Bazaar – php example code – part 12 – CAPTCHA login hardening

20.02.202125.12.2020 milan.ciljak

Article focus on implementation of CAPTCHA with tools available in PHP. Separate script generate randomly rotated and by scratches and lines distorted text consist from 6 alphanumeric characters stored in a picture. Also provide hash of generated pass_phrase stored in session variable. Login page read text from verification field, hash them and compare against value stored in session. If password and captcha pass_phrases are as expected. User is validated as logged in user.

Expectation from CAPTCHA hardened login

A CAPTCHA (/kæp.tʃə/, a contrived acronym for „Completely Automated Public Turing test to tell Computers and Humans Apart“) is a type of challenge–response test used in computing to determine whether or not the user is human. ( as is is mentioned in wiki, 25.12.2020).

Most common way how to implement captcha verification, is generating human readable picture containing alphanumeric characters that are displayed on pages with some type of prompt for a data. Our login page expect login name and password, rouge system (robot) can attempt for brute force access gain. Inserting third field for retyping code from provided image is a way how to eliminate automatized mechanism for gaining page access.

Our expectation from way how it will be implemented are:

simple CAPTCHA image generation only with resource available in PHP code
generated CAPTCHas must be hardly readable by captcha OCR software (image rotation, scratches, ping or red color text, random lines)
main captcha code stored separately from login or other pages implementing them
way how to delete used captcha images
quick implementation on login page without large code retyping

Captcha image is implemented in frontpage of login as it display next image.

If user type incorrect text, error message is displayed along with new CAPTCHA image as it is shown on next picture.

captcha.php generating code

Our CAPTCHA images generating ode is stored in separate script. This approach enable further improvements and transfer into other applications.

Leading part of code enable define dimensions of generated image, next variety of alphanumeric characters included in to a code and number of them in one image.

<!– ***************************************************************** –>

<!– PHP code generating verification captcha image –>

<!– ***************************************************************** –>

<!– Vrsion: 1.0 Date: 8. – 9.11.2020 by CDesigner.eu –>

<!– ***************************************************************** –>

<?php

//require_once(‚appvars.php‘); // including variables for database

// if included whole not necessary session_start(); // start the session – must be added on all pages for session variable accessing

// solution using SESSIONS with COOKIES for longer (30days) login persistency

/*if(!isset($_SESSION[‚users_id‘])) { // if session is no more active

if(isset($_COOKIE[‚users_id‘]) && isset($_COOKIE[‚username‘])) { // but cookie is set then renew session variables along them

$_SESSION[‚users_id‘] = $_COOKIE[‚users_id‘];

$_SESSION[‚username‘] = $_COOKIE[‚username‘];

$_SESSION[‚user_role‘] = $_COOKIE[‚user_role‘]; // added for role

}

} */

// important captcha constants

define(‚CAPTCHA_NUMCHARS‘, 6); // number of charakters in CAPTCHA

define(‚CAPTCHA_WIDTH‘, 200); // width of image

define(‚CAPTCHA_HEIGHT‘, 60); // height of image

// Set Correct Path to Font File

$fontPath=’C:\xampp_7_4_2020\htdocs\bazaar\images\courier_new_bold.ttf‘;

// generating passphrase by random numbers

$pass_phrase = „“;

for($i = 0; $i < CAPTCHA_NUMCHARS; $i++ ) {

$pass_phrase .= chr(rand(97, 122));

}

// store the encryption pass-phrase in a session variable

$_SESSION[‚pass_phrase‘] = sha1($pass_phrase);

//create the image

$img = imagecreatetruecolor(CAPTCHA_WIDTH, CAPTCHA_HEIGHT);

//set a white background with black text and gray graphics

$bg_color = imagecolorallocate($img, 255, 255, 255); //white

$text_color = imagecolorallocate($img, 255, 146, 130); //pale red

$graphic_color = imagecolorallocate($img, 64, 64, 64); //darkgray

$graphic_color_noise_red = imagecolorallocate($img, 255, 128, 128); //red noise pattern

$graphic_color_noise_green = imagecolorallocate($img, 128, 255, 128); //green noise pattern

// fill the background

imagefilledrectangle($img, 0, 0, CAPTCHA_WIDTH, CAPTCHA_HEIGHT, $bg_color);

// image edges rectangle drawing

imagerectangle ( $img , 0 , 0, CAPTCHA_WIDTH -1 , CAPTCHA_HEIGHT -1 , $graphic_color );

//draw some random lines

for($i = 0; $i < 5; $i++) {

imageline($img,0, rand() % CAPTCHA_HEIGHT, CAPTCHA_WIDTH, rand() % CAPTCHA_HEIGHT, $graphic_color);

