Generating Password Hash In PHP 5.5 And Setting Cost Option - php

Question

I know PHP 5.5 is in alpha but this class I am making is just being made in advance to utilize it's hashing feature by using function_exists().
I checked out the password_hash documentation. The 3rd argument is for $options which currently supports two options, 'salt' and 'cost'.
It states the following:
cost, which denotes the algorithmic cost that should be used. Examples
of these values can be found on the crypt() page.
When I go to the crypt() page the documentation it gives is:
Blowfish hashing with a salt as follows: "$2a$", "$2x$" or "$2y$", a
two digit cost parameter, "$", and 22 digits from the alphabet
"./0-9A-Za-z". Using characters outside of this range in the salt will
cause crypt() to return a zero-length string. The two digit cost
parameter is the base-2 logarithm of the iteration count for the
underlying Blowfish-based hashing algorithmeter and must be in range
04-31, values outside this range will cause crypt() to fail. Versions
of PHP before 5.3.7 only support "$2a$" as the salt prefix: PHP 5.3.7
introduced the new prefixes to fix a security weakness in the Blowfish
implementation. Please refer to » this document for full details of
the security fix, but to summarise, developers targeting only PHP
5.3.7 and later should use "$2y$" in preference to "$2a$".
I can't seem to get my head wrapped around this. It says PHP 5.3.7 and later should use $2y$, but what cost value do I use to get that one and is it the best value to choose? The example they provide uses a value of 7, but according to the above it can go up to 31, what difference does it make to use say 4 opposed to say 31?

Answer 1

The function password_hash() is just a wrapper around the function crypt(), and shall make it easier to use it correctly. It takes care of the generation of a safe random salt, and provides good default values.
The easiest way to use this function would be:
$hash = password_hash($password, PASSWORD_DEFAULT);
That means, the function will hash the password with BCrypt (algorithm 2y), generates a random salt, and uses the default cost (at the moment this is 10). These are good default values, particularly i would not recommend generating the salt of your own, it is easy to make mistakes there.
Should you want to change the cost parameter, you can do it like this:
$hash = password_hash($password, PASSWORD_BCRYPT, ["cost" => 11]);
Increasing the cost parameter by 1, doubles the needed time to calculate the hash value. The cost parameter is the logarithm (base-2) of the iteration count, which means:
$iterations = 2 ^ $cost;
Edit:
I missed the point, that you want to generate your own class. For PHP version 5.3.7 and later, there exists a compatibility pack, from the same author that made the password_hash() function. You can either use this code directly, or look at the well crafted implementation. For PHP versions before 5.3.7 there is no support for crypt with 2y, the unicode aware BCrypt algorithm. You can instead use 2a, which is the best alternative for earlier PHP versions. I did an example with a lot of comments, maybe you want to have a look at it too.
P.S. The expressions "salt" and "cost factor" are used correctly in password_hash(), the crypt() function though, uses the word salt for all crypt parameters together, that's a bit misleading.

Answer 2

Disclaimer: this is with PHP 5.3.10, but it seems not really different from your description.
The cost applies to the cost of computation. When you increase the cost value, it takes longer to hash the password
function blowfish_salt($cost)
{
$chars = './ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
$salt = sprintf('$2y$%02d$', $cost);
for ($i = 0; $i < 22; ++$i)
$salt .= $chars[rand(0,63)];
return $salt;
}
$password = 'My perfect password';
$cost = $argv[1];
$salt = blowfish_salt($cost);
$hash = crypt($password, $salt);
When I run this on my (old) machine as
php mycrypt.php 10
it returns immediately (~0.2 sec), whereas with
php mycrypt.php 16
it takes about 5.2 seconds.