I use the MachineKey.Protect() method to encrypt the id passed as a query string in my asp.net MVC application.

Here's the code I use to encrypt/decrypt:

public static string Encrypt(this string expression)
{
    if (string.IsNullOrEmpty(expression))
        return string.Empty;

    byte[] stream = Encoding.Unicode.GetBytes(expression);
    byte[] encodedValue = MachineKey.Protect(stream);            
    return HttpServerUtility.UrlTokenEncode(encodedValue);            
}

public static string Decrypt(this string expression)
{
    if (string.IsNullOrEmpty(expression))
        return string.Empty;

    byte[] stream = HttpServerUtility.UrlTokenDecode(expression);
    byte[] decodedValue = MachineKey.Unprotect(stream);
    return Encoding.Unicode.GetString(decodedValue);
}

And, here is the MachineKey element in my web.config file:

    .
    .
    .
    

The problem is the encrypted id is not persistent. Every time I call the method, I get a new encrypted expression. How do I make it persistent?