}

//sprinkle in some random green dots

for($i = 0; $i < 1000; $i++) {

imagesetpixel($img, rand() % CAPTCHA_WIDTH, rand() % CAPTCHA_HEIGHT, $graphic_color_noise_green);

}

// draw the pass-phrase string

imagettftext($img, 36, rand(0,10), rand(0, 12) , CAPTCHA_HEIGHT – rand(-5, 5), $text_color, $fontPath, $pass_phrase);

//sprinkle over in some random dots

for($i = 0; $i < 1000; $i++) {

imagesetpixel($img, rand() % CAPTCHA_WIDTH, rand() % CAPTCHA_HEIGHT, $graphic_color_noise_red);

}

// VERY IMPORTANT: Prevent any Browser Cache!! – older approach send by header

// header(„Cache-Control: no-store,

//no-cache, must-revalidate“);

// output the image as PNG using a header;

/* ob_clean();

header(„Content-type: image/jpg“);

imagejpg($img);*/

// creating filename and sending them through session and variable

$imageCaptchafilename = IMAGE_PATH . „captcha“.rand(1,1000).“.png“;

// debug echo $imageCaptchafilename;

$_SESSION[‚imageCaptchafilename‘] = $imageCaptchafilename;

//writting image to png

imagepng($img, $imageCaptchafilename, 5);

//clean up

imagedestroy($img);

Example of generated image for closer look follows

Improved login page with CAPTCHA

Link to generated CAPTCHA image and pass_phrase is available in session variables for login page scripts.

Existing form code is extended for verification field and is followed by CAPTCHA image. After unsuccessfully retyped code, error message is displayed formatted with bootstrap danger style.

Al parts implementing CAPTCH in login page are marked by orange for better understanding and distinguishing them from other text.

<!– ****************************************************************** –>

<!– PHP „self“ code handling login into the bazaar app –>

<!– ****************************************************************** –>

<!– Vrsion: 1.0 Date: 11.10-24.10.2020 by CDesigner.eu –>

<!– ****************************************************************** –>

<?php

require_once(‚appvars.php‘); // including variables for database

require_once(‚captcha.php‘); // including generator of captcha image

session_start(); // start the session

// two variables for message and styling of the mesage with bootstrap

$msg = “;

$msgClass = “;

$usr_username = “;

$usr_passwd = “;

$verified_human_by_CAPTCHA = -1; //

//get info that user is loged in, if not try it looking at cookies

//if(!isset($_COOKIE[‚s‘])) { old solution with cookies

if(!isset($_SESSION[‚users_id‘])) { //new with session variables

if(isset($_POST[‚submit‘])) {

/* Attempt MySQL server connection. */

$dbc = mysqli_connect(DB_HOST, DB_USER, DB_PW, DB_NAME);

// accessing user entered login data

$usr_username = htmlspecialchars($_POST[‚u_name‘]);

$usr_passwd = htmlspecialchars($_POST[‚u_pass‘]);

//implement CAPTCHA pass-phrase verification

$user_pass_phrase = sha1(htmlspecialchars($_POST[‚verify‘]));

$pass_phrase_now = htmlspecialchars($_POST[‚pass_phrase_now‘]);

$imageCaptchafilename_now = htmlspecialchars($_POST[‚imageCaptchafilename_now‘]); // name of current captcha photo file for deletion after usage

if($pass_phrase_now == $user_pass_phrase) {

$verified_human_by_CAPTCHA = 1;

@unlink($imageCaptchafilename_now); // delete captcha file

//debug echo „captcha ok“;

} else {

$verified_human_by_CAPTCHA = 0;

@unlink($imageCaptchafilename_now); // also delete captcha file because new one was created

$msgClass = ‚alert-danger‘;

$msgCAPTCHA = „Your CAPTCHA was written wrong, please correct it and resend.“;

};

if(!empty($usr_username) && !empty($usr_passwd) && $verified_human_by_CAPTCHA) {

// try lookup user database

$usr_passwd_SHA = sha1($usr_passwd);

$sql = „SELECT users_id, username, user_role FROM bazaar_user WHERE username = „.“‚$usr_username'“. “ AND pass_word = „.“‚$usr_passwd_SHA'“ ;

// debug output echo $usr_username;

// echo $usr_passwd;

//echo $usr_passwd_SHA;

$data = mysqli_query($dbc, $sql);

if(mysqli_num_rows($data) == 1) {

// login is ok, set user ID and username cookies and redirect to the homepage

$row = mysqli_fetch_array($data);

//setcookie(‚users_id‘, $row[‚users_id‘]); old solution with cookies

//setcookie(‚username‘, $row[‚username‘]);

$_SESSION[‚users_id‘] = $row[‚users_id‘]; // sloution with sessions

$_SESSION[‚username‘] = $row[‚username‘];

