Both the “eCK” model, by LaMacchia, Lauter and Mityagin, and the “CK01” model, by Canetti and Krawczyk, address the effect of leaking session specific ephemeral data on the security of key establishment schemes. The CK01-adversary is given a SessionStateReveal query to learn session specific private data defined by the protocol specification, whereas the eCK-adversary is equipped with an EphemeralKeyReveal query to access all ephemeral private input required to carry session computations. SessionStateReveal cannot be issued against the test session; by contrast EphemeralKeyReveal can be used against the test session under certain conditions. On the other hand, it is not obvious how EphemeralKeyReveal compares to SessionStateReveal. Thus it is natural to ask which model is more useful and practically relevant. While formally the models are not comparable, we show that recent analysis utilizing SessionStateReveal and EphemeralKeyReveal have a similar approach to ephemeral data leakage. First we pinpoint the features that determine the approach. Then by examining common 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 discrete logarithm values of the outgoing ephemeral public keys are leaked and offer a method to achieve security even if the values are exposed.
We extend the well-known Tree-Diffie-Hellman technique used for the design of group key exchange (GKE) protocols with robustness, i.e. with resistance to faults resulting from possible system crashes, network failures, and misbehavior of the protocol participants. We propose a fully robust GKE protocol using the novel tree replication technique: our basic protocol version ensures security against outsider adversaries whereas its extension addresses optional insider security. Both protocols are proven secure assuming stronger adversaries gaining access to the internal states of participants. Our security model for robust GKE protocols can be seen as a step towards unification of some earlier security models in this area.