want code of encryption and decryption

i want to pass '3' encrypt as 'erewwelrjwlkrjwlerjelrjlrjr'+' 3' and decrypt and get only '3' at other page
if pass '18' then get 18 at other end

MD5 is useless
I want encrypt and decrypt strong alpha numeric code

i have currnet code as: but it only encrypt and decrypt less digits at other end , I want more string len in query string in address bar

encry:=> 

string s = "Himanshu", Cypher = " ";
            for (int i = 0; i < s.Length; i++)
            {
                char c = (char)(s[i] + 2);
                Cypher = Cypher + new string(c, 1);
            }
            lblCypher.Text = Cypher;


decry=>

string s = lblCypher.Text, PlainText = "";
        for (int i = 0; i < s.Length; i++)
        {
            char c = (char)(s[i] - 2);
            PlainText = PlainText + new string(c, 1);
        }
        lblPlain.Text = PlainText;

---

-- Himanshu K