Cryptology ePrint Archive: Report 2016/350

Probabilistic Termination and Composability of Cryptographic Protocols

Ran Cohen and Sandro Coretti and Juan Garay and Vassilis Zikas

Abstract: When analyzing the round complexity of multi-party protocols, one often overlooks the fact that underlying resources, such as a broadcast channel, can by themselves be expensive to implement. For example, it is well known that it is impossible to implement a broadcast channel by a (deterministic) protocol in a sub-linear (in the number of corrupted parties) number of rounds.

The seminal works of Rabin and Ben-Or from the early 80's demonstrated that limitations as the above can be overcome by using randomization and allowing parties to terminate at different rounds, igniting the study of protocols over point-to-point channels with probabilistic termination and expected constant round complexity. However, absent a rigorous simulation-based definition, the suggested protocols are proven secure in a property-based manner or via ad hoc simulation-based frameworks, therefore guaranteeing limited, if any, composability.

In this work, we put forth the first simulation-based treatment of multi-party cryptographic protocols with probabilistic termination. We define secure multi-party computation (MPC) with probabilistic termination in the UC framework and prove a universal composition theorem for probabilistic-termination protocols. Our theorem allows to compile a protocol using deterministic-termination hybrids into a protocol that uses expected-constant-round protocols for emulating these hybrids, preserving the expected round complexity of the calling protocol.

We showcase our definitions and compiler by providing the first composable protocols (with simulation-based security proofs) for the following primitives, relying on point-to-point channels: (1) expected-constant-round perfect Byzantine agreement, (2) expected-constant-round perfect parallel broadcast, and (3) perfectly secure MPC with round complexity independent of the number of parties.

Category / Keywords: cryptographic protocols / probabilistic termination, Universal Composition, cryptographic protocol, randomized Byzantine agreement

Original Publication (with major differences): IACR-CRYPTO-2016

Date: received 1 Apr 2016, last revised 19 Feb 2018

Contact author: cohenrb at cs biu ac il

Available format(s): PDF | BibTeX Citation

Version: 20180220:031652 (All versions of this report)

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