Hash salting -- digression

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To: 'Eric Knight' <eric@swordsoft.com>, secprog@securityfocus.com
Date: Thu, 18 Dec 2003 14:50:43 -0500

Thanks, Eric. There are many discussion points in this post but I'm going
to seize on one, because I would like to know if anyone has access to
in-depth research on it:
> A single second as deemed "adequate".

Something that goes hand-in-hand with a salt is an iteration count.
Recently I have been doing some password security investigations and I tried
to approach the question of the actual value of an iteration count.

To quote PKCS #5:
"For the methods in this document, a minimum of 1000 iterations is
recommended. This will increase the cost of exhaustive search for passwords
significantly, without a noticeable impact in the cost of deriving
individiual keys."

Iteration counts seem to be valuable in two ways: they greatly increase the
difficulty and library size of precomputed hashes (just as a salt does), and
also increase the cost of brute force even when the attacker has access to
the hash+salt+IC.

At first glance, this seems very effective. If an IC causes the hash
computation time (on a modern desktop) to grow to 1 or 2 seconds, it makes
the cracker's job exceedingly difficult. It seems that with a good (and
enforced) password policy, and an adequate random salt, the threat of
password compromise could become limited to attackers with access to vast
computing resources.

***But is that true?***

  While I know many systems employ iteration counts, I have been unable to
find any good analysis on their value in this regard. There exists the
possibility that a hash function could be algorithmically 'shortcutted',
allowing for much faster computation of digest values.

This seems unlikely, but I am certainly no cryptanalyst. If anyone is aware
of any work that has been done in this area, I'd appreciate a point in the
right direction.

Cheers,
Ken Buchanan

-----Original Message-----
From: Eric Knight [mailto:eric@swordsoft.com]
Sent: Wednesday, December 17, 2003 7:24 PM
To: secprog@securityfocus.com
Cc: CraigSecurity@blazemail.com
Subject: Re: Values to use for a salt?

Craig,

Yes, the salt is meant to be a deterrent to guessing passwords, the purpose
is exactly to make using a dictionary attack computationally unfeasible.
However, once a salt is computed and the hashing algorithm is all that
stands between the attacker and the defender, the process speeds up
entirely.

The crypto-logic, based on Dr. Robert Morris Sr.'s approach, was that the
password would require 1 second of computational time on a PDP-8 computer.
These days, 1 second of computational time on desktop PC could do
tens-of-thousands of these. A single second as deemed "adequate". DES was
used as the hash function, and it wouldn't be useful entirely by itself if
it didn't have the one second computation, so the salt was added to the hash
to slow it down.

The salt also served the purpose of disguising the similar passwords in the
database when two users pick the same one. As a second purpose, the salt
provides better protection against casual observation than it does against
computational attacks against it. To ensure this, salts are computed
differently than the hash. If you were to, for example, use the password
key to create a salt to hide the hash, you are created a two-part hash
instead of a salt.

The magic formula should be something along the lines of: 1 sec =
T(S(SaltValue) + H(HashValue)) [where T is a time measurement, H is the
hash function, S is the salt function].

"Same Salt" attack is where the salt is calculated once, and the hash is
calculated multiple times. That is 1 sec = T(S(SaltValue) + H(HashValue)
for the first try, then its 1 sec - T(H(HashValue)) = T(S(SaltValue)

"Similar Salts" attack is where multiple entries with the same salt value so
logically, its the computation of O(sqrt(N)) (from the sort) added into the
"Same Salt" attack, subtracting each instance of a similar salt from the
overall required list. Although this can create a speed decrease in cases
of no repetitive salts, in most cases it yields at least a few.

There is strength inherent in using salts, but they have weaknesses as well.
The optimum use of salt+hash for passwords would have very little weight on
the Time factor on the salt, and very heavy weight on T for the hash.

As far as crypto is concerned, its hard to explain that the strength of any
crypto is dependant on the time it takes to break, and as we've expanded the
keys to sizes that are impossible for anyone but a computer to remember, the
hash itself is going to have to become more time consuming in order to have
validation of strength.

After having "broken" nearly a quarter-million passwords in the course of my
career pen-testing and research, its a semi-dead subject to me. As an
industry, we really need to be moving onward to solving other surrounding
issues to better enforce the password collections. The onion effect of
packing passwords in passwords, encrypting and hiding password files, and so
forth and so on is only as reasonable as the question of who knows the
process and how difficult is it to figure out.

This brings about the question of creating custom crypto with unpublished
implementations. I think of it was "security with inherent weakness", which
is a broad category that all crypto falls into. I use it all the time, but
that's mostly because I can control its implementation better and I can
change the standards at a whim. Obviously, this can't be done well with
ITAR restrictions and allowing people to program to your code, but the
"game" of crypto legalities is to hide and uncover secrets, and its neither
a blackhat nor a whitehat exclusive.

Well, I'm going to cut off here because otherwise I'd write another 100 page
book, there is a lot of depth to authentication components compared to
simple password hiding. In the future, especially with keys reliably
generated from biometric data, that shouldn't benefit much at all from
salts.

Take it easy,

Eric Knight

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