As I stated in the section that you just read, it's also possible to use different mathematical operations as challenge strings when protecting a web form from attacks. In this particular situation, I'm going to demonstrate how to apply this concept specifically to multiplication.
As this article's title suggests, before a user submits a targeted web form, he/she will be asked to enter the correct result of a challenge multiplication, which will be constructed dynamically with Ajax. Thus, I am first going to list the signature of the (X)HTML file that renders a sample HTML form, and that also performs HTTP requests through Ajax. Here it is:
(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 challenge strings 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 challenge strings 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="Multiply the above digits and enter the result" /></p>
<p><input type="submit" value="Send Data"></p>
As you can see, the above (X)HTML file looks nearly identical to what I coded in the previous section, except for some "title" attributes, which have been properly modified. Since you shouldn't have major problems understanding how this file works, it's time to code a new one, which will be responsible for dynamically generating the integer numbers that must be multiplied by users before submitting the web form. Here is the corresponding signature of this file:
echo $valuea.' x '.$valueb;
See how simple it is to create a PHP script that generates multiplication challenges? I bet you do! In this case, the above PHP file will send to the client a couple of random integers, and naturally users will be asked to enter the correct product of the pair before submitting the web form. Pretty simple to understand, right?
Now that you've grasped the logic that drives the previous PHP script, it's time to move on and list the full source code of this simple web form protection system, which generates multiplication equations as challenge strings via Ajax.
Jump ahead and read the section to come. It's only one click away.
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