Handling special characters like =,? while encrpting/decrypting querystring

I am using the below code for querystring encryption and decryption.

My problem is that while encrypting the method inserts special characters like '=', '?' in the encoded string which the decrypt program is unable to handle. It also gives an issue with the length of the encoded string. Can anybody let me know on how to handle this?

I need this urgently.
using System;
using System.Data;

using System.Configuration;

using System.Web;

using System.Web.Security;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Web.UI.WebControls.WebParts;

using System.Web.UI.HtmlControls;

using System.Security.Cryptography;

using System.IO;

using System.Text;

 

public class Encryption

{

string myKey = "somekeyhere";

TripleDESCryptoServiceProvider cryptDES3 = new TripleDESC ...