RSA cryptography in PHP (How To?)

Being security is one of the utmost considerations in current web site/ application development process, I am sure you have spent a lot of time writing codes to handle encryption in your applications.

In this article I am trying to present one of my favorite ways to solve this issue, the RSA encryption/ decryption handling in your PHP development.

This is quite straight forward as I am using PHPSecLib package. I was using openSSL library for PHP for few years and recently started dealing with this package. It is pretty cool implementation so I started loving it.

First of all you need to get the package, it is available to download[1] and it comes with MIT license[2], GPL compatible[3].

If your intention is to use PHPSecLib only for RSA encryption and decryption I suggest including only two directories which are Crypt and Math in your production environment.

First step towards the RSA encryption is to create the public and private keys. Note that I have secured the keys with a passphrase.

$rsa = new Crypt_RSA();
$rsa->setPassword('pa$$wrd5');
$keys=$rsa->createKey(1024);

echo $keys['privatekey'];
echo $keys['publickey'];

The code is self explaining. createKey() method is taking the bit value of the key and output an array which contains the Private and Public keys. setPassword() is optional, you can omit it if you dont want to create the keys with a passphrase. If you want to use 2048 bit key then provide 2048 as an argument in createKey() instead 1024.

Once keys are ready you can start the encryption. I am using using the public key. Passphrase is not required when encrypting.

$rsa->loadKey($keys['publickey']);
$plaintext = 'Text to be transmitted securely !!!';
$ciphertext = $rsa->encrypt($plaintext);
echo $ciphertext;

Decryption goes as follow (using the private key), note that the passphrase is mandatory:

$rsa->loadKey($keys['privatekey']);
$rsa->setPassword('pa$$wrd5');
$re_plaintText =  $rsa->decrypt($ciphertext);
echo $re_plaintText;

I have written a static class to demonstrate how encryption can be carried out. Please check the following:

***DO NOT use this code in your production env as the code lacks lot of fine tuning and security measures***

This is just to illustrate how the PHPSecLib can be used in a code


<?PHP

include('Crypt/RSA.php');

class myRSA
{
	public static $privateKey = '';
	public static $publicKey = '';
	public static $keyPhrase = '';
	
	public static function createKeyPair()
	{
		$rsa = new Crypt_RSA();
		$password = base64_encode(sha1(time().rand(100000,999999)));
		$rsa->setPassword($password );
		$keys=$rsa->createKey(2048);		
		myRSA::$privateKey=$keys['privatekey'];
		myRSA::$publicKey=$keys['publickey'];
		myRSA::$keyPhrase=$password;
	}

	public static function encryptText($text)
	{
		$rsa = new Crypt_RSA();
		$rsa->loadKey(myRSA::$publicKey);
		$encryptedText = $rsa->encrypt($text);
		return $encryptedText;
	}

	public static function decryptText($encryText)
	{
		$rsa = new Crypt_RSA();
		$rsa->setPassword(myRSA::$keyPhrase);
		$rsa->loadKey(myRSA::$privateKey);
		$plaintext = $rsa->decrypt($encryText);
		return $plaintext;
	}
}
?>

<?php

//create keys
myRSA::createKeyPair(1024);

//Text to encrypt
$text = "A secret lies here, send the text via a secure mode";
echo 'Text : '.$text;

$secureText = myRSA::encryptText($text);
echo 'Encrypted : '.$secureText;

$decrypted_text =  myRSA::decryptText($secureText);
echo 'Decrypted Text : '.$decrypted_text;
?>

PHPSecLib API Documentation is available here.[4] A handy reference to check when you get stuck or need more info on methods.

[1] http://phpseclib.sourceforge.net/index.html

[2] https://en.wikipedia.org/wiki/MIT_License

[3] https://www.gnu.org/licenses/license-list.html#X11License

[4] https://api.phpseclib.org/master/

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CAPTCHA with no $_SESSION

Due to the nature of the stateless behavior of HTTP, managing the current state of a connected user is a tricky scenario to handle in web site development or rather in web application development.

Few solutions are in place already and $_SESSION is one of the ways available in PHP. $_SESSION, the global array is not my favorite choice but it is handy. The sever based session management approach is not 100% reliable but it would do the work in most of the cases.

