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Re: Password policy publication
From: "Shalla, Kevin" <kshalla () UIC EDU>
Date: Tue, 28 Oct 2008 09:26:05 -0500

Doesn't this require stealing the password file, so that you can run the
brute-force attack?  Or are we protecting from sysadmins who already have
access to the password file?

On Mon, October 27, 2008 11:13 am, Valdis Kletnieks wrote:
On Sat, 25 Oct 2008 06:00:25 EDT, Geoff Nathan said:
Does publishing the standards for strong passwords (e.g. eight
characters, at
least one upper case, at least one numeral) constitute a security hazard
giving information to potential hackers?

"8 chars, at least one upper case and one numeral" isn't exactly what I'd
call "strong".  These days, I'd go for "at least 15-16, at least one
one numeric, and one special character".  Or go the way the Linux
module handles it - you get 1 point for each character, the sysadmin
how many extra points you get for each numeric, uppercase, lowercase, and
special chars, and a minimum total point score - so you could (for
score the site-required 20 points with a 15-char password that includes
3 uppercase and 2 special chars, *or* with a longer 20-char lowercase-only

Publishing password guidelines that do *not* constrain the search space,
convince hackers that brute force isn't worth it isn't a hazard.  And any
of publishing "your password must be this tall to ride the system" info is
far outweighed by the risk of *not* having a published policy (and
policy is just nuts - your help desk staff will lynch you after a week of
"why can't I change my password" calls...)

What *is* a hazard are guidelines  that *do* constrain the search space.
instance, if your guidelines said "*exactly* 8 chars, exactly 1 uppercase,
exactly 1 numeric", that allows an attacker to narrow down the brute-force
space a *lot*.  For "8 chars, at least one upper and numeric", the search
is (roughly) 62**8 or 218,340,105,584,896.  For the "exactly" version,
only (26**7)*10 or 80,318,101,760 - on the order of 2,718 times smaller.

For those who think that's enough that it doesn't matter - the EFF showed
how to brute-force the *entire* 2**56 DES keyspace in under 24 hours - and
that was years ago. Technology has moved along since then.  And 2**56
is 72,057,594,037,927,936 or 330 times bigger than 62**8.  So your average
8-char password can be brute forced in about 4 minutes.  Or less.

(Yes, I cheated slightly on the two values due to lack of caffeine.  Feel
to derive the actual correct formulas - the numbers don't change all that

Special note: publishing a rule that says "at least 8 chars long" when
legacy application in the system doesn't allow more than 8 chars is
saying "exactly 8".

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