RSA C＃ImportParameters“参数不正确”异常

Question

I have a server (python) and a client (c#), I need to communicate between them temporarily using assymetric rsa cryptography. When I connect to the server as the client, I send him my public key and he send me his. I use at the server the rsa library and I get there the server's public key parameters {n,e} I send those and receive them with a space between them. I seperate them and convert the modulus into a BigInteger using this function:

public static BigInteger GetBigInteger(string number)
{
    BigInteger bigNum = new BigInteger();
    for (int i = number.Length; i > 0; i--)
    {
        bigNum *= 10;
        bigNum += (int) number[number.Length-i];
    }
    return bigNum;
}

public static void Connect(IPAddress ipAddress, int port)
{
    try
    {                
        string[] message;
        byte[] data = new byte[1024];
        srvr.Receive(data); //Recieve the server's public key. 
        int length = int.Parse(Encoding.ASCII.GetString(data.Take(4).ToArray()));
        message = Encoding.ASCII.GetString(data.Skip(4).Take(length).ToArray()).Split(' ') ;
        RSACryptoServiceProvider RSAserver = new RSACryptoServiceProvider(1024);
        RSAParameters par = new RSAParameters();
        par.Modulus = GetBigInteger(message[0]).ToByteArray();  // Saves the server's public key.
        par.Exponent = new byte[] { 1, 0, 1 }; // Saves the server's public key.           
        RSAserver.ImportParameters(par);
        addresseeKey = RSAserver.ToXmlString(false);
        ...
    }
    ...
}

An exception is thrown on the ImportParameters line that says: "The parameter is incorrect" . What's wrong?

Answer 1

BigInteger.ToByteArray() exports the data in little-endian order. RSAParameters wants all of its data in big-endian order.

There's also an issue that the modulus value should have its most significant bit set, and therefore BigInteger.ToByteArray() needs to add in a padding byte to prevent the number from being reinterpreted as being negative, so you'll need to trim that off.

Assuming that your python export is representing the modulus as a base-10 positive integer, your code will work (though BigInteger.Parse(string) would be more efficient) once you align the data properly.

byte[] tmp = GetBigInteger(message[0]).ToByteArray();

// Array.Resize (to a smaller number) trims off the high index values,
// so it's slightly more compact code to do this before the resize.
if (tmp.Length > 0 && tmp[tmp.Length-1] == 0)
    Array.Resize(ref tmp, tmp.Length - 1);

Array.Reverse(tmp);
par.Modulus = tmp;