Biblio

The security of almost any real-world distributed system today depends on the participants having some "reasonably accurate" sense of current real time. Indeed, to name one example, the very authenticity of practically any communication on the Internet today hinges on the ability of the parties to accurately detect revocation of certificates, or expiration of passwords or shared keys.,,However, as recent attacks show, the standard protocols for determining time are subvertible, resulting in wide-spread security loss. Worse yet, we do not have security notions for network time protocols that (a) can be rigorously asserted, and (b) rigorously guarantee security of applications that require a sense of real time.,,We propose such notions, within the universally composable (UC) security framework. That is, we formulate ideal functionalities that capture a number of prevalent forms of time measurement within existing systems. We show how they can be realized by real-world protocols, and how they can be used to assert security of time-reliant applications - specifically, certificates with revocation and expiration times. This allows for relatively clear and modular treatment of the use of time consensus in security-sensitive systems.,,Our modeling and analysis are done within the existing UC framework, in spite of its asynchronous, event-driven nature. This allows incorporating the use of real time within the existing body of analytical work done in this framework. In particular it allows for rigorous incorporation of real time within cryptographic tools and primitives.

We report on our research on proving the security of multi-party cryptographic protocols using the EASYCRYPT proof assistant. We work in the computational model using the sequence of games approach, and define honest-butcurious (semi-honest) security using a variation of the real/ideal paradigm in which, for each protocol party, an adversary chooses protocol inputs in an attempt to distinguish the party's real and ideal games. Our proofs are information-theoretic, instead of being based on complexity theory and computational assumptions. We employ oracles (e.g., random oracles for hashing) whose encapsulated states depend on dynamically-made, nonprogrammable random choices. By limiting an adversary's oracle use, one may obtain concrete upper bounds on the distances between a party's real and ideal games that are expressed in terms of game parameters. Furthermore, our proofs work for adaptive adversaries, ones that, when choosing the value of a protocol input, may condition this choice on their current protocol view and oracle knowledge. We provide an analysis in EASYCRYPT of a three party private count retrieval protocol. We emphasize the lessons learned from completing this proof.