Hiding a queryString in an ASP.NET Webapplication

using System;
using System.Web;
using System.Web.Services;
using System.Web.Services.Protocols;

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class AuthenticateUserService : System.Web.Services.WebService
{
    [WebMethod]
    public bool AuthenticateUser(string username, string passhash)
    {
        // Fake authentication for the example
        return (username == "jon" && passhash == "SomeHashedValueOfFoobar");
    }

}

protected void Page_Load(object sender, EventArgs e)
{
    // Now we can easily authenticate user in our code
    AuthenticateUserService authenticationProxy =
         new AuthenticateUserService();
    bool isUserAuthenticated =
         authenticationProxy.AuthenticateUser("jon", SomeHashMethod("foobar"));
}