What's the algorithm that Linux uses to hash users' passwords? How can I implement that algorithm in PHP?
You might need to know some background information on Linux password storage formats - especially on shadowed password configuration before you can actually implement your own.
On Linux distributions using glibc2, the hash function has a 'magic bit' and salt prefixed to it.
The magic bit starts off with '$x$' and is used to determined the hash function that was used:
(Other unix systems like NetBSD might have different values for this).
The magic bit then followed by 8 bits that constitutes the salt and optionally is terminated by another "$". Between this and the next "$", you will find the actual password hash.
Almost all modern Linux systems these days do NOT store the passwords in the world-readable /etc/passwd. Instead the passwords are shadowed in /etc/shadow where only root is allowed read permission. If the shadowed password scheme in use, the /etc/passwd file shows a character such as '*', or 'x' instead of the password.
The format of a typical password in /etc/shadow would looks like this:
$a:$b:$c:$e:$f:$g:$h:$i
Where:
$a: username
$b: salt and hashed password (as explained above). If this is "NP" or "!" or null then it means that the account has no password. "LK" or "*" means the account is locked and the user will be unable to log-in. "!!" means that the password has expired
$c: Days since epoch of last password change
$d: Days until change allowed
$e: Days before change required
$f: Days warning for expiration
$g: Days before account inactive
$h: Days since epoch when account expires
$i: Reserved for future use.
An example of a shadowed password file could be found at: http://configuration.logfish.net/index.php/etc/shadow
References:
Try using the PHP crypt function.