What is the best practise for where password comparison should be performed

Question

When authenticating a user to a website, should the hash generation and comparison be done in the database or the website?

My argument is the website should pass the user supplied password (possibly encrypted by the web server) to the database. The database then re-encrypts it with the salt and compares the hash's. The database the responds to the web server whether the user's credentials are valid or not. This way, the very minimum ever leaves the database, essentially either a yes or no, none of the stored credential info. Downside is, the database has to do more work.

The other argument is that the work should be done in the web server. Here the web server would create the hash and request the stored hash from the database and compare. In this situation the salt needs to be passed from the database back to the web server for the hash to be created. but, work is shared as # of web servers increase.

Personally I see the second method as a potential security risk. Should the web server be compromised, salts and hashes can be requested from the database and easily cracked.

What is the best practise for performing the above operation? Am I overlooking/missing something?

Thanks

Answer 1

The first problem I suspect you will run into (and it's a big one) is that your database does not have a password hash function. Sure, it probably has MD5() and SHA1() but these are cryptographic hash functions. Does it have bcrypt() or scrypt() or PBKDF2()?

Using a cryptographic hash function rather than a password hash function is what meant that the LinkedIn passwords could be cracked so quickly. If you don't use one of the above functions then you will be similarly vulnerable if your hashes are leaked.

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Going on to answer your question assuming that your database does support a password hashing algorithm (using bcrypt simply because I have to pick one). The two alternatives are:

Hashing in the database:

$db->query("SELECT COUNT(*) FROM users WHERE username = '?' AND password = BCRYPT(?, (SELECT salt FROM user WHERE username = '?'))", $username, $password, $username);
if($row['count'] != 1)
{
  // Not authenticated.  Throw exception.
}

In this case, the raw password is sent to the database and a simple yes or no (1 or 0) is returned. This database communication can be encrypted. The hash and salt are never held in the application.

Hashing in the application:

$db->query("SELECT username, salt, password FROM users WHERE username = '?', $username);
if(bcrypt($password, $row['salt']) != $row['password'])
{
  // Not authenticated.  Throw exception.
}

In this case, the hash and salt are pulled from the database into the application and the hashing of the raw password and comparison is done there. The communication to the database can still be encrypted. The raw password is never held in the database memory.

For efficiency, we can assume that both hashing algorithms are written in C (or some compiled language) and are possibly provided by the OS so take the same time. The application hashing option receives more data over the wire and the database hashing option sends more and has a more complex query (essentially two queries, one to get the salt and one to effect the comparison). It may not be possible to use an index the way I have written that query but the query could be rewritten. Since the size of the data in both cases is likely still one TCP packet, the speed difference will be negligible. I would call this one a win for the application hashing option due to the subquery.

For exposure. I would consider the raw password to be more sensitive than the hash and the salt together. Therefore, limiting the exposure of the raw password seems like the safer bet, making application hashing the best practice.

Answer 2

There's a really good article on how to store passwords securely here:

http://throwingfire.com/storing-passwords-securely/