PHP : Encryption and Security - [14.7] Dealing with Lost Passwords

14.7.1 Problem

You want to issue a password to a user who claims he's lost his password.

14.7.2 Solution

Generate a new password and send it to the user's email address (which you should have on file):

// generate new password
$new_password = '';
$i = 8;
while ($i--) { $new_password .= chr(mt_rand(33,126)); }

// encrypt new password
$encrypted_password = crypt($new_password);

// save new encrypted password to the database
$dbh->query('UPDATE users SET password = ? WHERE username = ?',
            array($encrypted_password,$username));

// email new plaintext password to user
mail($email,"New Password","Your new password is $new_password");

14.7.3 Discussion

If a user forgets his password, and you store encrypted passwords as recommended in Section 14.5, you can't provide the forgotten password. The one-way nature of crypt( ) prevents you from retrieving the unencrypted password.

Instead, generate a new password and send that to his preexisting contact address. If you send the new password to an address you don't already have on file for that user, you don't have a way to verify that the new address really belongs to the user. It may be an attacker attempting to impersonate the real user.

Because the email containing the new password isn't encrypted, the code in the Solution doesn't include the username in the email message to reduce the chances that an attacker that eavesdrops on the email message can steal the password. To avoid disclosing a new password by email at all, let a user authenticate himself without a password by answering one or more personal questions (the answers to which you have on file). These questions can be "What was the name of your first pet?" or "What's your mother's maiden name?" — anything a malicious attacker is unlikely to know. If the user provides the correct answers to your questions, you can let him choose a new password.

One way to compromise between security and readability is to generate a password for a user out of actual words interrupted by some numbers.

$words = 
array('dished','mother','basset','detain','sudden','fellow','logged','sonora',
      'earths','remove','dustin','snails','direct','serves','daring','cretan',
      'chirps','reward','snakes','mchugh','uphold','wiring','gaston','nurses',
      'regent','ornate','dogmas','singed','mended','hinges','latent','verbal',
      'grimes','ritual','drying','hobbes','chests','newark','sourer','rumple');

mt_srand((double) microtime() * 1000000);
$word_count = count($words);

$password = sprintf('%s%02d%s',
                    $words[mt_rand(0,$word_count - 1)],
                    mt_rand(0,99),
                    $words[mt_rand(0,$word_count - 1)]);

print $password;

This code produces passwords that are two six-letter words with two numbers between them, like mother43hinges or verbal08chirps. The passwords are long, but remembering them is made easier by the words in them.