  //Implementation for calculating v naively taken from there, I don't understand
  //any of this.
  //https://github.com/web3j/web3j/blob/master/crypto/src/main/java/org/web3j/crypto/Sign.java
  var recId = -1;
  for (var i = 0; i < 4; i++) {
    final k = _recoverFromSignature(i, sig, messageHash, params);
    if (k == publicKey) {
      recId = i;
      break;
    }
  }

  if (recId == -1) {
    throw Exception(
        'Could not construct a recoverable key. This should never happen');
  }

  return MsgSignature(sig.r, sig.s, recId + 27);
}