CAPTCHA

The current, editable version of this book is available in Wikibooks, the open-content textbooks collection, at
https://en.wikibooks.org/wiki/CAPTCHA

Permission is granted to copy, distribute, and/or modify this document under the terms of the Creative Commons Attribution-ShareAlike 3.0 License.

Why?

There are advantages to using a CAPTCHA:

Preventing spam
Preventing abuse
Slowing abusers down

Usages

User X wants to send email to Y. They go to example.com/mail.php and notice some words. The form says that he has had over 100 spam emails sent via the form.
Spammer S edits a page on wiki W. The wiki had been spammed before. They notice that the page is not saved with their spamming software.

Integration

You can integrate a CAPTCHA into your website or form. Select the way you wish to do it:

Integration/Pre-made

Examples of Pre-made CAPTCHA

hCAPTCHA
reCAPTCHA

Integration/Roll your own

There are ways to roll your own CAPTCHA.

Examples currently covered in this book

Integration/The Anti-CAPTCHA

This is a CAPTCHA killer. It needs nothing on the form except a hidden field and some server-side code.

PHP Example

//... Your lovely form here
// Example, please adapt
echo "<input type=hidden name=url />";
//... Rest of your lovely form here

verification

// compare
if ($_POST['url'] != "") {
 // abort!
}
// processing code here

Integration/The Anti-CAPTCHA/Stronger

This version of the Anti-CAPTCHA is stronger.

Form Code

JavaScript

document.write("<input type=hidden name=code value="+Math.random()+" />");

This must be in your form.

Use in an HTML form:

<script type="text/javascript">
document.write("<input type=hidden name=code value="+Math.random()+" />");
</script>

Server-Side Code

Use this:

<?php

if (!$_POST['code']) {
   // abort
}

?>

Problems with this version

It hinders the usability, as people with JavaScript disabled (for example, for security reasons) won't be able to access your form.

Integration/Images and PHP

image.php

<?php

$width = 50;
$height = 25;

session_start();

unset($_SESSION['code']); // added security

$len = 6; // you can change this
mt_srand(time());
// generate random values
$r = 0;
$g = 0;
$b = 0;
$r = mt_rand(80, 255);
$g = mt_rand(80, 255);
$b = mt_rand(80, 255);

$s = 0;
$h = 0;
$c = 0;
$s = mt_rand(0, 80);
$h = mt_rand(80, 80);
$c = mt_rand(80, 100);

$code = mt_rand(100000,999999);

$size = 0.75 * 40;
$image = imagecreate($width, $height) or die("couldn't generate image");
$bg = imagecolorallocate($image, $s, $h, $c);
$c1 = imagecolorallocate($image, $r, $g, $b);

imagestring($image,2,3,3,$code,$c1);

header('Content-Type: image/png');

imagepng($image);
$_SESSION=$code;
imagedestroy($image);

?>

This is the CAPTCHA itself.

form.php

<?php

echo "<img src='/image.php' /><input name=code />";

?>

This is the form code.

validate.php

<?php

session_start();

if ($_POST['code'] != $_SESSION['code']) {
 //fail
}

?>

This will validate the CAPTCHA.

Integration/Plugins for software

WordPress

There are many CAPTCHA plugins available for WordPress. Search with Google.

DokuWiki

A DokuWiki plugin is available and has many types of CAPTCHAs available.

MediaWiki

ConfirmEdit is a extension for MediaWiki that provides a CAPTCHA and is configurable. It is well known as Wikimedia Foundation projects use them.

CAPTCHA/Printable version