Pico without public keys

B. Christianson, Frank Stajano, Mark Lomas, Graeme Jenkinson, Jeunese Payme, Quentin Stafford-Fraser, Max Spencer
Revised Selected Papers of the 23rd International Workshop on Security Protocols XXIII - Volume 9379, Pages 195-211
Pico is a user authentication system that does not require remembering secrets. It is based on a personal handheld token that holds the user’s credentials and that is unlocked by a “personal aura” generated by digital accessories worn by the owner. The token, acting as prover, engages in a public-key-based authentication protocol with the verifier. What would happen to Pico if success of the mythical quantum computer meant secure public key primitives were no longer available, or if for other reasons such as energy consumption we preferred not to deploy them? More generally, what would happen under those circumstances to user authentication on the web, which relies heavily on public key cryptography through HTTPS/TLS? Although the symmetric-key-vs-public-key debate dates back to the 1990s, we note that the problematic aspects of public key deployment that were identified back then are still ubiquitous today. In particular, although public key cryptography is widely deployed on the web, revocation still doesn’t work. We discuss ways of providing desirable properties of public-key-based user authentication systems using symmetric-key primitives and tamperevident tokens. In particular, we present a protocol through which a compromise of the user credentials file at one website does not require users to change their credentials at that website or any other. We also note that the current prototype of Pico, when working in compatibility mode through the Pico Lens (i.e. with websites that are unaware of the Pico protocols), doesn’t actually use public key cryptography, other than that implicit in TLS. With minor tweaks we adopt this as the native mode for Pico, dropping public key cryptography and achieving much greater deployability without any noteworthy loss in security.

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