End-to-end encryption with Zend Framework 3

by Enrico Zimuel

zend-crypt¹ 3.1.0 includes a hybrid cryptosystem², a feature that can be used to implement an end-to-end encryption³ schema in PHP.

A hybrid cryptosystem is a cryptographic mechanism that uses symmetric encyption (e.g. AES⁴) to encrypt a message, and public-key cryptography (e.g. RSA⁵) to protect the encryption key. This methodology guarantee two advantages: the speed of a symmetric algorithm and the security of public-key cryptography.

Before I present the PHP implementation, let's explore the hybrid mechanism in more detail. Below is a diagram demonstrating a hybrid encryption schema:

A user (the sender) wants to send a protected message to another user (the receiver). He/she generates a random session key (one-time pad) and uses this key with a symmetric algorithm to encrypt the message (in the figure, Block cipher represents an authenticated encryption⁶ algorithm). At the same time, the sender encrypts the session key using the public key of the receiver. This operation is done using a public-key algorithm, e.g., RSA. Once the encryption is done, the sender can send the encrypted session key along with the encrypted message to the receiver. The receiver can decrypt the session key using his/her private key, and consequently decrypt the message.

This idea of combining together symmetric and asymmetric (public-key) encryption can be used to implement end-to-end encryption (E2EE). E2EE is a communication system that encrypts messages exchanged by two users with the property that only the two users can decrypt the message. End-to-end encryption has become quite popular in the last years in software, and particularly messaging systems, such as WhatsApp⁷. More generally, when you have software used by many users, end-to-end encryption can be used to protect information exchanged by users. Only the users can access (decrypt) exchanged information; even the administrator of the system is not able to access this data.

Build end-to-end encryption in PHP

We want to implement end-to-end encryption for a web application with user authentication. We will use zend-crypt 3.1.0 to implement our cryptographic schemas. This component of Zend Framework uses PHP's OpenSSL extension⁸ for its cryptographic primitives.

The first step is to create public and private keys for each users. Typically, this step can be done when the user credentials are created. To generare the pairs of keys, we can use Zend\Crypt\PublicKey\RsaOptions. Below is an example demonstrating how to generate public and private keys to store in the filesystem:

use Zend\Crypt\PublicKey\RsaOptions;
use Zend\Crypt\BlockCipher;

$username = 'alice';
$password = 'test'; // user's password

// Generate public and private key
$rsaOptions = new RsaOptions();
$rsaOptions->generateKeys([
    'private_key_bits' => 2048
]);
$publicKey  = $rsaOptions->getPublicKey()->toString();
$privateKey = $rsaOptions->getPrivateKey()->toString();

// store the public key in a .pub file
file_put_contents($username . '.pub', $publicKey);

// encrypt and store the private key in a file
$blockCipher = BlockCipher::factory('openssl', array('algo' => 'aes'));
$blockCipher->setKey($password);
file_put_contents($username, $blockCipher->encrypt($privateKey));

In the above example, we generated a private key of 2048 bits. If you are wondering why not 4096 bits, this is questionable and depends on the real use case. For the majority of applications, 2048 is still a good key size, at least until 2030. If you want more security and you don't care about the additional CPU time, you can increase the key size to 4096. I suggest reading the following blog posts for more information on key key size:

In the example above, we did not generate the private key using a passphrase; this is because the OpenSSL extension of PHP does not support AEAD (Authenticated Encrypt with Associated Data) mode for ciphers yet, which is required in order to use passphrases.

The default passphrase encryption algorithm for OpenSSL is des-ede3-cbc¹⁰ using PBKDF2¹¹ with 2048 iterations for generating the encryption key from the user's password. Even if this encryption algorithm is quite good, the number of iterations of PBKDF2 is not optimal; zend-crypt improves on this in a variety of ways, out-of-the-box. As demonstrated above, I use Zend\Crypt\BlockCipher to encrypt the private key; this class provides encrypt-then-authenticate¹² using the AES-256 algorithm for encryption and HMAC-SHA-256 for authentication. Moreover, BlockCipher uses the PBKDF2¹¹ algorithm to derivate the encryption key from the user's key (password). The default number of iterations for PBKDF2 is 5000, and you can increase it using the BlockCipher::setKeyIteration() method.

In the example, I stored the public and private keys in two files named, respectively, $username.pub and $username. Because the private file is encrypted, using the user's password, it can be access only by the user. This is a very important aspect for the security of the entire system (we take for granted that the web application stores the hashes of the user's passwords using a secure algorithm such as bcrypt¹³).

