On Fairness in Simulatability-based Cryptographic Systems

Simulatability constitutes the cryptographic notion of a secure refinement and has asserted its position as one of the fundamental concepts of modern cryptography. Although simulatability carefully captures that a distributed protocol does not behave any worse than an ideal specification, it however does not capture any form of liveness guarantees, i.e., that something good eventually happens in the protocol. We show how one can extend the notion of simulatability to comprise liveness guarantees by imposing specific fairness constraints on the adversary. As the common notion of fairness based on infinite runs and eventual message delivery is not suited for reasoning about polynomial-time, cryptographic systems, we propose a new definition of fairness that enforces the delivery of messages after a polynomial number of steps. We provide strengthened variants of this definition by granting the protocol parties explicit guarantees on the maximum delay of messages. The variants thus capture fairness with explicit timeout signals, and we further distinguish between fairness with local timeouts and fairness with global timeouts. We compare the resulting notions of fair simulatability, and provide separating examples that help to classify the strengths of the definitions and that show that the different definitions of fairness imply different variants of simulatability

High Level Concurrency in C∀

Concurrent programs are notoriously hard to write and even harder to debug. Furthermore concurrent programs must be performant, as the introduction of concurrency into a program is often done to achieve some form of speedup. This thesis presents a suite of high-level concurrent-language features in the new programming language C∀, all of which are implemented with the aim of improving the performance, productivity, and safety of concurrent programs. C∀ is a non-object-oriented programming language that extends C. The foundation for concurrency in C∀ was laid by Thierry Delisle [15], who implemented coroutines, user-level threads, and monitors. This thesis builds upon that work and introduces a suite of new concurrent features as its main contribution. The features include a mutex statement (similar to a C++ scoped lock or Java synchronized statement), Go-like channels and select statement, and an actor system. The root ideas behind these features are not new, but the C∀ implementations extends the original ideas in performance, productivity, and safety

Notes on Theory of Distributed Systems

Notes for the Yale course CPSC 465/565 Theory of Distributed Systems