The final script - PHP

If you're working on a captcha system for your PHP-based website, you may be faced with an interesting challenge. How do you make your system too hard for spam bots to read, but not too hard for humans? This is especially worrying in the wake of bots that can harness OCR for reading captchas. This article explains how to increase the difficulty of a captcha system and test it to make sure it meets your requirements.

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By: Codex-M

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August 02, 2010

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The final script is shown below:

<?php

session_start();

$stringgen = mt_rand(1000, 9999);

$_SESSION['answer']=$stringgen;

$imagecreate = imagecreate(50, 50);

$background = imagecolorallocate($imagecreate, 0, 0, 255);

$textcolor = imagecolorallocate($imagecreate, 255, 255, 255);

for ($c = 0; $c < 200; $c++){

$x = rand(0,50-1);

$y = rand(0,50-1);

imagesetpixel($imagecreate, $x, $y, $textcolor);

}

$xlocation = rand(1,10);

$ylocation = rand(1,10);

imagestring($imagecreate, 5, $xlocation, $ylocation, $stringgen, $textcolor);

header("Content-type: image/png");

$image= imagepng($imagecreate);

Conclusions and Recommendations

There are still a lot of ways to increase the difficulty of captcha, such as distorting the text or having the characters appear connected to each other. However, this system can also substantially increase the difficulty humans will experience in solving it, and even humans can commit frequent mistakes when answering this type of challenge.

As shown in this tutorial, simply increasing the background noise can make the OCR unable to solve the challenge properly without necessarily adding too much difficulty for humans.