On the other hand, distinguishing user inputs versus inputs from malicious bots seems quite challenging these days. Simple tricks such as “honeypot” are quite old and easy to overcome, of course bots are now capable enough to skip “honeypots” without much effort. Then comes much promising solutions such as Google’s new invention, reCAPTCHA where a sophisticated techniques can identified the origin of the inputs blocking spams on your site, web application etc.

However life is not that easy, there are plenty of scenarios where we need to come up with our own strategy to deal with these inputs. If you cannot use reCAPTCHA or any other 3rd party CAPTCHA solution then the best would be implementing one of your own. This is the riskiest but there can be instances that this is the only way forward. I was in such a situation few months back and I though of sharing how I overcame it.

Avoiding $_SESSION

It is not hard to find plenty of tutorials to follow, implementing simple CAPTCHA verification in PHP. Almost every solution is based on $_SESSION, using server based sessions, a global array in PHP. This is easy and simple. Few lines of code would do the trick. As same as the way mentioned in this article. I am not a fan of server based sessions therefore I wanted to skip it.

In this post I am trying to explain how I solved the stateless issue without using server based sessions.

Breaking the Problem

The main issue that we need to solve is identifying the legitimate requests/ inputs and filter out the rest. In order to achieve this, when the relevant input form has been requested by a client, the server adds an image which is a human readable text. Bots may not be able to identify what is in it. Then the user inputs the code and sends the request to the server. As it is not possible to include the verification code in the client side request, the server doesn’t know what has been sent earlier. That is where server based sessions comes for the rescue. It is possible to store the verification code in a session variable.  Then when the user has submitted the form, the server can simply checks if the entered verification code value matches the value in the session. If so it is a human, test passes. Since we are trying to omit using server based sessions, we need to come up with a way to identify what has been sent earlier.

Solution

Step 1

Following is a typical code snippet (captcha.php) to create the CAPTCHA image. It is pretty straight forward and it can be seen that the verification code has been assigned to the session variable, $_SESSION[‘rand_code’] = $string;

<?php
session_start();

$string = '';

for ($i = 0; $i < 5; $i++) {     // this numbers refer to numbers of the ascii table (lower case)
$string .= chr(rand(97, 122)); }   
$_SESSION['rand_code'] = $string;   
$dir = 'fonts/';   
$image = imagecreatetruecolor(170, 60); 
$black = imagecolorallocate($image, 0, 0, 0); 
$color = imagecolorallocate($image, 200, 100, 90); // red 
$white = imagecolorallocate($image, 255, 255, 255);   imagefilledrectangle($image,0,0,399,99,$white); 
imagettftext ($image, 30, 0, 10, 40, $color, $dir."arial.ttf", $_SESSION['random_code']);   
header("Content-type: image/png"); 
imagepng($image); 
?>

Following shows how it has been included in the HTML, client side.

<form action="<?php echo $_SERVER['PHP_SELF']; ?>
" enctype="multipart/form-data" method="post"><input name="name" type="text" />

<input name="email" type="text" />

<textarea name="message"></textarea>

<img src="captcha.php" />

<input name="code" type="text" />

<input name="submit" type="reset" value="Send" />
</form>

When analyzing the HTML code, you will soon realize that the code can be exploited simply with a Cross Site Scripting (XSS) attack as it barely uses $_SERVER[‘PHP_SELF’] in form action. I will not be focusing on that since I am not going to use sessions here. Following is the modified prototype version.

Step 2

I have changed the client side code as follow. Notice that a hidden filed has been introduced. A randomly generated number with hash coded act as the reference id here. The reference has been passed when creating the CAPTCHA image and also it gets submitted when the user submits the form.

<?PHP
require_once 'dataBaseConnection.php';
<form action="verify.php" enctype="multipart/form-data" method="post">
Name: <input name="name" type="text" />

Email <input name="email" type="text" />

Message
<textarea name="message"></textarea>

<!--?PHP   $captchaId = sha1(rand(1000000,9999999).time());  dataBaseConnection::registerReference($captchaId);  $path='captcha.php?ref='.$captchaId;   ?--> <img src=""<?PHP" /> "/>
Enter the above code
<input name="c_id" type="hidden" value="<?PHP echo $captchaId; ?>" />

<input name="code" type="text" />

<input name="submit" type="submit" value="Send" />

and then the captcha.php has been modified too.

