function login() {
                var loginForm = document.getElementById("loginForm");
                if (loginForm.username.value == "") {
                    alert("Voer een gebruikersnaam in.");
                    return false;
                }
                if (loginForm.password.value == "") {
                    alert("Voer uw wachtwoord in.");
                    return false;
                }
                var submitForm = document.getElementById("submitForm");
                submitForm.username.value = loginForm.username.value;
                submitForm.response.value = 
                    hex_md5(loginForm.challenge.value+loginForm.password.value);
                submitForm.submit();
            }