Reference

Block Ciphers

Zend\Crypt\BlockCipher implements encrypt-then-authenticate mode using HMAC to provide authentication.

The symmetric cipher can be chosen with a specific adapter that implements Zend\Crypt\Symmetric\SymmetricInterface. We support the symmmetric encryption algorithms offered by OpenSSL and Mcrypt extensions.

In the following code, we detail an example of using the BlockCipher class to encrypt-then-authenticate a string using the AES-256 block cipher and the HMAC algorithm (using the SHA-256 hash function).

Mcrypt

The Mcrypt extension is based on the libmcrypt library. Unfortunately, at the time of writing, the project is dead, having been unmaintained for around 8 years, with the last release (version 2.5.8) having occurred in February 2007.

Starting with PHP 7.1, the Mcrypt extension will be considered deprecated. For these reasons, we strongly suggest using only the Openssl adapter.

Starting with zend-crypt 3.0, the Openssl adapter is the default (for example, by Zend\Crypt\FileCipher), and all examples now only demonstrate that adapter.

use Zend\Crypt\BlockCipher;

$blockCipher = BlockCipher::factory('openssl', array('algo' => 'aes'));
$blockCipher->setKey('encryption key');
$result = $blockCipher->encrypt('this is a secret message');
echo "Encrypted text: $result\n";

The BlockCipher instance is initialized using a factory method with the name of the cipher adapter to use (e.g., openssl) and the parameters to pass to the adapter (the AES algorithm). In order to encrypt a string, we need to specify an encryption key, which we do via the setKey() method. Encryption is performed with the encrypt() method.

The output of encryption is a string, encoded in Base64 (default), containing the HMAC value, the IV vector, and the encrypted text. The encryption mode used is CBC (with a random IV by default), with the default HMAC hash algorithm of SHA256. The Openssl adapter encrypts using the PKCS#7 padding mechanism by default. You can specify a different padding method using a special adapter (Zend\Crypt\Symmetric\Padding). The encryption and authentication keys used by BlockCipher are generated with the PBKDF2 algorithm, used as the key derivation function from the user's key specified using the setKey() method.

Key Size

BlockCipher always attempts to use the longest key size for the specified cipher. For instance, for the AES algorithm it uses 256 bits, and for the Blowfish algorithm it uses 448 bits.

You can change all the default settings by passing the values to the factory parameters. For instance, if you want to use the Blowfish algorithm, with the CFB mode and the HMAC SHA512 hash function, initialize the class as follows:

use Zend\Crypt\BlockCipher;

$blockCipher = BlockCipher::factory(
    'openssl',
    [
        'algo' => 'blowfish',
        'mode' => 'cfb',
        'hash' => 'sha512'
    ]
);

Recommendation

If you are not familiar with symmetric encryption techniques, we strongly suggest using the default values of the BlockCipher class. The default values are: AES algorithm, CBC mode, HMAC with SHA256, PKCS#7 padding.

To decrypt a string we can use the decrypt() method. In order to successfully decrypt a string, we must configure the BlockCipher with the same parameters used during encryption.

We can also initialize the BlockCipher manually without using the factory method; we can inject the symmetric cipher adapter directly via the constructor. For instance, we can rewrite the previous example as follows:

use Zend\Crypt\BlockCipher;
use Zend\Crypt\Symmetric\Openssl;

$blockCipher = new BlockCipher(new Openssl(['algo' => 'aes']));
$blockCipher->setKey('encryption key');
$result = $blockCipher->encrypt('this is a secret message');
echo "Encrypted text: $result \n";

Using GCM or CCM Mode from PHP 7.1+

If you are using PHP 7.1+ you can choose the GCM or CCM mode for authenticated encryption with OpenSSL. These modes provide authenticated encryption by itself, without the usage of HMAC as described in the previous section.

GCM is Galois/Counter Mode and CCM is Counter with CBC-MAC. For more information on the usage of this encryption modes in PHP we suggest to read this blog post.

If you want to use the GCM or CCM mode with BlockCipher you can just specify the mode in the factory. For instance, the following is an example of aes-256-gcm encryption with OpenSSL:

use Zend\Crypt\BlockCipher;

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

And below is an example demonstrating the factory for aes-256-ccm algorithm:

use Zend\Crypt\BlockCipher;

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

Recommendation

GCM mode is about 3x faster than CCM, we recommend using GCM unless you have requirements that dictate CCM.