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">
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Ajax-based Random Sums Generator</title>
font: bold 16pt Arial, Helvetica, sans-serif;
font: bold 9pt Arial, Helvetica, sans-serif;
font: bold 18pt Arial, Helvetica, sans-serif;
border: 1px solid #999;
border: 1px solid #999;
// get verification code with Ajax
<h1>Ajax-based Random Sums Generator</h1>
<form action="check_form.php" method="post">
<p>First Name <input type="text" class="inputbox" title="Enter your first name" /></p>
<p>Last Name <input type="text" class="inputbox" title="Enter your last name" /></p>
<p>Email <input type="text" class="inputbox" title="Enter your email address" /></p>
<p>Enter your comments below:</p>
<p><textarea title="Enter your comments" rows="10" cols="10"></textarea></p>
<p>Verification Code: <input type="text" name="code" class="checkingcode" title="Sum the above digits and enter the result" /></p>
<p><input type="submit" value="Send Data"></p>
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:
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.
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