The security of modern password expiration: an algorithmic framework and empirical analysis, by Yingian Zhang, Fabian Monrose and Michael Reiter. (ACM DOI link)
This paper presents the first large-scale study of the success of password expiration in meeting its intended purpose, namely revoking access to an account by an attacker who has captured the account’s password. Using a dataset of over 7700 accounts, we assess the extent to which passwords that users choose to replace expired ones pose an obstacle to the attacker’s continued access. We develop a framework by which an attacker can search for a user’s new password from an old one, and design an efficient algorithm to build an approximately optimal search strategy. We then use this strategy to measure the difficulty of breaking newly chosen passwords from old ones. We believe our study calls into question the merit of continuing the practice of password expiration.
This is the sort of work that we at the New School love. Take a best practice recommended by just about everyone for what seems like excellent reasons, and take notice of the fact that human beings are going to game your practice. Then get some actual data, and see how effective the practice is.
Unfortunately, we lack data on rates of compromise for organizations with different password change policies. So it’s hard to tell if password policies actually do any good, or which ones do good. However, we can guess that not making your default password “stratfor” is a good idea.
ACM gets a link because they allow you to post copies of your own papers, rather than inhibiting the progress of science by locking it all up.