RC4.cs

class RC4
{
    public static byte[] rc4(byte[] input, byte[] key)
    {
        byte[] encrypted = new byte[input.Length];
        int x, y, j = 0;
        int[] box = new int[256];

        for (int i = 0; i < 256; i++) box[i] = i;

        for (int i = 0; i < 256; i++)
        {
            j = (key[i % key.Length] + box[i] + j) % 256;
            x = box[i];
            box[i] = box[j];
            box[j] = x;
        }

        for (int i = 0; i < input.Length; i++)
        {
            y = (i + 1) % 256;
            j = (box[y] + j) % 256;
            x = box[y];
            box[y] = box[j];
            box[j] = x;

            encrypted[i] = (byte)(input[i] ^ box[(box[y] + box[j]) % 256]);
        }

        return encrypted;
    }
}