Comparing SessionStateReveal and EphemeralKeyReveal for Diffie-Hellman Protocols

Both the “eCK” model, by LaMacchia, Lauter and Mityagin, and the “CK01” model, byCanetti and Krawczyk, address the effect of leaking session specific ephemeral data on thesecurity of key establishment schemes. The CK01-adversary is given a SessionStateRevealquery to learn session specific private data defined by the protocol specification, whereasthe eCK-adversary is equipped with an EphemeralKeyReveal query to access all ephemeralprivate input required to carry session computations. SessionStateReveal cannot be issuedagainst the test session; by contrast EphemeralKeyReveal can be used against the test ses-sion under certain conditions. On the other hand, it is not obvious how EphemeralKeyRevealcompares to SessionStateReveal. Thus it is natural to ask which model is more useful andpractically relevant.While formally the models are not comparable, we show that recent analysis utilizingSessionStateReveal and EphemeralKeyReveal have a similar approach to ephemeral dataleakage. First we pinpoint the features that determine the approach. Then by examiningcommon motives for ephemeral data leakage we conclude that the approach is meaningful,but does not take into account timing, which turns out to be critical for security. Lastly,for Diffie-Hellman protocols we argue that it is important to consider security when discretelogarithm values of the outgoing ephemeral public keys are leaked and offer a method toachieve security even if the values are exposed.