Paper 2019/402 Original: eprint.iacr.org/2019/402 ↗

ILC: A Calculus for Composable, Computational Cryptography

Abstract

The universal composability (UC) framework is the established standard for analyzing cryptographic protocols in a modular way, such that security is preserved under concurrent composition with arbitrary other protocols. However, although UC is widely used for on-paper proofs, prior attempts at systemizing it have fallen short, either by using a symbolic model (thereby ruling out computational reduction proofs), or by limiting its expressiveness. In this paper, we lay the groundwork for building a concrete, executable implementation of the UC framework. Our main contribution is a process calculus, dubbed the Interactive Lambda Calculus (ILC). ILC faithfully captures the computational model underlying UC---interactive Turing machines (ITMs)---by adapting ITMs to a subset of the pi-calculus through an affine typing discipline. In other words, well-typed ILC programs are expressible as ITMs. In turn, ILC's strong confluence property enables reasoning about cryptographic security reductions. We use ILC to develop a simplified implementation of UC called SaUCy.