PHP & MySQL compare password

Question

How does one check to see if a user has typed in the right password to log in?

This is what (out of a bunch of combinations...) I am doing:

<?

$login = $_POST['login'];
$password = $_POST['password'];

mysql_connect('localhost', 'root', 'abc123');

mysql_select_db('aun_vox') or die(mysql_error());

$q = mysql_query("SELECT password FROM customer WHERE login='$login'");
$db_pass = mysql_result($q, 0);

if(md5($password) == $db_pass)
{
    echo "You did it.";
}

else echo "Wrong.";

?>

As I can see from the ouput, there's something wrong in the mysql_result bit, but I can't figure out the right way.

Can someone please help.

Answer 1

I see you are storing a hash of the password in the database, but for the benefit of other readers, never store passwords in plain text in the database. You don't want to be like Monster.com.uk!

You should use a stronger hashing function than MD5(). Ideally you should use SHA256. This hash method is available in PHP using the hash() function.

You should also apply a random salt to the password. Store a different salt value for each user's account. This helps to defeat dictionary attacks and rainbow table attacks.

You should learn to use the mysqli extension instead of the old mysql extension. Mysqli supports parameterized queries, so you can reduce vulnerability to some SQL injection attacks.

Here is some example code. I haven't tested it, but it should be pretty close to working:

$input_login = $_POST['login'];
$input_password = $_POST['password'];

$stmt = $mysqli->prepare("SELECT password, salt FROM customer WHERE login = ?");
$stmt->bind_param("s", $input_login);
$stmt->execute();
$stmt->bind_result($password_hash, $salt);

while ($stmt->fetch()) {
  $input_password_hash = hash('sha256', $input_password . $salt);
  if ($input_password_hash == $password_hash) {
    return true;
  }
  // You may want to log failed password attempts here,
  // for security auditing or to lock an account with
  // too many attempts within a short time.
}
$stmt->close();

// No rows matched $input_login, or else password did not match
return false;

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Some other people suggest the query should test for login = ? AND password = ? but I don't like to do that. If you do this, you can't know if the lookup failed because the login didn't exist, or because the user provided a wrong password.

Of course you shouldn't reveal to the user which caused the failed login attempt, but you may need to know, so you can log suspicious activity.

---

@Javier says in his answer that you shouldn't retrieve the password (or password hash in this case) from the database. I don't agree.

Javier shows calling md5() in PHP code and sending that the resulting hash string to the database. But this doesn't support salting the password easily. You have to do a separate query to retrieve this user's salt before you can do the hash in PHP.

The alternative is sending the plaintext password over the network from your PHP app to your database server. Anyone wiretapping your network can see this password. If you have SQL queries being logged, anyone who gains access to the logs can see the password. Motivated hackers can even dumpster-dive to find old filesystem backup media, and might read the log files that way!

The lesser risk is to fetch the password hash string from the database into the PHP app, compare it to the hash of the user's input (also in PHP code), and then discard these variables.

Answer 2

Firstly, make sure you properly escape your variables before using them in the query - use mysql_real_escape_string().

Then, why not use the MySQL MD5 function to check for a valid login in your query?

SELECT login FROM customer WHERE login='$login' AND password = MD5('$password')

Then just use mysql_num_rows() to count the number of returned rows.