IACR News item: 27 February 2015
Jeremiah Blocki, Manuel Blum, Anupam Datta
ePrint Reportmanagement schemes} --- systematic strategies to help users create and remember multiple
passwords. In the same way that security proofs in cryptography are based on
complexity-theoretic assumptions (e.g., hardness of factoring and discrete logarithm), we quantify
usability by introducing \\emph{usability assumptions}. In particular, password management relies
on assumptions about human memory, e.g., that a user who follows a particular rehearsal
schedule will successfully maintain the corresponding memory. These assumptions are informed by research in cognitive science and can be tested empirically. Given rehearsal requirements and a user\'s
visitation schedule for each account, we use the total number of extra rehearsals that
the user would have to do to remember all of his passwords as a measure of the usability of
the password scheme. Our usability model leads us to a key observation: password reuse benefits users not only by reducing the number of passwords that the user has to memorize, but more importantly by increasing the natural rehearsal rate for each password. We also present a security model which accounts for the complexity of password
management with multiple accounts and associated threats,
including online, offline, and plaintext password leak attacks. Observing that current
password management schemes are either insecure or unusable, we present Shared Cues--- a new scheme in which the underlying secret is strategically
shared across accounts to ensure that most rehearsal requirements are satisfied naturally while
simultaneously providing strong security. The construction uses the Chinese Remainder Theorem to achieve these competing goals.
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