How to XOR two strings in C#?
<p>I am making a programme to encrypt a message using a vegnier cypher.
But i am not sure what is wrong with it as when I xor the two variables it just outputs a question mark in a box any help will be appreciated.</p>

<pre><code>    static string xor(string plaintext, string pad)
    {
        string cyphertext = "";
        for (int i = 0; i &lt; (plaintext.Length); i++)
        {
            char p1 = Convert.ToChar(plaintext.Substring(i, 1));
            char c1 = Convert.ToChar(pad.Substring(i, 1));
            char ct1 = (char)(p1 ^ c1);
            cyphertext = cyphertext + (Convert.ToString(ct1));

        }
        return cyphertext;
    }
</code></pre>