Displaying basic mathematical sums as challenge strings - AJAX

As you know, Ajax is a technology that can be used to perform all sorts of clever tasks; this includes building web forms that are less vulnerable to attacks from malicious web bots. Indeed, it’s pretty simple to develop certain mechanisms that permit the dynamic generation of verification codes via Ajax, which must be entered manually by a user before submitting an HTML form. This is the second part of a four-part series that shows you how to make your web forms safer with Ajax.

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By: Alejandro Gervasio

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March 25, 2009

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Another approach that can be used to protect a targeted web form is based upon displaying elemental sums as challenge strings. Of course, in this case, a user must enter the correct result of this mathematical operation before submitting the form, which can be an interesting variation of the example that I showed you in the previous section.

Fortunately, to implement this alternative security mechanism with a specific HTML form, most of the source files already created will remain practically the same. However, it's necessary to redefine the "get_checkingcode.php" file coded earlier, since it will now be responsible for dynamically displaying the basic sums.

So, first here's the file that renders the sample web form, and also retrieves these challenge mathematical operations from the web server:

(definition of 'sample_form.htm' file)

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<head>

<title>Ajax-based Random Sums Generator</title>

body{

padding: 0;

margin: 0;

background: #fff;

}

h1{

font: bold 16pt Arial, Helvetica, sans-serif;

color: #000;

}

font: bold 9pt Arial, Helvetica, sans-serif;

color: #000;

}

#formbox{

width: 380px;

text-align: right;

padding: 10px;

background: #eee;

}

#codebox{

font: bold 18pt Arial, Helvetica, sans-serif;

color: #00f;

}

.inputbox,textarea{

width: 300px;

border: 1px solid #999;

}

.checkingcode{

width: 50px;

border: 1px solid #999;

}

</style>

$(document).ready(function(){

// get verification code with Ajax

$.get('get_checkingcode.php',{data:'getting code'},function(checkingcode){$('#codebox').html(checkingcode);});

});

</script>

</head>

<body>

<h1>Ajax-based Random Sums Generator</h1>

<p>First Name <input type="text" class="inputbox" title="Enter your first name" /></p>

<p>Email <input type="text" class="inputbox" title="Enter your email address" /></p>

<p>Enter your comments below:</p>

<p>Verification Code: <input type="text" name="code" class="checkingcode" title="Sum the above digits and enter the result" /></p>

</form>

</div>

</body>

</html>

Since this file hasn't been modified, I won't spend time explaining how it works. Instead, I'd like you to pay attention to the following one, which is charged with displaying different sums on the browser that must be correctly calculated by users before submitting the web form. Here is its short signature:

<?php

session_start();

$valuea=rand(1,10);

$valueb=rand(1,10);

$_SESSION['checkcode']=$valuea+$valueb;

echo $valuea.'+'.$valueb;

That was extremely simple to code, wasn't it? As you can see, the above PHP file dynamically generates several mathematical sums, whose results are stored on a session variable to be checked later on. However, the operation itself is sent to the client as a challenge string, meaning that a user will have to enter its correct result before submitting the online form.

At this point, you'll surely have grasped the logic that stands behind this simple protection mechanism. The best way to understand how this Ajax application really works, however, is by showing its complete source code, including the modifications that you saw before.

Thus, this is precisely what I'm going to do in the following section. So, please jump forward and read the next few lines.

Next: The Ajax application's full source code >>