Comments on: Username as password salt

By: Jeremy

Jeremy — Wed, 05 Nov 2008 07:16:13 +0000

Or if you use uniqueidentifiers, use that for the salt

By: Callum

Callum — Sun, 15 Jun 2008 02:52:12 +0000

@Morgan Tocker: Thanks for chiming in.

It’s true that you could build your own hash database, but would it not be prohibitively expensive (time and disk wise)? I thought the beauty of rainbow table attacks is that you precompute the rainbow table once, then you can store it indefinitely. Whereas I haven’t heard the argument that the principle of hashing in itself is flawed.

I understood that passwords were stored as salted hashes so that a rainbow table would be useless. In effect, an attacker would need to generate a new rainbow table for every user, which would be prohibitively expensive.

I think I’ll go with something like md5( username . password . domain ). It would be possible from that data to break the passwords, but it would require a *huge* amount of work.

By: Morgan Tocker

Morgan Tocker — Sun, 15 Jun 2008 02:45:18 +0000

It’s not devoid of use, but As Alex pointed out - it’s probably better to keep that known constant hidden and treat it like a password in itself. That way nobody else can build their own hash database (easily) by just starting from MD5(salt . ‘a’) .. MD5(salt . ‘b’) etc.

FWIW, the largest hash database I know of is Gdata (http://gdataonline.com/). Presumably from their homepage they have 670M hashes - which is just a bit less than 64^5.

The storage cost for 64^5 is 4G just for the hashes - and for 64^6 it’s 256G, so it starts to get expensive quickly.

By: Callum

Callum — Fri, 13 Jun 2008 08:43:58 +0000

An excellent point Alex, I knew you’d chime in with something useful!

So simply combining the two would not provide adequate protection against rainbow attacks. Very true. But the two combined with a known constant, such as the site URL, would presumably overcome this issue. Each record would be individually unique, and the whole namespace would also be globally unique.

By: Alex

Alex — Fri, 13 Jun 2008 08:40:38 +0000

… only that the username is frequently a known quantity, particularly for users such as ‘admin’ or ‘root’ that you might reasonably want to attack. Since the point of salt is to eliminate pre-calculated hashes being used in a dictionary attack, it doesn’t help when a set of pre-calculated hashes can be created with the salt ‘admin’ or ‘root’.