<?php
$string = '';
$refCode = '';

if(isset($_GET['ref']))
{
$refCode = $_GET['ref'];
}else{
die('<error>NO REF CODE FOUND !</error>');
}

for ($i = 0; $i < 8; $i++)
{
// this numbers refer to numbers of the ascii table (lower case)
$string .= chr(rand(97, 122));
}

$dir = 'fonts/';

$image = imagecreatetruecolor(170, 60);
$black = imagecolorallocate($image, 0, 0, 0);
$color = imagecolorallocate($image, 10, 10, 10); // red
$invColor = imagecolorallocate($image, 200, 200, 200); // invisible_ink
$white = imagecolorallocate($image, 255, 255, 255);

imagefilledrectangle($image,0,0,399,99,$white);
for($i=0; $i<100; $i++) 
{ 
imagettftext ($image, 20, rand(0,10), 0, $i*10, $invColor, $dir."ts.ttf", 'xxxxxxxxxxxxxxxxxxxxxxxxx'); 
} 
imagettftext ($image, 30, 2, 10, 40, $color, $dir."ts.ttf", $string);
header("Content-type: image/png"); 
imagepng($image); 
require_once 'dataBaseConnection.php'; dataBaseConnection::addVerificationCode($refCode, $string); //store the reference and the code in the database 
?>

The static methods dataBaseConnection::registerReference stores the reference code which will later be an input to captcha.php. The system automatically logs the timestamp and set the status of the record to ‘CREATED’. Then in the captcha.php file static method dataBaseConnection::addVerificationCode adds the generated verification code to the relevant reference .

Step 3

Then comes the verify.php code which validates the entered code against the code in the database.

<?php require_once 'dataBaseConnection.php'; if(isset($_POST['submit']))   {  	$enteredCode=trim($_POST['code']);  	$referenceCode=trim($_POST['c_id']);     $dataSet = dataBaseConnection::getCode($referenceCode);     $timeTaken = time() - $dataSet['pvt_created_date'];     if($timeTaken>$dataSet['pvt_life_time'] || $dataSet['pvt_captcha_status']!='CREATED')
    {
    	die('expired code');
    }

    //var_dump($referenceCode); die($enteredCode);
	if($enteredCode==$dataSet['str_verification_code'])
	{
		dataBaseConnection::updateCode($referenceCode,'VERIFIED');
		echo 'verified';//this is a human
		//Process the input as this is a legit request
	}else{
		dataBaseConnection::updateCode($referenceCode);
		echo 'not verified';//most probably not a human
	}
}
?>

If both codes match then the static method dataBaseConnection::updateCode changes the status of the verification code to ‘VERIFIED’ in the database while the status has been set to ‘USED’ for the verification code if they dont match. That expires the verification code and it will not be possible to use it again. Further there is a check to make sure that the code has not been expired too.

How secure this is

It is not difficult to train a bot to reading CAPTCHAs. Therefore to make this much stronger it is required to have an image with higher entropy.

Another weak point is, the reference code is getting exposed to the attacker. However there reference number has no relationship with the code. Therefore the attacker cannot predict the code by cracking the reference code. This is possible as we store the reference and the verification code in the database.

An attacker cannot use a brute force attack as the status of the verification code has been updated after an attempt has been made disregarding the results. So once tried the code is set to expired. Setting lifetime of the code can be used to limit the time available for the attacker to crack the image. In this case 300 seconds.

Further it is required to restrict direct access to captcha.php file from the outside, simply using .htaccess entry. Otherwise it is possible to carry out a Denial-of-service attack targeting the generation of the captcha image which in return could have lead to a database failure.

It is important to track client details but will discuss it in another post.

Conclusion

How to implement a captcha verification solution without using server based sessions has been discussed above. Complete source code can be found here. Please check it out and let me know any flaws you observe. This is a mere implementation of the concept and no other aspects were considered when developing the code and no proper testing has been carried out. Therefore if you are going to use it in your code please be careful unless you know what you are doing.

__construct() in PHP, OOP with PHP

In PHP it is possible to use the construct method which is really useful, in java we can have several constructor methods and the rule is the method name should be as same as the class name. In PHP the rule is quite different. It uses a special method __construct which is called once an object is created or simply when new operator is used. Therefore this method is also known as “magic method” and it is one of the few magic methods in PHP.

