rsaDecrypt function

Future<Uint8List> rsaDecrypt(RSAPrivateKey privateKey, Uint8List ciphertext)

Implementation

Future<Uint8List> rsaDecrypt(
    RSAPrivateKey privateKey, Uint8List ciphertext) async {
  if (!checkPub(privateKey)) {
    throw "wrong public key";
  }
  var k = (privateKey.n!.bitLength + 7) ~/ 8;
  var hLen = 32;
  if (ciphertext.length > k || k < hLen * 2 + 2) {
    throw "wrong data len";
  }

  var c = bytesToBigInt(ciphertext);

  var m = decrypt(privateKey, c);

  var em = fillBytes(m, k);

  var firstByteIsZero = em[0] == 0 ? 1 : 0;

  var seed = em.sublist(1, hLen + 1);
  var db = em.sublist(hLen + 1);

  // mgf1XOR(db, 0, db.length, h, 0, h.length);
  mgf1XOR(seed, 0, seed.length, db, 0, db.length);
  mgf1XOR(db, 0, db.length, seed, 0, seed.length);

  // The remainder of the plaintext must be zero or more 0x00, followed
  // by 0x01, followed by the message.
  //   lookingForIndex: 1 iff we are still looking for the 0x01
  //   index: the offset of the first 0x01 byte
  //   invalid: 1 iff we saw a non-zero byte before the 0x01.
  int lookingForIndex = 1;
  int index = 0;
  int invalid = 0;
  var rest = db.sublist(hLen);

  for (int i = 0; i < rest.length; i++) {
    var equals0 = rest[i] == 0 ? 1 : 0;
    var equals1 = rest[i] == 1 ? 1 : 0;
    if (lookingForIndex & equals1 == 1) {
      index = i;
    }
    if (equals1 == 1) {
      lookingForIndex = 0;
    }
    if (lookingForIndex & (~equals0) == 1) {
      invalid = 1;
    }
  }

  int checkResult = firstByteIsZero & (~invalid) & (~lookingForIndex);

  if (checkResult != 1) {
    throw "Error RSA decryption";
  }

  return Uint8List.fromList(rest.sublist(index + 1));
}