In October 2016, UCL’s Information Services Division (ISD) implemented a new password policy to encourage users to choose stronger passwords. The policy links password lifetime (the time before the password expires) to password strength: The stronger the password, the longer the lifetime.

We (Ingolf Becker, Simon Parkin and M. Angela Sasse) decided to collaborate with the Information Services Division to study the effect of this policy change, and the results were published at USENIX Security this week. We find that users appreciate the choice and respond to the policy by choosing stronger passwords when changing passwords. Even after 16 months the mean password lifetime at UCL continues to increase, yet stronger passwords also lead to more password resets.

The new policy

In the new policy, passwords with Shannon Information Entropy of 50 bits receive a lifetime of 100 days, and passwords with 120 bits receive a lifetime of 350 days:

Password expiry by entropy

Additionally, the new policy penalises the lifetime of passwords containing words from a large dictionary.

Users play the game

We analysed the password lifetime – what we will refer to from here on in as the ‘password strength’ – of all password change and reset events of all pseudonymised users at UCL. The following figure shows the mean password expiration of all users over time, smoothed by 31-day moving averages:

Password expiration over time for all users and new users.

A small drop in password strength was observed between November ’16 and February ’17, as users were moved on to and generally became accustomed to the new system; the kinds of passwords they would have been used to using were at that point not getting them as many days as before (hence the drop). After February ’17, the mean strength increases from 145 days to 170 days in 12 months – an increase of 6.9 bits of entropy. This strongly suggests that users have generally adapted slowly to the new password policy, and eventually make use of the relatively new ability to increase password lifetime by expanding and strengthening their passwords.

The evolution of the mean password strength is underpinned by cyclical behaviours. A quarter of users have a password lifetime of less than 110 days and have to change their passwords on average every 80 days, but every time they do, they increase their average password strength.

This manifests twice in this figure: at the start of the deployment of the new system where there are no existing users (the increase in password strength is delayed until February ’17); and again, with the enrolment of over 10,000 new students who set their first password around September ’17, in time for the start of the new academic year. As this large number of users have all set their initial passwords in a short time frame, their first regular password change occurs from November ’17 onwards.

Changes and Resets

Over the 16 months of data collected, we observe that 66% of users had to reset their passwords. On average, there are 1.1 resets per user, and 2.4 changes per user. There is a strong positive correlation between password strength and likelihood of reset before expiration: A user with 300 days lifetime is 4 times as likely to forget their password than a user with a lifetime of 100 days.

Also, the more times a user resets a password, the weaker their password choices.

Password expiration over time grouped by number of password resets.

In the figure above we plot the average password lifetime of unexpired passwords grouped by the number of password resets a user has performed. While the average password lifetime of all groups is increasing as the users renew their password, the division between users with 0 or 1 resets and users with more resets is pronounced, separated by at least 10 days of lifetime.
This analysis suggests that one reset per year does not affect the system’s performance, but two or more resets do (which applies to 27% of users).

The effect of expiration warnings

At UCL, we are sent a reminder of a password’s impending expiration 5 times: 30, 20, 10, 4 and 1 day(s) in advance. The following figure plots the frequency of password changes and resets
relative to the time of password expiration:

Change and reset frequency relative to the time of password expiration.

Every time users receive an expiration reminder, approximately 10% of all users act upon the reminder and change their password within 24 hours. This implies that users on average change their password 22 days before expiration. Essentially the frequent reminders causes users to voluntarily reduce the lifetime of their passwords.

A useful policy?

The intervention was clearly successful: users – of all user groups – have been choosing stronger passwords in return for longer lifetime. However, from a cost-benefit analysis the intervention is counterproductive: All passwords at UCL fall into what Florencio et al. call the ‘Don’t care region of password strength’ wherein any increase in password strength provides no additional security. The weakest possible password (100 days) is strong enough to resist an online attack; while the strongest possible password (350 days) is not strong enough to resist an offline attack. This ought to be considered alongside an increase in costs to the user to memorise and use more complex passwords. We also observed that stronger passwords cause a higher reset frequency, which increases interactions with online self-help and helpdesk support.

In light of this analysis, one argument could be that this policy is useful when it encourages users to choose passwords that are ‘usefully stronger’, giving a limited lifetime to passwords weaker than the online guessing threshold (or some other threshold that is supported by threat models) and removing expiration of passwords beyond that threshold, in line with current guidance. We have continued to have productive discussions on the password system and authentication management with UCL’s ISD since completing the analysis.

Further details can be found in the full paper: The Rewards and Costs of Stronger Passwords in a University: Linking Password Lifetime to Strength.

This blog post first appeared on Bentham’s Gaze, the blog of UCL’s Information Security Group.