[unisog] [Fwd: Is the current password std flawed?]

Valdis.Kletnieks at vt.edu Valdis.Kletnieks at vt.edu
Fri Feb 25 05:05:14 GMT 2005

On Thu, 24 Feb 2005 20:30:29 CST, "Clinton E. Troutman" said:

> CS's statement would be correct if:
> - it is known that a particular character position in any particular password
> *must* contain *only* a numeric, or

But actually, you *DO* know that for many cases.  For instance, if there
is a *requirement* that at least 1 position have a numeric, you can not bother
trying all the combinations that don't have at least 1 digit.  So if you're
brute-forcing, and the min length is 8, and you're testing  'aaaaaaa' and
another character, you can only try 10 and be done, rather than all 96 printables.
Similarly for *aaaaaaa, a*aaaaaa, aa*aaaaa, and so on...

To mathematically model it, let's say you have 8 positions and 96 usable chars.
If all 8 are free, you have 96^8.  If you force a digit, you only have 10*96^7
(or only about 10% of the space).  If you force a digit and a "special", you're
down to 10*96^6*34, or about 3% of the original space.

A *better* way is the way that Fedora Core's 'pam_cracklib' does it:

        minlen=N        The minimum simplicity count for a good password.

        ocredit=N       Weight, digits, upper, lower, other characters with
                        count N. Use these values to compute the
                        'unsimplicity' of the password.

So you can say, for instance, that you need to score at least 15 points. Let's
say we have d/u/l/o credit of 2/2/1/3 - so you can get there with a password of
15 lower case chars, or 10 lower case, a digit, and a 'other', or 11 lower case
and 2 digits, or....  If you use a minlen of 20 or so with the weights I
listed, you're creating a *HUGE* space an attacker has to choose through - and
users can still come up with some easily memorable passphrases or whatever.  If
they don't want to type a lot, they can get to 21 points with just 7 special
characters and no letters/numbers at all. ;)

This way, you lose a *lot* less entropu, because no one position is "forced"
because there's more than one way to get the needed points....
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