Reference

Key Derivation

In cryptography, a key derivation function (or KDF) derives one or more secret keys from a secret value such as a master key, or known information — such as a password or passphrase — using a pseudo-random function. For instance, a KDF function can be used to generate encryption or authentication keys from a user password. Zend\Crypt\Key\Derivation implements a key derivation function using specific adapters.

User passwords are not really suitable to be used as keys in cryptographic algorithms, since users normally choose keys they can write with a keyboard. These passwords use only 6 to 7 bits per character (or less). It is highly recommended always to use a KDF function to transform a user's password in a cryptographic key.

The output of the following key derivation functions is a binary string. If you need to store the value in a database or a different persistent storage, we suggest converting it to Base64 format, using the base64_encode() function, or to hex format, using the bin2hex() function.

Pbkdf2 Adapter

Pbkdf2 is a KDF that applies a pseudorandom function, such as a cryptographic hash, to the input password or passphrase along with a salt value and repeats the process many times to produce a derived key, which can then be used as a cryptographic key in subsequent operations. The added computational work makes password cracking much more difficult, and is known as key stretching.

In the example below we demonstrate typical usage of the Pbkdf2 adapter.

use Zend\Crypt\Key\Derivation\Pbkdf2;
use Zend\Math\Rand;

$pass = 'password';
$salt = Rand::getBytes(32, true);
$key  = Pbkdf2::calc('sha256', $pass, $salt, 10000, 32);

printf ("Original password: %s\n", $pass);
printf ("Derived key (hex): %s\n", bin2hex($key));

The Pbkdf2 adapter takes the password ($pass) and generates a binary key of 32 bytes. The syntax is calc($hash, $pass, $salt, $iterations, $length) where $hash is the name of the hash function to use, $pass is the password, $salt is a pseudo random value, $iterations is the number of iterations of the algorithm, and $length is the size of the key to be generated. We use the Rand::getBytes() function from the class Zend\Math\Rand to generate a random string of 32 bytes for the salt, using a strong generator (the true value means the usage of a cryptographically strong generator).

The number of iterations is a very important parameter for the security of the algorithm; bigger values guarantee more security. There is no fixed value for the parameter because the number of iterations depends on CPU power. You should always choose a number of iterations that prevents brute force attacks.

SaltedS2k Adapter

The SaltedS2k algorithm uses a hash function and a salt to generate a key based on a user's password. This algorithm doesn't use a parameter to specify the number of iterations, and for that reason it's considered less secure compared to Pbkdf2. We suggest using the SaltedS2k algorithm only if you really need it (for instance, due to hardware limitations).

The following demonstrates usage of the SaltedS2k adapter to generate a 32 byte key.

use Zend\Crypt\Key\Derivation\SaltedS2k;
use Zend\Math\Rand;

$pass = 'password';
$salt = Rand::getBytes(32, true);
$key  = SaltedS2k::calc('sha256', $pass, $salt, 32);

printf ("Original password: %s\n", $pass);
printf ("Derived key (hex): %s\n", bin2hex($key));

Scrypt Adapter

The scrypt algorithm uses the Salsa20/8 core algorithm and Pbkdf2-SHA256 to generate a key based on a user's password. This algorithm has been designed to be more secure against hardware brute-force attacks than alternative functions such as Pbkdf2 or bcrypt.

The scrypt algorithm is based on the idea of memory-hard algorithms and sequential memory-hard functions. A memory-hard algorithm is an algorithm which asymptotically uses almost as many memory locations as it uses operations¹. A natural way to reduce the advantage provided by an attacker’s ability to construct highly parallel circuits is to increase the size of a single key derivation circuit — if a circuit is twice as large, only half as many copies can be placed on a given area of silicon — while still operating within the resources available to software implementations, including a powerful CPU and large amounts of RAM.

"From a test executed on modern (2009) hardware, if 5 seconds are spent computing a derived key, the cost of a hardware brute-force attack against scrypt is roughly 4000 times greater than the cost of a similar attack against bcrypt (to find the same password), and 20000 times greater than a similar attack against Pbkdf2."

— Colin Percival (author of the scrypt algorithm)

This algorithm uses 4 parameters to generate a key of 32 bytes:

• salt, a random string;
• N, the CPU cost;
• r, the memory cost;
• p, the parallelization cost.

Following is a usage example for the Scrypt adapter:

use Zend\Crypt\Key\Derivation\Scrypt;
use Zend\Math\Rand;

$pass = 'password';
$salt = Rand::getBytes(32, true);
$key  = Scrypt::calc($pass, $salt, 2048, 2, 1, 32);

printf ("Original password: %s\n", $pass);
printf ("Derived key (hex): %s\n", bin2hex($key));

Performance of the Scrypt Implementation

The aim of the scrypt algorithm is to generate a secure derived key that prevents brute force attacks. Just like the other derivation functions, the more time (and memory) spent executing the algorithm, the more secure the derived key will be. Unfortunately a pure PHP implementation of the scrypt algorithm is very slow compared with the C implementation (this is always true, if you compare execution time of C with PHP). If you want use a faster scrypt algorithm, we suggest installing the PECL scrypt extension. The Scrypt adapter we provide is able to recognize if the PECL extension is loaded and will use it instead of the pure PHP implementation.

Footnotes

• ¹ See Colin Percival's slides on scrypt from BSDCan'09.