PHP : Encryption and Security - [14.5] Storing Passwords

14.5.1 Problem

You need to keep track of users' passwords so they can log in to your web site.

14.5.2 Solution

When a user signs up, encrypt her chosen password with crypt( ) and store the encrypted password in your database of users:

// encrypt the password
$encrypted_password = crypt($_REQUEST['password']);

// store $encrypted_password in the user database 
$dbh->query('INSERT INTO users (username,password) VALUES (?,?)',
            array($_REQUEST['username'],$encrypted_password));

Then, when that user attempts to log in to your web site, encrypt the password she supplies with crypt( ) and compare it to the stored encrypted password. If the two encrypted values match, she has supplied the correct password:

$encrypted_password = 
    $dbh->getOne('SELECT password FROM users WHERE username = ?',
                 array($_REQUEST['username']));

if (crypt($_REQUEST['password'],$encrypted_password) == $encrypted_password) {
  // successful login
} else {
  // unsuccessful login
}

14.5.3 Discussion

Storing encrypted passwords prevents users' accounts from becoming compromised if an unauthorized person gets a peek at your username and password database. (Although such unauthorized peeks may foreshadow other security problems.)

When the password is initially encrypted, crypt( ) supplies two randomly generated characters of salt that get prepended to the encrypted password. Passing $encrypted_password to crypt( ) when testing a user-supplied password tells crypt( ) to use the same salt characters again. The salt reduces your vulnerability to dictionary attacks, in which someone compares encrypted passwords with encrypted versions of common words. Still, it's a good idea to prevent users from choosing passwords that are simple words or other easier-to-crack combinations. Section 14.6 provides a function to filter out easily guessable passwords.

The crypt( ) function uses a one-way algorithm. This means it's currently impossible (or at least prohibitively computationally expensive) to turn a crypt( )-generated ciphertext back into plain text. This makes your stored passwords somewhat more secure, but it also means that you can't get at the plaintext of users' passwords even if you need to. So, for example, if a user forgets his password, you won't be able to tell him what it is. The best you can do is to reset the password to a new value and then tell the user the new password. A method for dealing with lost passwords is covered in Section 14.7.