Symmetric-Key Encryption using openSSL

I wrote few blog posts on Asymmetric Key encryption using PHPSecLib library recently and this blog post is on Symmetric-Key Encryption and I ll be using PHP openSSL extension for the implementation. OpenSSL is a open source library which implements SSL and other major cryptographic functions. It is extensively used in Linux, Windows and other operating systems. Luckily PHP got an extension, so we can straightaway use it.

Instead of PHPSecLib, openSSL can be used to implement RSA cryptography functions in PHP as well.

The major drawback in Symmetric-Key Encryption is the usage of the same key to encrypt and decrypt data. However unlike in Asymmetric-Key encryption (I would say RSA hereafter) there is no limit on the data size that can be encrypted and it is relatively faster than RSA. As a result RSA is used to exchange Symmetric Keys securely and then the client and the server continues to communicate based on symmetric-key encryption. Not only for communication but also we can use symmetric key encryption in data storage too. For an example before storing data on a database, it is a good practice to encrypt those data, in such a scenario symmetric key encryption is much more suitable than RSA.

There are plenty of other libraries which provide symmetric key encryption, however it is recommended to use libraries like openSSL or PHPSecLib as they are extensively tested and well maintained. My advice is to use openSSL as much as possible, however due to limitations on your server, it may not be possible to use openSSL for RSA but it can always be used for symmetric key encryption.

Will check the PHP code for the task. It is simple !

My PHP version is PHP Version 7.0.10 and using OpenSSL/1.0.2h, if you want to check yours you can use phpinfo(); function which gives all information regarding your PHP installation.

<?PHP 
$text = 'This is a secret message that should be encrypted before saving it to the database so that even the database is compromised the attacker wont be able to steal data'; 

$encryption_key = 'password'; $iv = 'randomdigit16bit'; 

$encrypted_data = openssl_encrypt($text, 'aes-256-cbc', $encryption_key, OPENSSL_RAW_DATA, $iv); 

$encrypted_data = base64_encode($encrypted_data); 
var_dump($encrypted_data); 
?>

The code is self explaining, openssl_encrypt is the function that should be used and $text is the data that should be encrypted, then the next parameter is the cipher method, $encryption_key is the key to decrypt the encrypted text. This needs to be stored as it is required later to decrypt the data. I ll discuss about storing keys securely in a future blog post, as it is quite out of the scope for this topic.

Decryption is to be carried out as follow:

<?PHP

$text = 'This is a secret message that should be encrypted before saving it to the database so that even the database is compromised the attacker wont be able to steal data';

$encryption_key = 'password';
$iv = 'randomdigit16bit';

$encrypted_data = openssl_encrypt($text, 'aes-256-cbc', $encryption_key, OPENSSL_RAW_DATA, $iv);

//$encrypted_data = base64_encode($encrypted_data);

$decrypted_data = openssl_decrypt($encrypted_data, 'aes-256-cbc', $encryption_key, OPENSSL_RAW_DATA, $iv);

var_dump($decrypted_data);

?>

Same as the openssl_encrypt function openssl_decrypt function could be used. Note that same $encryption_key and the $iv have been used.

Same as the $encryption_key, $iv should be same in encryption and decryption. $iv is known as the Initialization Vector which adds an extra layer of protection. It can be a random 16 bits string and the value should be stored as it requires in the decryption process as same as the encryption key.

The role of the $iv (Initialization Vector) is to differentiate the encrypted text. For an example the encryption key could be common per user or your web site etc. But you can use a random $iv per each encryption. As a result if you encrypt the same value using the same key, the result is going to be different when you use two different $iv.

Following method can be used to generate 16 bits IV. Make sure you store the IV otherwise decrypted data will not be able to decrypt back.

$iv = openssl_random_pseudo_bytes (16);

That is it, simple and handy isnt it?

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