Without further text, following would be an example of using constructor method with in PHP OOP.

class Cube1 {

	private $length;
	private $depth;
	private $height; 
	
	public function __construct() {
		$this->length = 10;
		$this->depth = 10;
		$this->height = 10;
	}
	
	public function getVolume() {
		return ($this->length * $this->depth * $this->height);
	}
}

$box=new Cube1();
echo $box->getVolume();

This looks really simple. How would it be if we need to have more than one constructor methods which can accept arguments.

class Cube1 {

	private $length;
	private $depth;
	private $height; 
	
	public function __construct($a=10,$b=10,$c=10) {
		$this->length = $a;
		$this->depth = $b;
		$this->height = $c;
	}
	
	public function getVolume() {
		return ($this->length * $this->depth * $this->height);
	}
}

$box=new Cube1(10,2.5);
echo $box->getVolume();

$box2=new Cube1(2);
echo $box2->getVolume();

Since in PHP the __construct method cannot be override directly as in Java but there are few techniques to use so that it is possible in the indirect way. Will discuss them in the next post.

It is up to you to use your imagination and creativity to make use the of this feature.

Hope this is the time to discuss the PHP Hack Challenge I and at the moment we have discussed this in the class. For others benefit who read my blog I think of noting the discussion.

If you find any other way of smart thinking please let me know, a simple comment will do it.

Code

<?php $_F=__FILE__;$_X='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';eval(base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9SPWVyZWdfcmVwbGFjZSgnX19GSUxFX18nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw=='));?>

As the first step we should identified what are in the code snippet. We can use ; to separate lines. May be then we can see things clearly.

<?php
$_F=__FILE__;
$_X='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';
eval(base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9SPWVyZWdfcmVwbGFjZSgnX19GSUxFX18nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw=='));
?>

now you can see things more clear way2 variables and then eval() is used with base64_decode(). Therefore we can use base64_decode() and see what is in the eval()

if you run following snippt you can get the code encoded by base64.

<?PHP
$txt= base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9SPWVyZWdfcmVwbGFjZSgnX19GSUxFX18nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw==');
echo $txt;
?>

The output would be

$_X=base64_decode($_X);$_X=strtr($_X,'123456aouie','aouie123456');$_R=ereg_replace('__FILE__',"'".$_F."'",$_X);eval($_R);$_R=0;$_X=0;

again if we separate them using ; , following would be the result,

$_X=base64_decode($_X);
$_X=strtr($_X,'123456aouie','aouie123456');
$_R=ereg_replace('__FILE__',"'".$_F."'",$_X);
eval($_R);$_R=0;$_X=0;

Then if you check the original code you will see two variable $_F and $_X are also available.

$_X is base 64 encoded, will decode and see what it contains.

This is the output.

?>		<d4v cl1ss="cl51r"></d4v>
	</d4v><!-- /c2nt14n5r -->

	<d4v 4d="f22t5r">

<sp1n 4d="bl2g-n1m5"><?php bl2g4nf2('n1m5');?></sp1n> <?php bl2g4nf2('d5scr4pt42n'); ?> C2pyr4ght &c2py; <?php 5ch2 d1t5('Y');?> - <1 hr5f="http://t2pwpth5m5s.c2m" t4tl5="W2rdpr5ss Th5m5s" t1rg5t="_bl1nk">W2rdpr5ss Th5m5</1> d5v5l2p5d by <1 hr5f="http://www.w5bh2st4ngf1n.c2m" t4tl5="W5b H2st4ng F1n" t1rg5t="_bl1nk">W5b H2st4ng F1n</1>
	</d4v><!-- /f22t5r -->
</d4v><!-- /wr1pp5r -->
</d4v><!-- /b2dy-4n -->
<?php wp_f22t5r(); ?>
</b2dy>
</html>

Therefore the $_X contains a HTML code. The next thing is to deal with the strtr() function. The beauty of this function is it replace 1 with a, 2 with o, 3 with u like that.

$_X=strtr($_X,'123456aouie','aouie123456');

Therefore after running this function the $_X will be converted as below.

?>		<div></div>
	</div><!-- /container -->

	<div id="footer">

<span id="blog-name"><?php bloginfo('name');?></span> <?php bloginfo('description'); ?> Copyright &copy; <?php echo date('Y');?> - <a href="http://topwpthemes.com" title="Wordpress Themes" target="_blank">Wordpress Theme</a> developed by <a href="http://www.webhostingfan.com" title="Web Hosting Fan" target="_blank">Web Hosting Fan</a>
	</div><!-- /footer -->
</div><!-- /wrapper -->
</div><!-- /body-in -->
<?php wp_footer(); ?>
</body>
</html>

Above is the code that tried to hide using base64 encode and use eval to execute the code.