Once we have the public and private keys for the users, we can start using the hybrid cryptosystem provided by zend-crypt. For instance, imagine Alice wants to send an encrypted message to Bob:

use Zend\Crypt\Hybrid;
use Zend\Crypt\BlockCipher;

$sender   = 'alice';
$receiver = 'bob';
$password = 'test'; // bob's password

$msg = sprintf('A secret message from %s!', $sender);

// encrypt the message using the public key of the receiver
$publicKey  = file_get_contents($receiver . '.pub');
$hybrid     = new Hybrid();
$ciphertext = $hybrid->encrypt($msg, $publicKey);

// send the ciphertext to the receiver

// decrypt the private key of bob
$blockCipher = BlockCipher::factory('openssl', ['algo' => 'aes']);
$blockCipher->setKey($password);
$privateKey = $blockCipher->decrypt(file_get_contents($receiver));

$plaintext = $hybrid->decrypt($ciphertext, $privateKey);

printf("%s\n", $msg === $plaintext ? "The message is: $msg" : 'Error!');

The above example demonstrates encrypting information between two users. Of course, in this case, the sender (Alice) knows the message because she wrote it. More in general, if we need to store a secret between multiple users, we need to specify the public keys to be used for encryption.

The hybrid component of zend-crypt supports encrypting messages for multiple recipients. To do so, pass an array of public keys in the $publicKey parameter of Zend\Crypt\Hybrid::encrypt($data, $publicKey).

Below demonstrates encrypting a file for two users, Alice and Bob.

use Zend\Crypt\Hybrid;
use Zend\Crypt\BlockCipher;

$data    = file_get_contents('path/to/file/to/protect');
$pubKeys = [
  'alice' => file_get_contents('alice.pub'),
  'bob'   => file_get_contents('bob.pub')
];

$hybrid     = new Hybrid();

// Encrypt using the public keys of both alice and bob
$ciphertext = $hybrid->encrypt($data, $pubKeys);

file_put_contents('file.enc', $ciphertext);

$blockCipher = BlockCipher::factory('openssl', ['algo' => 'aes']);

$passwords = [
  'alice' => 'password of Alice',
  'bob'   => 'password of Bob'
];

// decrypt using the private keys of alice and bob, one at time
foreach ($passwords as $id => $pass) {
  $blockCipher->setKey($pass);
  $privateKey = $blockCipher->decrypt(file_get_contents($id));
  $plaintext  = $hybrid->decrypt($ciphertext, $privateKey, null, $id);
  printf("%s for %s\n", $data === $plaintext ? 'Decryption ok' : 'Error', $id);
}

For decryption, I used a hard coded password for the users. Usually, the user's password is provided during the login process of a web application and should not be stored as permanent data; for instance, the user's password can be saved in a PHP session variable for temporary usage. If you use sessions to save the user's password, ensure that data is protected; the PHP-Secure-Session¹⁴ library or the Suhosin¹⁵ PHP extension will help you do so.

To decrypt the file, I used the Zend\Crypt\Hybrid::decrypt() method, where I specified the $privateKey, a null passphrase, and finally the $id of the privateKey. This parameters are necessary to find the correct key to use in the header of the encrypted message.

Footnotes

¹. https://github.com/zendframework/zend-crypt ↩

². https://docs.zendframework.com/zend-crypt/hybrid/ ↩

³. https://en.wikipedia.org/wiki/End-to-end_encryption ↩

⁴. https://en.wikipedia.org/wiki/Advanced_Encryption_Standard ↩

⁵. https://en.wikipedia.org/wiki/RSA_%28cryptosystem%29 ↩

⁶. https://en.wikipedia.org/wiki/Authenticated_encryption ↩

⁷. https://www.whatsapp.com/faq/en/general/28030015 ↩

⁸. http://php.net/manual/en/book.openssl.php ↩

⁹. https://wiki.php.net/rfc/openssl_aead ↩

¹⁰. https://en.wikipedia.org/wiki/Triple_DES ↩

¹¹. https://en.wikipedia.org/wiki/PBKDF2 ↩

¹². http://www.daemonology.net/blog/2009-06-24-encrypt-then-mac.html ↩

¹³. https://en.wikipedia.org/wiki/Bcrypt ↩

¹⁴. https://github.com/ezimuel/PHP-Secure-Session ↩

¹⁵. https://suhosin.org ↩

End-to-end encryption with Zend Framework 3

End-to-end encryption with Zend Framework 3

Build end-to-end encryption in PHP

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