RE: A doable frequent password change policy?

["Craig Wright" <Craig.Wright () bdo com au>]

Hi
There are 3 aspects to password chaning:
1   Crack of the password and the length it is valid over a network use case.
2   Capture and crack of the password database offline
3   Leakage of the password through other means - eg brute force grinding of the admin password over Terminal services 
open tro the Internet.
 
These are a function of the controls. First to address issues 1 and 2. These require a capture of a secret. Issue 1 is 
easy to capture, but limited in what should be captured. Issue 2 is difficult to do (or should be so) but gives a large 
amount of information.
 
There are other more effective controls over authentication for issues 1 and 2 than enforced password changes. The use 
of IPSec for instance (issue 1) or the monitoring and lockdown of the authentication server (issue 2).
 
Issue 3 is generally the one people are most worried about. However, this is the issue that is the least likely to be 
addressed from a password change regime. Mathematically, the likelihood function derived from a password change being 
effective in stopping an attacker guessing the password subsequent to the change is related to the strength of the new 
password relative to the old and not the frequency of change. The frequency of change is insignificant even at the 
alpha = 1% level and may be ignored. In fact, the issues with users having to remember passwords and the resultant 
issue of having them write them etc is actually of larger effect mathematically and more than negates the benifit of 
password changing with issue 3
 
What we are left with is the function of the likelihood of a password being guessed from the old password weighted 
against the same likelihood on the new password. So there is thus no chance in likelihood of compromise for an "equal" 
password, less for a simpler (mathematically) one and a larger likelihood for a more complex one.
 
This is possible to model mathematically, but due to the complexity of issues 1 and 2 and the number of network 
controls, computationally difficult. It is not as if it is in the NP hard range though. I have started working on a 
Bayesian model which can take a large variety of controls to model all three issues and derive the optimum password 
change time. I do not expect to have this finished before this time next year though. 
 
What matters most is the strength of the password and controls to prevent access to it, not the age. The reason to 
change passwords is based on an incomplete understanding of our systems (which is understandable due to complexity), 
the incompleteness of controls over the passwords and the threat of passwords having been compromised already and thus 
limiting the time that this threat can act as an exposure.
 
Regards,
Craig



Craig Wright
Manager of Information Systems

Direct : +61 2 9286 5497
Craig.Wright () bdo com au
+61 417 683 914

BDO Kendalls (NSW)
Level 19, 2 Market Street Sydney NSW 2000
GPO BOX 2551 Sydney NSW 2001
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www.bdo.com.au

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________________________________


From: listbounce () securityfocus com on behalf of WALI
Sent: Fri 29/06/2007 3:20 PM
To: security-basics () securityfocus com
Subject: A doable frequent password change policy?



Yes I am aware of the importance of advising users on changing their
passwords frequently, be it their AD passwords or passwords on other
independent applications (ERP) etc.

But I don't want to enforce a policy that comes crashing down. I
personally, cannot keep changing my password every month making sure that
it differs from the last two in history (at least).

Even Cisco on it's CCO account only makes it's users aware that their
password hasn't been changed for quite some time and giving them an option
of either changing it or just do a 'No Thanks' option and carry on with
their old password. This sounds like a doable compliance to me.

Your thoughts??