Are hashed and salted passwords secure against dictionary attacks?

Question

I understand that salts make the same password hash to different values. However, salts are usually stored in the database with the password. So let\'s say I am attacker,

Answer 1

Nothing keeps an attacker from just guessing the password.

Salts just make it harder by forcing an attacker to hash the dictionary on a per-user (effectively, per-salt) basis.

To improve security, a tunable hash function is your best bet. Crank the time-per-hash up, making dictionary attacks impractical on whatever hardware your attacker is likely to have available.

Basically, read this.

Answer 2

Your logic is sound, but in reality, with enough computing power and time, there is no protection against dictionary/brute-force attacks.

Answer 3

That's correct. If someone got the password material, a dictionary attack would be effective.

To guard against this:

• Make sure your passwords aren't subject to dictionary attacks.
• Make sure your password file (/etc/shadow) is readable only by root.

Answer 4

Without salt, the attacker can generate hashes for every word in his dictionnary then run the new dictionnary against your passwords list

With salt, each password is hashed with a random string so even with the prior hashed dictionnary knowledge, he still have to re-create a new hashed dictionnary containing the salt for every different salt in your database.

Just think of dictionnaries tables as a subset (small portion) of the rainbow tables. While rainbow tables can contain billions of entries, dictionnaries contain "known words", so maybe a few million entries at most.

The reason why rainbow tables fail against salt is because the re-creation process would be "billions of entries" of recalculation while dictionnary attacks are still "few millions of entries". The salt just blocks precomputed values

Answer 5

Salt doesn't prevent dictionary attacks, just precalculated dictionary attacks. In particular, it protects against rainbow tables (http://en.wikipedia.org/wiki/Rainbow_table) and also ensures that cracking one user's password doesn't automatically let you crack any user who shares that password.

The article I linked to mentions some ways to improve upon salting, incudling key strengthening (http://en.wikipedia.org/wiki/Key_strengthening).