Question

I need to encrypt a text string in a .net application using a known text key, and then decrypt it in Ruby using the same known key…however I’m having trouble making this happen. I think it has to do with string encoding issues. .Net to Ruby RC4 examples have proven to be elusive.

I’m receiving and invalid decryption on the Ruby side. The encryption/decryption works fine on the .net implementation. But when I copy the encrypted value over to the Ruby implementation and use the same key, I don’t get my original value.

Below is my .net RC4 implementation (this does not require the highest level of security, but some is nice) 🙂

On the ruby side I’m using ruby-rc4 https://github.com/caiges/Ruby-RC4

       public string keytext = "thisismykey";
    public Form1()
    {
        InitializeComponent();
    }
    public void RC4(ref Byte[] bytes, Byte[] key)
    {

        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];
        }
    }
    static byte[] GetBytes(string str)
    {
        Byte[] bytes = new byte[str.Length * sizeof(char)];
        System.Buffer.BlockCopy(str.ToCharArray(), 0, bytes, 0, bytes.Length);
        return bytes;
    }
    static string GetString(byte[] bytes)
    {
        char[] chars = new char[bytes.Length / sizeof(char)];
        System.Buffer.BlockCopy(bytes, 0, chars, 0, bytes.Length);
        return new string(chars);
    }
    private void button1_Click(object sender, EventArgs e)
    {
        Byte[] content = Encoding.ASCII.GetBytes(textBox1.Text);
        RC4(ref content, Encoding.ASCII.GetBytes(keytext));
        textBox2.Text = Encoding.ASCII.GetString(content);
        RC4(ref content, Encoding.ASCII.GetBytes(keytext));
        label1.Text = Encoding.ASCII.GetString(content);
    }

Answer 1

The problem is likely occurring because ASCII encoding is 7-bit and has trouble recreating characters for values above 127. Try encoding with UTF-8 or convert the byte array to a Base64 string.