Modeling Computational Security in Long-Lived Systems

  • Authors:

  • Ran Canetti;Ling Cheung;Dilsun Kaynar;Nancy Lynch;Olivier Pereira

  • Affiliations:

  • IBM T. J. Watson Research Center, and Massachusetts Institute of Technology,;Massachusetts Institute of Technology,;Carnegie Mellon University,;Massachusetts Institute of Technology,;Université catholique de Louvain,

  • Venue:
  • CONCUR '08 Proceedings of the 19th international conference on Concurrency Theory
  • Year:
  • 2008

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Abstract

For many cryptographic protocols, security relies on the assumption that adversarial entities have limited computational power. This type of security degrades progressively over the lifetime of a protocol. However, some cryptographic services, such as timestamping services or digital archives, are long-livedin nature; they are expected to be secure and operational for a very long time (i.e.super-polynomial). In such cases, security cannot be guaranteed in the traditional sense: a computationally secure protocol may become insecure if the attacker has a super-polynomial number of interactions with the protocol.This paper proposes a new paradigm for the analysis of long-lived security protocols. We allow entities to be active for a potentially unbounded amount of real time, provided they perform only a polynomial amount of work per unit of real time. Moreover, the space used by these entities is allocated dynamically and must be polynomially bounded. We propose a new notion of long-term implementation, which is an adaptation of computational indistinguishability to the long-lived setting. We show that long-term implementation is preserved under polynomial parallel composition and exponential sequential composition. We illustrate the use of this new paradigm by analyzing some security properties of the long-lived timestamping protocol of Haber and Kamat.