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Waters Signatures with Optimal Security Reduction

  • Dennis Hofheinz
  • Tibor Jager
  • Edward Knapp
Part of the Lecture Notes in Computer Science book series (LNCS, volume 7293)

Abstract

Waters signatures (Eurocrypt 2005) can be shown existentially unforgeable under chosen-message attacks under the assumption that the computational Diffie-Hellman problem in the underlying (pairing-friendly) group is hard. The corresponding security proof has a reduction loss of O(ℓ·q), where ℓ is the bitlength of messages, and q is the number of adversarial signature queries. The original reduction could meanwhile be improved to \(O(\sqrt{\ell}\cdot q)\) (Hofheinz and Kiltz, Crypto 2008); however, it is currently unknown whether a better reduction exists. We answer this question as follows:

  1. (a)

    We give a simple modification of Waters signatures, where messages are encoded such that each two encoded messages have a suitably large Hamming distance. Somewhat surprisingly, this simple modification suffices to prove security under the CDH assumption with a reduction loss of O(q).

     
  2. 1

    We also show that any black-box security proof for a signature scheme with re-randomizable signatures must have a reduction loss of at least Ω(q), or the underlying hardness assumption is false. Since both Waters signatures and our variant from (a) are re-randomizable, this proves our reduction from (a) optimal up to a constant factor.

     

Understanding and optimizing the security loss of a cryptosystem is important to derive concrete parameters, such as the size of the underlying group. We provide a complete picture for Waters-like signatures: there is an inherent lower bound for the security loss, and we show how to achieve it.

Keywords

Digital signatures Waters signatures provable security black-box reductions 

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Copyright information

© International Association for Cryptologic Research 2012

Authors and Affiliations

  • Dennis Hofheinz
    • 1
  • Tibor Jager
    • 1
  • Edward Knapp
    • 2
  1. 1.Institut für Kryptographie und SicherheitKarlsruhe Institute of TechnologyGermany
  2. 2.Department of Combinatorics and OptimizationUniversity of WaterlooCanada

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