Encryption algorithm using CTR mode I am using CTR mode (it is a cipher in itself) in this code. I just wanted to see what you thought about it before I finish it.

(Yes, I know that the plaintext length and key length must be highly specific. I will take care of that with padding later.)

I just wanted to know if you thought if this code was cryptographically secure and if I could speed it up a little.

class Encrypt
{
    public string CTR(string p, string key)
    {
        StringBuilder sb = new StringBuilder();

        for (uint c = 0; c &lt; (p.Length / key.Length); c++ )
        {
            char[] kc = key.ToCharArray();
            char[] E = new char[key.Length];

            char[] cc = c.ToString().ToCharArray();
            int inc = 0;
            foreach (char ch in kc)
            {
                E[inc] = (char)(ch ^ cc[0]); // Something goes wrong here when there are 2 digits in uint c. (CodeReview plz help)
                inc++;
            }

            char[] pc = p.ToCharArray();


            for (int i = 0; i &lt; E.Length; i++)
            {
                sb.Append((char)(pc[i] ^ E[i]));
            }
        }

        return sb.ToString();
    }
}
It looks like you are XORing a mildly modified key with the plaintext, then mildly modifying the key every 10 blocks.  As @SimonForsberg has noted, such simplistic schemes just are amateur-grade ciphers, and certainly not cryptographically secure.  (Cryptanalysis would be beyond the scope of Code Review, though.)

Only a few experts are capable of producing cryptographically secure algorithms — and even then, they submit their schemes to merciless peer review.  There's always someone out there who is smarter than you are.  Attacks only get better, never worse.  If you want a cryptographically secure scheme, don't roll your own crypto.