Re: password aging

[Paul McNabb <mcnabb () argus-systems com>]

But this doesn't work either.  You have to assume that your users
have an idea of what a "well formed password" is.  If they don't,
I suspect you'll have a lot of unhappy users who struggle with
finding an acceptable password.  Certainly an attacker will know
or can find out what is well formed.  If a password is rejected
it will be fairly easy to determine why.

But the whole idea of a system-wide password history is useless
at best and dangerous in practice.  You gain *absolutely nothing*
with system-wide histories.  Each person's password history had
better be completely independent of every other user's history,
or else you have a much, much bigger problem.  If people are
picking passwords from a bad (easily guessed) set, then all you
should do is tighten up your definition for acceptable passwords
to exclude the weak set of passwords.  System-wide password histories
just tell everyone that someone is using those weak passwords.
Since reusable passwords (and their attendant weaknesses) represent
the biggest single threat to most systems, system-wide password
histories make your weakest link even weaker.

Over the years most vendors were convinced to never store the actual
passwords, only hashed passwords.  We then were successful in getting
most vendors to completely hide the hashed passwords.  System-wide
password histories are a giant leap back in time to give users info
about what passwords are or have been used on the system by other
users.

System wide password histories are one of those things that sound
good at first, but the more you look at it the more problems you
begin to see.  It's like the site that was doing an XOR to "encrypt"
their data, and decided that to make it stronger they'd XOR the data
twice with the key.  I suppose that doing "quadruple encryption" would
seem very impressive and sound incredibly secure to management.  ;^)

Again, system-wide password histories are a security hole and
should be avoided at all cost.  If you are ever doing a security
audit of a site and find they are using a system wide password
history, you had better flag it as a problem and recommend that
they close that hole in their system.

paul

>  From steve () aztech net  Fri Aug 28 09:49:49 1998
>  
>  System-wide password histories shouldn't be used unless
>  you are also doing dictionary/pattern checks.  Given those
>  constraints, and the soundex + hashing that I mentioned in
>  my previous message, it will be difficult for an end-user to
>  determine exactly why their new password choice was
>  rejected by the system. Your example of "sleepy7" could
>  have been rejected at any stage of the "sanity checks".
>  The specific reason for rejecting the new password should
>  not be reported to the user, they only need to be told that
>  the new password is probably weak.
>  
>  Think of the global history as an adjunct to a dictionary check.
>  The dictionary, in this case just happens to match your user-
>  base's actual use very closeley, and it changes over time.
>  
>  Paul McNabb wrote:
>  
>  > >  From: "Stephen P. Gibbons" <steve () aztech net>
>  > >
>  > >  Storing the hashes system-wide so that user2 wouldn't be
>  > >  allowed to use a password that had been previously used
>  > >  by user1 or user3 is another option.  Use a strong hash, and
>  > >  chmod 0600 the history file if you do this.  Yeah, an HMAC
>  > >  would work better, but then you can't easily combine histories
>  > >  and have to protect the key used.
>  >
>  > Having a "system wide" password history is an EXTREMELY bad thing
>  > to do!  It is much worse than doing nothing at all!  The reason
>  > for a password history mechanism in the first place is because you
>  > want to break up (as much as possible) the password selection
>  > mechanism so that passwords are more random (and harder to guess)
>  > for a user.
>  >
>  > If your users are not telling each other their passwords, then any
>  > passwords reused by other people are purely random anyway.  And if
>  > a user ever gets a "hit" on a password, then he/she knows something
>  > about the password selection habits of someone on the system.  For
>  > example, if a user can't choose the password "sleepy7" because someone
>  > else has used it, it may be a reasonable guess that someone is using
>  > a "Snow White" theme in choosing passwords.
>  >
>  > System wide password history mechanisms remove randomness from the
>  > password set being used, and they *secretly* pass information about
>  > one user's passwords to another user.
>  >
>  > It should be obvious that if the password history includes the current
>  > passwords, then if I get a "hit" when trying to select a new password,
>  > I am fairly sure that some account on the system has the password I
>  > just tried!
>  >
>  > The moral?  NEVER, NEVER, NEVER USE SYSTEM WIDE PASSWORD HISTORIES!!

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Paul McNabb                     Argus Systems Group, Inc.
Vice President and CTO          1809 Woodfield Drive
mcnabb () argus-systems com        Savoy, IL 61874 USA
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