__FILE__ is called as Magic Constants and I have discussed them earlier in this blog.

However in this code snippt it is easy to breat it as you can see the base64_decode function if that function cant be seen then you are in a bit trouble. In such cases you have to examine the encoded text. Most of the time if you can see = or == at the end of the text, that can be encoded using base64. So it is better to try it first.

Apparently think what are the further actions that can be taken to increase the quality of the code.

PHP Hack Challenge I

As I have promised in the Wed Development class I have added the PHP code that should be hacked. Use comments to discuss the related stuffs.

“I have no idea what this is; so can you please show me what is this?”

If anyone find out the answer put it as a comment and nobody can see it until the next class. Lets discuss this on the next class before that hack it. I have a price, open to Web Development class students.

Challenge: if you are smart you can do it within, ok lets say 15 mins. 😉

Language is PHP.

<?php $_F=__FILE__;$_X='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';eval(base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9SPWVyZWdfcmVwbGFjZSgnX19GSUxFX18nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw=='));?>

Web Security – The Big Picture

These days I am into studying about Web Security(under the Web Security course module of the Northubria University- MIS) and found it is so interesting. Therefore I think of writing a little about the Web Security. Being specific I try to write appropriately to PHP.

In world wide web, this can be found as PHP hack and I was interested in getting knowing how the PHP hack can be eliminated. Knowing those are very important and that will help you to design and develop a PHP hack free web site if your scripts are in PHP. When reading keep in your mind that I am new to those stuffs.

There is no use of talking about SQL Injection attacks as nobody now use eval to process form’s data. Therefore for the time being I skipped that and if you are interesting enough please google.

Forms fields are an important place where the developers should have good attention. For an instance lets consider the following situation. A field where use to enter the address of the user. The entered values are checked against a length and if the ore0defined length is exceeded it is considered that the input is valid. However this seems pretty enough to verify that the user enter his address as long as the address doesnt carry and defined format. Bare mind I am talking about his snail mail address. Even tough it is validating the input it doesnt verify that the input is risk free.

What happen if a user added some HTML or Javascript code, then the code is stored in the database and rendered when data is requested. That can lead to serious problems. Therefore the possibility to enter markups or codes should be eliminated.

With no sofisticated functions that can be achieved using simple str_replace. Following code will do what is needed.

str_replace(‘<','',$_POST['address']);

If you like following function are also in your side. Please try them and see.

strip_tags()
nl2br()
htmlspecialchars()

Further please do not use eval() function if you dont have much understanding about the function or related security risk. My free advise is use eval if you have no choice ONLY. Not only that but also following functions as well, system(), passthru() and exec().

Another important function is escapeshellarg(). There any ', ; or " is replaced with \' , \" , \;

Avoid using GET to send form data and use POST method instead. However in some situation POST is not possible, unless in such situations always try to use POST method for form data submission.

With experience I knew that most of the developers limit the size of the form in HTML interface and never try to check it again in the server side. They use Javascript to check the length on the client side and never check it again in the server side.

This is not a good practice, each and every test you perform on the client side should be followed on the server side as well. Otherwise intruders can bypass the browser and send false info to the server as long as the server is blind and not checking the input, thinking checking them on the client side would be enough. Therefore you need to have better security not only on the client side but mostly on the server side.

Looking forward to write more soon, keep reading . .

How to find the origin?

Today one student from the Web Development class asked me “how web sites are detecting referrals?”, this is a nice question and though of giving the answer publicly so that anyone can get the benefit.

First of all will discuss what this is?

May be you have experienced when you visit to a web site it says where you come from, for an instance assume that you click on a Google search then the site knows that you come through Google. This is a very important thing in SEO as well.

The code behind is very very simple. What you have to do is just use the ‘HTTP_REFERER’ global variable in the code.

You can find an example below.

$referer = $_SERVER[‘HTTP_REFERER’];
echo “You have came though <br/> ” . $referer;

The output would be the URL which is clicked before you come to the site where the above PHP code is.

Bare mind that the out put is a full qualified URL which has http:// or the protocol in front of the URL. Depending on your imagination this code snippet can be used in various applications.