Skip to main content

How CAPTCHA Works? And a Simple Script in PHP

[For this post I'm presuming that you are familiar with CAPTCHA, if not please read this Introduction to CAPTCHA]

CAPTCHA Image Generated by Our ScriptToday we are going to see how CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) works and how it minimizes automatic sign-up of forms. We will also be creating a simple CAPTCHA script in PHP to illustrate this.

Basically CAPTCHA works in the following manner:

  1. Create Random Value: Some random string is generated, random values are often hard to guess and predict.

  2. Generate an Image: Images are used as these are generally a lot harder to read for computers while being nice and readable to humans. This is also the most important step as simple text in images can be read (and CAPTCHA cracked) quite easily. To make it difficult for them, developers employ different techniques so that the text in the image becomes hard to read for computers. Some create zig-zag lines for background while others twist-and-turn individual characters in the image. Possibilities are many and new techniques are being developed all the time as crackers are always into finding ways to break them.

  3. Store it: The random string generated (which is also in the image) is stored for matching the user input. The easiest way to do so is to use the Session variables.

  4. Matching: After the above step, the CAPTCHA image is generated and shown on some form which we want to protect from being abused. The users fills in the form along with the CAPTCHA text and submits it. Now we have the following:

    1. All submitted form data.

    2. CAPTCHA string (from form), input by user.

    3. CAPTCHA string (real one, generated by us), from session variable. Session variable is generally used as it can keep stored values across page requests. Here, we needed to preserve stored values from one page (form page) to another (action page-that receives form data).

  5. If both match, it's okay otherwise not, in that case we can give the user a message that the CAPTCHA they had entered was wrong and their form could not be submitted. You could also ask them to verify it again.

The following image might illustrates this better:

CAPTCHA Generation and Matching
How CAPTCHA is Generated and Matched

From the above image it's quite clear that when someone requests the form page, the CAPTCHA text is generated and sent back to requesting user, but only in the form of an image. If the requester is a human he'd not have much difficulty reading the image and inputting the text when asked but if it's a bot it might face difficulties guessing whats in the image. In the next step when we match the string generated and the one the user had input, we can restrict automated form submissions.

The following is the code that does this, it'll just output the CAPTCHA image to the browser when the script is requested:

<?php
/********************************************************
 * File:        captcha.php                             *
 * Author:      Arvind Gupta (www.arvindgupta.co.in)    *
 * Date:        12-Mar-2009                             *
 * Description: This file can be embedded as image      *
 *              to show CAPTCHA/                        *
 ********************************************************/

// The number of characters you
// want your CAPTCHA text to have
define('CAPTCHA_STRENGTH', 5);

/****************************
 *        INITIALISE        *
 ****************************/
// Tell PHP we're going to use
// Session vars
session_start();

// Md5 to generate the random string
$random_str md5(microtime());

// Trim required number of characters
$captcha_str substr($random_str, 0, CAPTCHA_STRENGTH);

// Allocate new image
$width = (CAPTCHA_STRENGTH * 10)+10;
$height 20;

$captcha_img =ImageCreate($width$height);

// ALLOCATE COLORS
// Background color-black
$back_color ImageColorAllocate($captcha_img000);

// Text color-white
$text_color ImageColorAllocate($captcha_img255255255);

// Line color-red
$line_color ImageColorAllocate($captcha_img25500);

/****************************
 *     DRAW BACKGROUND &    *
 *           LINES          *
 ****************************/
// Fill background color
ImageFill($captcha_img00$back_color);

// Draw lines accross the x-axis
for($i = 0; $i <
$width; $i += 5)
    
ImageLine($captcha_img, $i, 0, $i, 20, $line_color);

// Draw lines accross the y-axis
for($i = 0; $i < 20; $i += 5)
   
ImageLine($captcha_img, 0, $i,
$width, $i , $line_color);

/****************************
 *      DRAW AND OUTPUT     *
 *          IMAGE           *
 ****************************/
// Draw the random string
ImageString($captcha_img552$captcha_str$text_color);

// Carry the data (KEY) through session
$_SESSION['key'] = $captcha_str;

// Send data type
header("Content-type: image/jpeg");

// Output image to browser
ImageJPEG($captcha_img);

// Free-Up resources
ImageDestroy($captcha_img);
?>

Okay, this it for this, in the next one we'll integrate this CAPTCHA script into one form and see how it works. Till then goodbye!

Popular posts from this blog

Fix For Toshiba Satellite "RTC Battery is Low" Error (with Pictures)

RTC Battery is Low Error on a Toshiba Satellite laptop "RTC Battery is Low..." An error message flashing while you try to boot your laptop is enough to panic many people. But worry not! "RTC Battery" stands for Real-Time Clock battery which almost all laptops and PCs have on their motherboard to power the clock and sometimes to also keep the CMOS settings from getting erased while the system is switched off.  It is not uncommon for these batteries to last for years before requiring a replacement as the clock consumes very less power. And contrary to what some people tell you - they are not rechargeable or getting charged while your computer or laptop is running. In this article, we'll learn everything about RTC batteries and how to fix the error on your Toshiba Satellite laptop. What is an RTC Battery? RTC or CMOS batteries are small coin-shaped lithium batteries with a 3-volts output. Most laptops use

The Best Way(s) to Comment out PHP/HTML Code

PHP supports various styles of comments. Please check the following example: <?php // Single line comment code (); # Single line Comment code2 (); /* Multi Line comment code(); The code inside doesn't run */ // /* This doesn NOT start a multi-line comment block /* Multi line comment block The following line still ends the multi-line comment block //*/ The " # " comment style, though, is rarely used. Do note, in the example, that anything (even a multi-block comment /* ) after a " // " or " # " is a comment, and /* */ around any single-line comment overrides it. This information will come in handy when we learn about some neat tricks next. Comment out PHP Code Blocks Check the following code <?php //* Toggle line if ( 1 ) {      // } else {      // } //*/ //* Toggle line if ( 2 ) {      // } else {      // } //*/ Now see how easy it is to toggle a part of PHP code by just removing or adding a single " / " from th

Designing a Simple Order Form Application in PHP

Ok guys, for this post we’re going to create a simple yet complete order form page. Order forms are used on many sites to take customers order online. Order forms should have the capability to take orders from visitors regarding what items they want to purchase and store the information for further processing. For this post’s example, we are going to create an order form for a Book Seller. The form will be designed to take order of five different items (books). Our order form application should be able to take order of five different items in any separate quantities tht user wants, it should also ask for shipping address and name of the customer. It should then store the information provided in a file along with the date and time order was placed. The application should also be able to take any number of orders and store them all linearly for further human processing. For this, we need a front end of a HTML form to which the user would interact a