From: pcl@oxford.ac.uk (Paul Leyland)
Newsgroups: comp.security.misc,alt.security,comp.unix.ultrix
Subject: Re: Problem with npasswd??
Date: 28 Jul 92 10:46:38 GMT
	<1992Jul27.184324.14697@hubcap.clemson.edu>
Organization: Oxford University Computing Service, 13 Banbury Rd, Oxford, OX2
	6NN
In-reply-to: hubcap@hubcap.clemson.edu's message of 27 Jul 92 18:43:24 GMT

In article <1992Jul27.184324.14697@hubcap.clemson.edu> hubcap@hubcap.clemson.edu (System Janitor) writes:

> Unless I am mistaken, npasswd doesn't check for everything crack does.

You are correct.

> Even dropping the time consuming crypt part, it seems like it would
> take an unacceptable amount of time to change your password if it were 
> checked against, say, an 800,000 word dictionary with 300 transmogrification
> rulesets.

I don't know.  I could experiment, I suppose.  Looking up a word in a
800k entry dictionary only takes about 30% more time than looking it
up in the 25k entry /usr/dict/words.  (A binary search is used, on the
assumption that the dictionary is sorted, so we're comparing 2**20
against 2**15: 20/15 = 1.33).  Some rulesets could be dismissed
without doing the dictionary check.  For instance "append 2" will fail
if the candidate does not end in 2.  I suspect that full Crack testing
would not greatly increase checking time if optimisations like this
were included.

> The point is, even if you use npasswd, a cracker will still get some of
> your passwords. So what if they only get 10 instead of 200, they'll still

In my experience, the efficacy of Crack drops to near zero when it has
been used to crack a system's passwords *and the broken accounts are
disabled*.  Users really do learn to choose Crack-resistant passwords
if they are required to come in with proof of identity before they can
use their account again.  Npasswd helps them to choose Crack-resistant
passwords.

In my experience, a naive Unix site will have 20-30% guessable
passwords.  A Crack- and npasswd -free site, but otherwise fairly
security concious will yield 10-15%.  After the first iteration of
Crack and account disabling, I found four passwords out of several
hundred -- around 1%.  After two iterations I didn't break a single
account.

> have some userids from which to launch their nefarious plans.
> So, aren't you fooling yourself to think that npasswd like schemes 
> enhance your system security enough to make them worthwhile? 

I don't think so.  It is a relatively cheap addition to password
security.  Admittedly, it isn't perfect; nothing in this life is.

However, it has two important benefits.  First, it ensures that the
simple Crack rules won't work.  Minor benefit, as one can always
re-order the rules to do the bizarre ones first.

Secondly, and much more importantly in my view: it gets the user to
*think* about password security.  If forced to use non-trivial
passwords, people tend to become quite creative.  They also tend to
carry this creativity to other systems where it is not strictly
ncessary, but still desirable.

> I realize there is a need for this kind of security, and of course, I
> don't have the right answer. But it doesn't seem like npasswd is it. 
> Discussion?

Npasswd does *not* solve all of life's problems.  It is a useful tool;
nothing more.  Shadow passwords, password aging and other such
techniques are also useful tools.  People who believe they can solve
the security problem are deluding themselves.  The best we can do with
current technology is to increase the cost/benefit ratio for crackers,
to the point where it is (probably) not worth the effort, without
increasing it too much for authorised users.  What "too much" means
depends strongly on local circumstances.

Paul

--
Paul Leyland <pcl@oxford.ac.uk>          | Hanging on in quiet desperation is
Oxford University Computing Service      |     the English way.
13 Banbury Road, Oxford, OX2 6NN, UK     | The time is come, the song is over.
Tel: +44-865-273200  Fax: +44-865-273275 | Thought I'd something more to say.
Newsgroups: comp.security.misc