International Association for Cryptologic Research

International Association
for Cryptologic Research

IACR News item: 27 February 2015

Jeremiah Blocki, Manuel Blum, Anupam Datta
ePrint Report ePrint Report
We introduce quantitative usability and security models to guide the design of \\emph{password

management 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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