Biblio

Cryptographic channels aim to enable authenticated and confidential communication over the Internet. The general understanding seems to be that providing security in the sense of authenticated encryption for every (unidirectional) point-to-point link suffices to achieve this goal. As recently shown (in FSE17/ToSC17), however, the security properties of the unidirectional links do not extend, in general, to the bidirectional channel as a whole. Intuitively, the reason for this is that the increased interaction in bidirectional communication can be exploited by an adversary. The same applies, a fortiori, in a multi-party setting where several users operate concurrently and the communication develops in more directions. In the cryptographic literature, however, the targeted goals for group communication in terms of channel security are still unexplored. Applying the methodology of provable security, we fill this gap by defining exact (game-based) authenticity and confidentiality goals for broadcast communication, and showing how to achieve them. Importantly, our security notions also account for the causal dependencies between exchanged messages, thus naturally extending the bidirectional case where causal relationships are automatically captured by preserving the sending order. On the constructive side we propose a modular and yet efficient protocol that, assuming only point-to-point links between users, leverages (non-cryptographic) broadcast and standard cryptographic primitives to a full-fledged broadcast channel that provably meets the security notions we put forth.

We consider the problem of verifying the security of finitely many sessions of a protocol that tosses coins in addition to standard cryptographic primitives against a Dolev-Yao adversary. Two properties are investigated here - secrecy, which asks if no adversary interacting with a protocol P can determine a secret sec with probability textgreater 1 - p; and indistinguishability, which asks if the probability observing any sequence 0$øverline$ in P1 is the same as that of observing 0$øverline$ in P2, under the same adversary. Both secrecy and indistinguishability are known to be coNP-complete for non-randomized protocols. In contrast, we show that, for randomized protocols, secrecy and indistinguishability are both decidable in coNEXPTIME. We also prove a matching lower bound for the secrecy problem by reducing the non-satisfiability problem of monadic first order logic without equality.