Cryptographic Protocols: Surprising Vistas for Communication

Arto K. Salomaa

Matthew Franklin, Mark A. Gondree, Payman Mohassel

We design communication efficient two-party and multi-party protocols for the longest common subsequence (LCS) and related problems. Our protocols achieve privacy with respect to passive adversaries, under reasonable cryptographic assumptions. We benefit from the somewhat surprising interplay of an efficient block-retrieval PIR (Gentry-Ramzan, ICALP 2005) with the classic “four Russians” algorithmic design. This result is the first improvement to the communication complexity for this application over generic results (such as Yao’s garbled circuit protocol) and, as such, is interesting as a contribution to the theory of communication efficiency for secure two-party and multiparty applications.

Ling Dong, Kefei Chen

Richard A. DeMillo, Nancy Ann Lynch, Michael Merritt

Rajagopal Nagarajan, Nickolas K. Papanikolaou

In the 1980s it was first realized that quantum–mechanical phenomena can be exploited directly for the manipulation, storage and transmission of information. The discovery of quantum algorithms for prime factorization [18] and unstructured search [7], which outperformed the best classical algorithms for these tasks significantly, opened up new vistas for computer science and gave an initial thrust to the emerging field of quantum computation. To implement a quantum algorithm, however, a large scale quantum computer is necessary and such a device has yet to be built. Research in quantum information, on the other hand, has shown that quantum effects can be harnessed to provide efficient and highly secure communication channels, which can be built using current technology. Entangled quantum states, superpositions and quantum measurement are among the characteristics of the subatomic world which nature puts at our disposal; these and related phenomena enable the development of novel techniques for computation and communication with no rival in classical computing and communication theory. The focus in this paper is on communication protocols involving the use of such phenomena. Quantum protocols have particularly important applications in cryptography. Several quantum protocols have been proposed for cryptographic tasks such as oblivious transfer, bit commitment and key distribution [8,13]. The BB84 protocol for quantum key distribution [2,14], which allows two users to establish a common secret key using a single quantum channel, has been shown to be unconditionally secure against all attacks [11]. Other quantum protocols include procedures for

Michael Backes A, Dominique Unruh

Abstract. We investigate the security of protocols with logarithmic communication complexity. We show that for the security definitions with environment, i.e., Reactive Simulatability and Universal Composability, computational security of logarithmic protocols implies statistical security. The same holds for advantage-based security definitions as commonly used for individual primitives. While this matches the folklore that logarithmic protocols cannot be computationally secure unless they are already statistically secure, we show that under realistic complexity assumptions, this folklore does surprisingly not hold for the stand-alone model without auxiliary input, i.e., there are logarithmic protocols that are statistically insecure but computationally secure in this model. The proof is conducted by showing how to transform an instance of an NP-complete problem into a protocol with two properties: There exists an adversary such that the protocol is statistically insecure in the stand-alone model, and given such an adversary we can find a witness for the problem instance, hence yielding a computationally secure protocol assuming the hardness of finding a witness. The proof relies on a novel technique that establishes a link between cryptographic definitions and foundations of computational