When writing web applications, or any application for that manner, any passwords should be stored securely. As a rule of thumb, one should never store passwords as clear text in the database for the following reasons:
- If the database ever gets leaked out, then all accounts are compromised until every single user resets his password. Imagine that you’re an MMORPG developer; leaking out the database with clear text passwords allows the attacker to delete every player’s characters.
- Many people use the same password for multiple sites. Imagine that the password stored in your database is also used for the user’s online banking account. Even if the database does not get leaked out, the password is still visible to the system administrator; this can be a privacy breach.
There are several “obvious” alternatives, which aren’t quite secure enough:
- Storing passwords as MD5/SHA1/$FAVORITE_ALGORITHM hashes
- These days MD5 can be brute-force cracked with relatively little effort. SHA1, SHA2 and other algorithms are harder to brute-force, but the attacker can still crack these hashes by using rainbow tables: precomputed tables of hashes with which the attacker can look up the input for a hash with relative ease. This rainbow table does not have to be very large: it just has to contain words from the dictionary, because many people use dictionary words as passwords.
Using plain hashes also makes it possible for an attacker to determine whether two users have the same password.
- Encrypting the password
- This is not a good idea because if the attacker was able to steal the database, then there’s a possibility that he’s able to steal the key file as well. Plus, the system administrator is able to read everybody’s passwords, unless he’s restricted access to either the key file or the database.
The solution is to store passwords as salted hashes. One calculates a salted hash as follows:
salted_hash = hashing_algorithm(salt + cleartext_password)
salt is a random string. After calculating the salted hash, one should store the salted hash in the database, along with the (cleartext) salt. It is not necessary to keep the salt secret or to obfuscate it.
When a user logs in, one can verify his password by re-computing the salted hash and comparing it with the salted hash in the database:
salted_hash = hashing_algorithm(salt_from_database + user_provided_password) if (salted_hash == salted_hash_from_database): user is logged in else: password incorrect
The usage of the salt forces the attacker to either brute-force the hash or to use a ridiculously large rainbow table. In case of the latter, the sheer size of the required rainbow table can make it unpractical to generate. The larger the salt, the more difficult it becomes for the cracker to use rainbow tables.
However, even with salting, one should still not use SHA1, SHA2, Whirlpool or most other hashing algorithms because these algorithms are designed to be fast. Although brute forcing SHA2 and Whirlpool is hard, it’s still possible given sufficient resources. Instead, one should pick a hashing algorithm that’s designed to be slow so that brute forcing becomes unfeasible. Bcrypt is such a slow hashing algorithm. A speed comparison on a MacBook Pro with 2 Ghz Intel Core 2 Duo:
- SHA-1: 118600 hashes per second.
- Bcrypt (with cost = 10): 7.7 hashes per second.
Theoretically it would take 4*10^35 years for a single MacBook Pro core to crack an SHA-1 hash, assuming that the attacker does not harness any weaknesses in SHA-1. To crack a bcrypt hash one would need 6*10^39 years, or 10000 more times. Therefore, we recommend the use of bcrypt to store passwords securely.
There’s even a nice Ruby implementation of this algorithm: bcrypt-ruby! Up until recently, bcrypt-ruby was only available for MRI (“Matz Ruby Interpreter”, the C implementation that most people use). However, we’ve made it compatible with JRuby! The code can be found in our fork at Github. The current version also has issues with Ruby 1.9, which we’ve fixed as well. The author of bcrypt-ruby has already accepted our changes and will soon release a new version with JRuby and Ruby 1.9 support.
Further recommended reading
How to Safely Store a Password by Coda Hale.