🚀 go-pugleaf

Some general comments about password expiration.   Assumption: the
intruder is trying to guess a password by already having access to the
encrypted form.

Suppose T1 is the time it takes for the intruder to determine a password
by some sort of search (guessing, exchaustive, whatever).  Suppose T2 is
the time it takes for an intruder to actually *begin using* a password
once he has guessed it.  So if the intruder acquires an encrypted
password at time t0, he can begin using it at time t0 + T1 + T2.

It makes sense to set the password expiration period to be less than T1 +
T2.  Then, by the time the intruder is ready to use the password, it has
been changed.

So what are useful values for T1 and T2?

My guess is that T2 can be as little as 15 seconds, the time it takes to
log in.

For well-chosen Unix passwords T1 is at least decades, if not several
centuries.  We need to expire a password in (some decades + 15 seconds).
It should be reasonable to expire a password in about a year.

For *poorly* chosen passwords, T1 might be as short as a day.  In that
case, passwords should expire every few hours.

CONCLUSION 1:  If passwords are well-chosen, password expiration adds
nothing to security.  If passwords are poorly-chosen, password expiration
must occur too rapidly to be of much use.

However, consider the intruder's point of view.  If the intruder can
immediately get access to the encrypted form a password when it is
changed, then password expiration becomes irrelevant.  Mathematically
speaking, if the intruder is always trying the *current* encrypted
password, his chances of guessing it neither increase nor decrease if the
encrypted password changes every 6 hours (or however often).

CONCLUSION 2:  If the intruder has immediate access to changed encrypted
passwords, then password expiration has no beneficial effect at all.

FINAL CONCLUSION:  Password expiration sounds good, may impress
customers, and certainly makes a sysadmin feel powerful and in control
(the VAX/VMS syndrome).  But it's far, far better to use a "passwd"
program that doesn't allow poor passwords in the first place.  And if you
do so, shadow password files aren't really needed, though they are still
a good thing.
-- 
Rahul Dhesi <dhesi@cirrus.com>
also: dhesi@rahul.net
"He's metabolically challenged, Jim."