Encrypt session data in PHP


by Enrico Zimuel

As promised in my last post I present an example of strong cryptography in PHP to secure session data.
This is a very simple implementation that can be used to improve the security of PHP applications especially in shared environments where different users have access to the same resources. As you know, the PHP session data are managed by default using temporary files. In shared environment a malicious user that is able to access to these temporary files can easly read the session data because they are stored in plaintext (data in a session file is theserialization of the array $_SESSION).
In theory, the session data should be stored in folders that are accessible only by the owner of the web site, but never say never (btw, you can manage the location of the session data using the session_save_path function or changing the session.save_path in the php.ini).

To secure the session data I used strong cryptography to encrypt the content using the mcrypt extension of PHP. I chosen the simmetric cipher Rijandel-256 to encrypt the session data and the openssl_random_pseudo_bytes() function to generate a random key of 256 bit.
The idea is to use a cookie variable to store the key that will be used to encrypt the session data. In this way the key is stored only in the client (the browser) and only the client is able to decrypt the session data on the server. Each time we encrypt the session data we re-generate the IV vector in a random way using the mcrypt_create_iv() function. It is very important to generate a unique IV in each encryption. This best practice increase the security of the encryption algorithm.
It’s important to note that this implementation is not secure against session hijacking attack. If someone is able to capture the cookie variable of a client and have access to the temporary session files, in the server, he/she will be able to decrypt the session data. Our goal is to protect session data against attacks on shared environments.

The idea to encrypt the session data is not new, for instance Chris Shiflett proposed an implementation in his book “Essential PHP Security” (O’Reilly, 2006). Shiflett used a $_SERVER variable to store the key used to encrypt the session data. Kevin Schroeder, my colleague at Zend Technologies, implemented a very similar session encryption algorithm extending the Zend_Session class of Zend Framework (you can find it here). In my solution, I used some of the best practices related to strong cryptography to implement a secure session handler.

You can download the source code of my implementation here.

The implementation uses a SecureSession class that you need to inlucde in your PHP project before the session_start function.
After that you can use the PHP session as usual.