Question

I’ve been working on converting a C++ crypting method to C#. The problem is, I cant get it to encrypt/decrypt the way I want it to.

The idea is simple, I capture a packet, and decrypt it. The output will be:
Packet Size – Command/Action – Null (End)

(The decryptor cuts off the first and last 2 bytes)

The C++ code is this:

// Crypt the packet with Xor operator
void cryptPacket(char *packet)
{
    unsigned short paksize=(*((unsigned short*)&packet[0])) - 2;

    for(int i=2; i<paksize; i++)
    {
         packet[i] = 0x61 ^ packet[i];
    }
}

So I thought this would work in C# if I didn’t want to use pointers:

public static char[] CryptPacket(char[] packet)
{
    ushort paksize = (ushort) (packet.Length - 2);

    for(int i=2; i<paksize; i++)
    {
        packet[i] = (char) (0x61 ^ packet[i]);
    }

    return packet;
}

-but it isn’t, the value returned is just another line of rubish instead of the decrypted value. The output given is: ..O♦&/OOOe.

Well.. atleast the ‘/’ is in the right place for some reason.

Some more information:

• The test packet I’m using is this:

Hex value: 0C 00 E2 66 65 47 4E 09 04 13 65 00

Plain text: …feGN…e.

Decrypted: XX/hereXX

X = Unknown value, I cant really remember, but it doesn’t matter.

• Using Hex Workshop you can decrypt the packet this way:
  1. Special Paste the hex value as CF_TEXT, make sure the ‘treat as hexidecimal value’ box is checked.
  2. Afterwards, select everything from the hexidecimal value you just pasted, except the first and last 2 bytes.
  3. Go to Tools>Operations>Xor.
  4. Select ‘Treat data as 8 bit data’ and set value to ’61’.
  5. Press ‘OK’, and you’r done.

That’s all the information I can give at the moment, because I’m writing this off the top of my head.

Thank you for your time.

In case you don’t see a question in this:

It would be great if someone could take a look at the code to see what’s wrong with it, or if there’s another way to do it. I’m converting this code because I’m horrible with C++, and want to create a C# application with that code.

Ps: The code tags and such were a pain, so I’m sorry if the spacing etc. is a little messed up.

Answer 1

I just tried your function and it seems ok:

class Program
{
    // OP's method: http://stackoverflow.com/questions/4815959
    public static byte[] CryptPacket(byte[] packet)
    {
        int paksize = packet.Length - 2;
        for (int i = 2; i < paksize; i++)
        {
            packet[i] = (byte)(0x61 ^ packet[i]);
        }
        return packet;
    }

    // http://stackoverflow.com/questions/321370 :)
    public static byte[] StringToByteArray(string hex)
    {
        return Enumerable.Range(0, hex.Length).
               Where(x => 0 == x % 2).
               Select(x => Convert.ToByte(hex.Substring(x, 2), 16)).
               ToArray();
    }

    static void Main(string[] args)
    {
        string hex = "0C 00 E2 66 65 47 4E 09 04 13 65 00".Replace(" ", "");
        byte[] input = StringToByteArray(hex);
        Console.WriteLine("Input: " + ASCIIEncoding.ASCII.GetString(input));
        byte[] output = CryptPacket(input);
        Console.WriteLine("Output: " + ASCIIEncoding.ASCII.GetString(output));
        Console.ReadLine();
    }
}

Console output:

Input: ...feGN.....
Output: ...../here..
(where '.' represents funny ascii characters)

It seems a bit smelly that your CryptPacket method is overwriting the initial array with the output values. And that irrelevant characters are not trimmed. But if you are trying to port something, I guess you should know what you are doing.

You could also consider trimming the input array, to remove the unwanted characters first, and then use a generic ROT13 method (like this one). This way you have your own “specialized” version with 2-byte offsets inside the crypt function itself, instead of something like:

public static byte[] CryptPacket(byte[] packet)
{
    // create a new instance
    byte[] output = new byte[packet.Length];

    // process ALL array items
    for (int i = 0; i < packet.Length; i++)
    {
        output[i] = (byte)(0x61 ^ packet[i]);
    }

    return output;
}