$_SESSION[‚user_role‘] = $row[‚user_role‘]; // added user_role session variable

// new cookies for login persistency that expires after 30 days without logout combination SESSION with COOKIES is awailable

setcookie(‚users_id‘, $row[‚users_id‘], time()+(60+60*24*30));

setcookie(‚username‘, $row[‚username‘], time()+(60+60*24*30));

setcookie(‚user_role‘, $row[‚user_role‘], time()+(60+60*24*30)); // cookie for user_role of loged in user added

$home_url = ‚http://‘. $_SERVER[‚HTTP_HOST‘] . dirname($_SERVER[‚PHP_SELF‘]) . ‚/index.php‘;

header(‚Location:‘. $home_url);

// Free result set

mysqli_free_result($data);

// Close connection

mysqli_close($dbc);

} else {

// urename/ password are incorrect – error meesage is displayed

$msg = „Incorrect username or password. Login denied! „;

$msgClass = ‚alert-danger‘;

}

} else {

// username/ password were not entered – display error message

$msg = „Sorry, you must eneter username and password along with correct CAPTCHA phrase to log in. „;

$msgClass = ‚alert-danger‘;

}

<!– **************************************** –>

<!– HTML code containing Form for submitting –>

<!– **************************************** –>

<!DOCTYPE html>

<html>

<head>

<title> Bazaar login page </title>

</head>

<body>

<?php

require_once(‚headerlogo.php‘);

<a class=“navbar-brand“ href=“index.php“>Bazaar – Login page</a>

</div>

</nav>

<?php if($msg != “): ?>

<br>

<?php endif; ?>

<?php

//if(empty($_COOKIE[‚users_id‘])) { solution with cookies

if(empty($_SESSION[‚users_id‘])) { // solution with sessions

// only show for if session with name users_id does not exist

//echo ‚ <br> ‚;

//echo ‚<p class=“alert alert-danger“>‘ . $msg . ‚</p>‘;

<br>

<br>

<label>Username:</label>

<input type=“text“ onfocus=“this.value='<?php echo isset($_POST[‚u_name‘]) ? “ : “; ?>'“ name=“u_name“ class=“form-control“ value=“<?php echo isset($_POST[‚u_name‘]) ? ‚Please reenter‘ : ‚Login name‘; ?>“>

<label>Password:</label>

<input type=“password“ onfocus=“this.value='<?php echo isset($_POST[‚u_pass‘]) ? “ : “; ?>'“ name=“u_pass“ class=“form-control“ value=“<?php echo isset($_POST[‚u_pass‘]) ? ‚Please reenter‘ : ‚Login name‘; ?>“>

<label for=“verify“>Verification – enter text from image below:</label>

<input type=“text“ onfocus=“this.value='<?php echo isset($_POST[‚verify‘]) ? “ : “; ?>'“ name=“verify“ class=“form-control“ value=“<?php echo isset($_POST[‚verify‘]) ? “ : ‚Enter the CAPTCHA verify code‘; ?>„>

<br>

<?php if(($verified_human_by_CAPTCHA == 0) ): //error messaging if wrong CAPTCHA?>

<br>

<?php endif; ?>

<center> <img src=“<?php echo $imageCaptchafilename ; ?>“ alt=“Verification pass-phrase“ > </center>

<!– ass a hidden is sent sha actualy generated captcha pass-phrase only this way it is producet in same run –>

<!– as a hidden is sentname of captcha file for deletion after use –>

</div>

<br>

</form>

<?php } else {

// successfull login

// cookie solution echo ‚<p class=“alert alert-success“> You are loged in as ‚ . $_COOKIE[‚username‘]. ‚</p>‘;

echo ‚<br>‘;

echo ‚<p class=“alert alert-success“> You are loged in as <em>‘ . $_SESSION[‚username‘]. ‚</em></p>‘; // session solution

echo ‚<p class=“alert alert-success“> If you will logout or login with anither credentials, please first <a href=“logout.php“>logout!. </a></p>‘;

}

</div>

<?php // footer include code

require_once(‚footer.php‘); // including footer

generate_footer(580); // function from footer.php for seting width, you can use 580 and 1060px width

</body>

</html>

As it was mentioned before we must have way how to signal generated codes for verification, they are stored in session variable as pass_phrase, also image name. Second problem is, how to deleted unused images. Our implementation is simple, but if app run with many connected clients then in a short time can by generated many pictures. There is small posibility for generate one with same numbering part. Wider number is for consideration, also new problems will arisen in heavy loads that we must take in mind.

Conclusion

CAPTCHA verified login provide new level of security for gaining access to our application. Please keep in mind, that verification user and distinguishing them for automatized scripts is now a must have thing. It is a bare minimal for supporting basic level of security for today apps.

Full code of our bazaar education project can be obtained from github here.

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