Where do you store your salt strings?

Question

I've always used a proper per-entry salt string when hashing passwords for database storage. For my needs, storing the salt in the DB next to the hashed password has always worked fine.

However, some people recommend that the salt be stored separately from the database. Their argument is that if the database is compromised, an attacker can still build a rainbow table taking a particular salt string into account in order to crack one account at a time. If this account has admin privileges, then he may not even need to crack any others.

From a security perspective, is it worth it to store salts in a different place? Consider a web application with the server code and DB on the same machine. If the salts are stored in a flat file on that machine, chances are that if the database is compromised, the salts file will be, too.

Are there any recommended solutions to this?

Answer 1

The point of rainbow tables is that they're created in advance and distributed en masse to save calculation time for others - it takes just as long to generate rainbow tables on the fly as it would to just crack the password+salt combination directly (since effectively what's being done when generating rainbow tables is pre-running the calculations for brute-forcing the hash), thus the argument that by knowing the salt someone could "generate a rainbow table" is spurious.

There's no real point in storing salts in a separate file as long as they're on a per-user basis - the point of the salt is simply to make it so that one rainbow table can't break every password in the DB.

Answer 2

Often, they are prepended to the hash and stored in the same field.

There is no need to store them separately - the point is to use a random salt for each password so that a single rainbow table can't be used against your entire set of password hashes. With random salts, an attacker must brute-force each hash separately (or compute a rainbow table for all possible salts - vastly more work).

If you had a more secure storage location, it would make sense to just store the hashes there.

Answer 3

If it's per entry then it's a nonce