Comparing passwords with crypt() in PHP

Question

I need to get the basics of this function. The php.net documentation states, for the blowfish algorithm, that:

Blowfish hashing with a salt as follows: "$2a$", a two digit cost parameter, "$", and 22 base 64 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

So this, by definition, should not work:

echo crypt('rasmuslerdorf', '$2a$07$usesomadasdsadsadsadasdasdasdsadesillystringforsalt$');

However, it spits out:

$2a$07$usesomadasdsadsadsadaeMTUHlZEItvtV00u0.kb7qhDlC0Kou9e

Where it seems that crypt() has cut the salt itself to a length of 22. Could somebody please explain this?

Another aspect of this function I can't get my head around is when they use crypt() to compare passwords. http://php.net/manual/en/function.crypt.php (look at ex. #1). Does this mean that if I use the same salt for all encrypting all my passwords, I have to crypt it first? ie:

$salt = "usesomadasdsadsadsadae";
$salt_crypt = crypt($salt);

if (crypt($user_input, $salt) == $password) {
   // FAIL WONT WORK
}

if (crypt($user_input, $salt_crypt) == $password) {
   // I HAVE TO DO THIS?
}    

Thanks for your time

Answer 1

Following code example may answer your questions.

To generate hashed password using Blowfish, you first need to generate a salt, which starts with $2a$ followed by iteration count and 22 characters of Base64 string.

$salt = '$2a$07$usesomadasdsadsadsadasdasdasdsadesillystringfors';
$digest = crypt('rasmuslerdorf', $salt);

Store the whole $digest in database, it has both the salt and digest.

When comparing password, just do this,

  if (crypt($user_input, $digest) == $digest)

You are reusing the digest as salt. crypt knows how long is the salt from the algorithm identifier.