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Researchers: Lujo Bauer, Lorrie Cranor, Nicolas Christin

Cross Cutting Thrusts: Usable Privacy and Security


To combat both the inherent and user-induced weaknesses of text-based passwords, administrators and organizations typically institute a series of rules – a password policy – to which users must adhere when choosing a password. There is consensus in the literature that a properly-written password policy can provide an organization with increased security. There is, however, less accord in describing just what such a well-written policy would be, or even how to determine whether a given policy is effective. Although it is easy to calculate the theoretical password space that corresponds to a particular password policy, it is difficult to determine the practical password space. Users may, for example, react to a policy rule requiring them to include numbers in passwords by overwhelmingly picking the same number, or by always using the number in the same location in their passwords. There is little published empirical research that studies the strategies used by actual users under various password policies. In addition, some password policies, while resulting in stronger passwords, may make those passwords difficult to remember or type. This may cause users to engage in a variety of behaviors that might compromise the security of passwords, such as writing them down, reusing passwords across different accounts, or sharing passwords with friends. Other undesirable side effects of particular password policies may include frequently forgotten passwords. In fact, the harm caused by users following an onerously restrictive password policy may be greater than the harm prevented by that policy. In this project, we seek to advance understanding of the factors that make creating and following appropriate password policies difficult, collect empirical data on password entropy and memorability under various password policies, and propose password policy guidelines to simultaneously maximize security and usability of passwords. We also explore the security and usability of some new types of passwords.