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Majumdar R.,Max Planck Institute for Software Systems (Saarbrucken)
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2016

Computers have come a long way from their roots as fast calculating devices. We live in a world in which computers collect, store, and analyze huge volumes of data. We are seeing the beginnings of a new revolution in the use of computers. In addition to collecting and analyzing data, computers are influencing the physical world and interacting autonomously, and in complex ways, with large groups of humans. These cyber-physical-social systems have the potential to dramatically alter the way we lead our lives. However, designing these systems in a reliable way is a difficult problem. In this paper, we enumerate a set of research challenges that have to be overcome in order to realize the potential of cyber-physical-social systems. © Springer-Verlag Berlin Heidelberg 2016.

Abdalla M.,Ecole Normale Superieure de Paris | Catalano D.,University of Catania | Fiore D.,Max Planck Institute for Software Systems (Saarbrucken)
Journal of Cryptology | Year: 2014

In this paper we show a relation between the notions of verifiable random functions (VRFs) and identity-based key encapsulation mechanisms (IB-KEMs). In particular, we propose a class of IB-KEMs that we call VRF-suitable, and we propose a direct construction of VRFs from VRF-suitable IB-KEMs. Informally, an IB-KEM is VRF-suitable if it provides what we call unique decapsulation (i.e., given a ciphertext C produced with respect to an identity ID, all the secret keys corresponding to identity ID′, decapsulate to the same value, even if ID*ID′), and it satisfies an additional property that we call pseudo-random decapsulation. In a nutshell, pseudo-random decapsulation means that if one decapsulates a ciphertext C, produced with respect to an identity ID, using the decryption key corresponding to any other identity ID′, the resulting value looks random to a polynomially bounded observer. Our construction is of interest both from a theoretical and a practical perspective. Indeed, apart from establishing a connection between two seemingly unrelated primitives, our methodology is direct in the sense that, in contrast to most previous constructions, it avoids the inefficient Goldreich-Levin hardcore bit transformation. As an additional contribution, we propose a new VRF-suitable IB-KEM based on the decisional â.,"-weak Bilinear Diffie-Hellman Inversion assumption. Interestingly, when applying our transformation to this scheme, we obtain a new VRF construction that is secure under the same assumption, and it efficiently supports a large input space. © 2013 International Association for Cryptologic Research.

Chistikov D.,Max Planck Institute for Software Systems (Saarbrucken)
Leibniz International Proceedings in Informatics, LIPIcs | Year: 2014

We determine the descriptional complexity (smallest number of states, up to constant factors) of recognizing languages {1n} and {1tn: t = 0, 1, 2, . . .} with state-based finite machines of various kinds. This task is understood as counting to n and modulo n, respectively, and was previously studied for classes of finite-state automata by Kupferman, Ta-Shma, and Vardi (2001). We show that for Turing machines it requires log n/ log log n states in the worst case, and individual values are related to Kolmogorov complexity of the binary encoding of n. For deterministic pushdown and counter automata, the complexity is log n and √n, respectively; for alternating counter automata, we show an upper bound of log n. For visibly pushdown automata, i. e., if the stack movements are determined by input symbols, we consider languages {anbn} and {atnbtn: t = 0, 1, 2, . . .} and determine their complexity, of √n and min(n1 + n2), respectively, with minimum over all factorizations n = n1n2.

Chargueraud A.,Max Planck Institute for Software Systems (Saarbrucken)
Journal of Automated Reasoning | Year: 2012

This paper provides an introduction to the locally nameless approach to the representation of syntax with variable binding, focusing in particular on the use of this technique in formal proofs. First, we explain the benefits of representing bound variables with de Bruijn indices while retaining names for free variables. Then, we explain how to describe and manipulate syntax in that form, and show how to define and reason about judgments on locally nameless terms. © Springer Science+Business Media B.V. 2011.

Backes M.,Saarland University | Backes M.,Max Planck Institute for Software Systems (Saarbrucken) | Unruh D.,Saarland University
Journal of Computer Security | Year: 2010

The abstraction of cryptographic operations by term algebras, called Dolev-Yao models, is essential in almost all tool-supported methods for proving security protocols. Recently significant progress was made in proving that Dolev-Yao models offering the core cryptographic operations such as encryption and digital signatures can be sound with respect to actual cryptographic realizations and security definitions. Recent work, however, has started to extend Dolev-Yao models to more sophisticated operations with unique security features. Zero-knowledge proofs arguably constitute the most amazing such extension. In this paper, we first identify which additional properties a cryptographic (non-interactive) zero-knowledge proof needs to fulfill in order to serve as a computationally sound implementation of symbolic (Dolev-Yao style) zero-knowledge proofs; this leads to the novel definition of a symbolically-sound zero-knowledge proof system. We prove that even in the presence of arbitrary active adversaries, such proof systems constitute computationally sound implementations of symbolic zero-knowledge proofs. This yields the first computational soundness result for symbolic zero-knowledge proofs and the first such result against fully active adversaries of Dolev-Yao models that go beyond the core cryptographic operations. © 2010 - IOS Press and the authors. All rights reserved.

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