PHP Unconference Europe 2015

password_needs_rehash

(PHP 5 >= 5.5.0)

password_needs_rehashVérifie si la table de hachage fournie correspond aux options fournies

Description

boolean password_needs_rehash ( string $hash , integer $algo [, array $options ] )

Cette fonction vérifie si la table de hachage fournie implémente l'algorithme et les options fournis. Si ce n'est pas le cas, il est supposé que la table de hachage doit être re-généré.

Liste de paramètres

hash

Un hachage créé par la fonction password_hash().

algo

Une constantes de l'algorithme de mot de passe représentant l'algorithme à utiliser lors du hachage du mot de passe.

options

Un tableau associatif contenant les options. Voir aussi les constantes de l'algorithme de mot de passe pour une documentation sur les options supportées pour chaque algorithme.

Valeurs de retour

Retourne TRUE si la table de hachage doit être re-générée pour correspondre aux paramètres algo et options fournis, ou FALSE sinon.

add a note add a note

User Contributed Notes 5 notes

up
22
nick at nickstallman dot net
1 year ago
ydroneaud this would be used on a login page, not at any other time.

So if you have a site with MD5 passwords for example, and wish to upgrade to SHA256 for additional security you would put this check in the login script.

This function will take a user's hash and say if it is SHA256, if it isn't then you can take the user's password which you still have as plaintext and rehash it as SHA256.

This lets you gradually update the hashes in your database without disrupting any features or resetting passwords.
up
9
solariane
5 months ago
the correct use case is given here  ( https://github.com/ircmaxell/password_compat )

if (password_verify($password, $hash)) {
        if (password_needs_rehash($hash, $algorithm, $options)) {
            $hash = password_hash($password, $algorithm, $options);
            /* Store new hash in db */
        }
    }
up
3
php dot net at muer dot nl
5 months ago
nick, this function cannot check if a string is a MD5 or SHA1 hash. It can only tell you if a password, hashed using the password_hash function, needs to be put through the hashing function again to keep up to date with the new defaults.

The only time you can use this function is when your user logs in and you have already checked by means of password_verify that the password entered is actually correct. At that point, if password_needs_rehash returns true, you can put the plain text password through the password_hash function.
up
0
admin at torntech dot com
1 month ago
Some other use-cases for the password_needs_rehash function is when you have specified using the PASSWORD_DEFAULT algorithm for password_hash.
As mentioned on the Password Hashing Predefined Constants and password_hash pages, the algorithm used by PASSWORD_DEFAULT is subject to change as different versions of PHP are released.
Additionally password_needs_rehash would be used if you have changed the optional cost or static salt (DO NOT USE A STATIC SALT) requirements of your password_hash options.

Full example:

<?php

$new
= [
   
'options' => ['cost' => 11],
   
'algo' => PASSWORD_DEFAULT,
   
'hash' => null
];

$password = 'rasmuslerdorf';

//stored hash of password
$oldHash = '$2y$07$BCryptRequires22Chrcte/VlQH0piJtjXl.0t1XkA8pw9dMXTpOq';

//verify stored hash against plain-text password
if (true === password_verify($password, $oldHash)) {
   
//verify legacy password to new password_hash options
   
if (true === password_needs_rehash($oldHash, $new['algo'], $new['options'])) {
       
//rehash/store plain-text password using new hash
       
$newHash = password_hash($password, $new['algo'], $new['options']);
        echo
$newHash;
    }
}
?>

The above example will output something similar to:
$2y$11$Wu5rN3u38.g/XWdUeA6Wj.PD.F0fLXXmZrMNFyzzg2UxkVmxlk41W
up
-4
ydroneaud at opteya dot com
1 year ago
According to the documentation, it's checking if the given hashed password string is compatible with the provided algorithm (and options, but not salt), eg. it's checking if the hashed password string was generated with the provided algorithm (and options, but not salt).

There's nothing to 'rehash' in its parameters ... especially not the already hashed password string, and the password "stored" in the hashed password string is not supposed to be known, it's not in clear, it's a secret.

The name of the function seems misleading, this function should have been called "password_hash_compatible()" instead.

This function could be use to check if a password database/a hashed password string (hashed by function "password_hash()") need to be upgraded to a stronger password hashing/storage scheme: if the function returns false,  a new password will have to be set for the user, hashed with the new, stronger, algorithm/options.

One should carefully think before using this function to support multiple algorithms/options in one database, eg. support "legacy scheme" passwords + "new scheme" ...
To Top