Encryption between Python and C#

I do a lot of work in restricted environments, where I still need to secure internet traffic. I chose to use the RC4 algorithm because it is simple to implement and relatively fast. It is actually easy enough that a simple programmer like myself could write matching Python and C# implementations!

RC4 is also a stream cipher, which means you don't need to worry about breaking your cleartext into blocks, padding them out... and other related pain.

If you do use RC4, you should be aware that it has some vulnerabilities.

Python:

def RC4(data, key):
    x = 0 
    s = range(256)
    for i in range(256):
        x = (x + s[i] + ord(key[i % len(key)])) % 256 
        s[i], s[x] = s[x], s[i]
    x = y = 0 
    out = ""
    for c in data:
        x = (x + 1) % 256 
        y = (y + s[x]) % 256 
        s[x], s[y] = s[y], s[x]
        out += chr(ord(c) ^ s[(s[x] + s[y]) % 256])
    return out

C#:

using System;


public class RC4
{   
    static public void Cipher (ref byte[] bytes, string skey)
    {   
        var key = System.Text.ASCIIEncoding.ASCII.GetBytes(skey);
        byte[] s = new byte[256];
        byte[] k = new byte[256];
        byte temp;
        int i, j;

        for (i = 0; i < 256; i++) {
            s[i] = (byte)i;
            k[i] = key[i % key.GetLength (0)];
        }

        j = 0;
        for (i = 0; i < 256; i++) {
            j = (j + s[i] + k[i]) % 256;
            temp = s[i];
            s[i] = s[j];
            s[j] = temp;
        }

        i = j = 0;
        for (int x = 0; x < bytes.GetLength (0); x++) {
            i = (i + 1) % 256;
            j = (j + s[i]) % 256;
            temp = s[i];
            s[i] = s[j];
            s[j] = temp;
            int t = (s[i] + s[j]) % 256;
            bytes[x] ^= s[t];
        }
    }
}