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RE: ADS Password Storage Protection
From: "Roger A. Grimes" <roger () banneretcs com>
Date: Tue, 18 Jul 2006 14:16:57 -0400

Being "subject to" and being successful in cracking a 15 character
password are two different things. One's theorectical, the other's real
life.  If no one I've challenged cracks the 15 character challenges, it
means something...what exactly, I don't know...but it's probably enough
to say that 15 character passwords without complexity are sufficient for
most medium-security businesses.  If they do get cracked, it means one
of two things:

1. 15-character no complexity password isn't sufficient
2. Maybe a 15-character password is sufficient if the attacker didn't
know that it contained no complexity, only contained alpha or
alpha-numeric, and all the other hints I gave.

In real life, using the same password would give even more security,
because the attacker can't be guaranteed any of the hints I gave, which
would greatly increase the possible keyspace. 

But I'm interested in testing the first premise, is a 15-character no
complexity password sufficent enough for most businesses?  This won't
prove it either way, but the results will be something to consider.

-----Original Message-----
From: Gregory Rubin [mailto:grrubin () gmail com] 
Sent: Monday, July 17, 2006 5:43 PM
To: Roger A. Grimes
Cc: eric.baechle () dhs gov; security-basics () securityfocus com
Subject: Re: ADS Password Storage Protection

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

While I agree that length is far superior to complexity, I must disagree
that 15 char is sufficient.

(Pure theory to follow)
Each additional letter in English provides approximately 1.1 bits of
entropy.  Even grossly overestimating this at 2 bits, the total entropy
of a 15 char passphrase is only 30 bits or the equivelent of a complex
password of length 3 to 4.  Thus, the passphrase remains vulnerable to
dictionary attacks.

For secure systems, the user should type a sentance.  That will easily
provide around 20 or more characters.  At that length, the entropy at
the word level (as opposed to just the letter) starts to really come
into play and the pass phrase becomes secure.  For administrators, it
doesn't even need to be much longer, but they could throw in a little
complexity as they are likely to be more competant.

For low security systems, the users are going to pick weak stuff no
matter what, so is it worth the added inconvience?

Greg

P.S. Signed with a 40+ char pass-phrase.
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=81Gb
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