Captcha Implementation in sap enterprise portal 7.3 Logon Page

Hi ,

We are trying to implement CAPTCHA in portal 7.3 Logon Page.

we are not able to restrict login , when condition return false by using below code. please suggest how to restrict login when captcha is invalid

Jsp Code:

<inputstyle="height:4ex; font-weight:bold; color:black; background-color:#edf2f2;"  class="urBtnStdNew"type="submit"name="uidPasswordLogon"value="Login"onclick="ChkKeyboard(false);ValidCaptcha();"/>

Java script:

function DrawCaptcha()
    {
        var a = Math.ceil(Math.random() * 9)+ '';
        var b = Math.ceil(Math.random() * 9)+ '';      
        var c = Math.ceil(Math.random() * 9)+ ''; 
        var d = Math.ceil(Math.random() * 9)+ ''; 
        var e = Math.ceil(Math.random() * 9)+ ''; 
        var code = a + ' ' + b + ' ' + ' ' + c + ' ' + d + ' ' + e;
        
        document.getElementById("txtCaptcha").value = code;
    }

     
    var captcha_field;

    function ValidCaptcha(){
         
        var str1 = removeSpaces(document.getElementById('txtCaptcha').value);
        var str2 = removeSpaces(document.getElementById('txtInput').value);
        var struser = removeSpaces(document.getElementById('logonuidfield').value);
        var strpass = removeSpaces(document.getElementById('logonpassfield').value);
       
        if ((struser!=null||strpass!=null)&&(str1==str2))
        {
            return true;  
        }

      else
         {

          // captcha invalid
            window.alert("Conditionfailed");
       return false;
  }
     }

   // Remove the spaces from the entered and generated code
    function removeSpaces(string)
    {
        return string.split(' ').join('');
    }

Regards,

Srinivas