Encrypt query string

From http://www.infoexpediters.com/SecureQueryString.cs:

public string encrypt(string serializedQueryString)
    {
        byte[] buffer = Encoding.ASCII.GetBytes(serializedQueryString);
        TripleDESCryptoServiceProvider des = new TripleDESCryptoServiceProvider();
        MD5CryptoServiceProvider MD5 = new MD5CryptoServiceProvider();
        des.Key = MD5.ComputeHash(ASCIIEncoding.ASCII.GetBytes(cryptoKey));
        des.IV = IV;
        return Convert.ToBase64String(
            des.CreateEncryptor().TransformFinalBlock(
                buffer,
                0,
                buffer.Length
            )
        );
    }

    public string decrypt(string encryptedQueryString)
    {
        try
        {
            byte[] buffer = Convert.FromBase64String(encryptedQueryString);
            TripleDESCryptoServiceProvider des = new TripleDESCryptoServiceProvider();
            MD5CryptoServiceProvider MD5 = new MD5CryptoServiceProvider();
            des.Key = MD5.ComputeHash(ASCIIEncoding.ASCII.GetBytes(cryptoKey));
            des.IV = IV;
            return Encoding.ASCII.GetString(
                des.CreateDecryptor().TransformFinalBlock(
                    buffer,
                    0,
                    buffer.Length
                )
            );
        }
        catch (CryptographicException)
        {
            throw new InvalidQueryStringException();
        }
        catch (FormatException)
        {
            throw new InvalidQueryStringException();
        }
    }

Use a POST not a GET, that way it is not visible in the url. Implement some small encryption done on the POST body before sending, and decrypt upon receiving.

I assume you are using javascript on the pages? Provide more info on what languages you are using (php/python etc?) for something more specific (and code)

You may or may not really need encryption, but assuming you do, you can do this in PHP using triple des (or whatever you want) like this:

// Init mcrypt stuff
$descriptor = mcrypt_module_open('tripledes', '', MCRYPT_MODE_ECB, '');
$key = substr(md5('put your secret here'), 0, mcrypt_enc_get_key_size($descriptor));
$vector = mcrypt_create_iv(mcrypt_enc_get_iv_size($descriptor), MCRYPT_RAND);
mcrypt_generic_init($descriptor, $key, $vector);

// Encrypt id
$encryptedEmployeeId = mcrypt_generic($descriptor, $_GET['EmployeeId']);

// Clean up mcrypt
mcrypt_generic_deinit($descriptor);
mcrypt_module_close($descriptor);

The reverse process is similar except using mdecrypt_generic(). Of course, I've made the assumption that you're using PHP :).