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Assumptions Related to Discrete Logarithms: Why Subtleties Make a Real Difference
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Assumptions Related to Discrete Logarithms: Why Subtleties Make a Real Difference
Ahmad-Reza Sadeghi5 and Michael Steiner5 
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Fachrichtung Informatik, Universität des Saarlandes, D-66123 Saarbrücken, Germany |
Abstract
The security of many cryptographic constructions relies on assumptions related to Discrete Logarithms (DL), e.g., the Diffie-Hellman,
Square Exponent, Inverse Exponent or Representation Problem assumptions. In the concrete formalizations of these assumptions
one has some degrees of freedom offered by parameters such as computational model, the problem type (computational, decisional)
or success probability of adversary. However, these parameters and their impact are often not properly considered or are simply
overlooked in the existing literature. In this paper we identify parameters relevant to cryptographic applications and describe
a formal framework for defining DL-related assumptions. This enables us to precisely and systematically classify these assumptions.
In particular, we identify a parameter, termed granularity, which describes the underlying probability space in an assumption.
Varying granularity we discover the following surprising result:We prove that two DL-related assumptions can be reduced to
each other for medium granularity but we also show that they are provably not reducible with generic algorithms for high granularity.
Further we show that reductions for medium granularity can achieve much better concrete security than equivalent high-granularity
reductions.
Keywords Complexity Theory - Cryptographic Assumptions - Generic Algorithms - Discrete Logarithms - Diffie-Hellman - Square Exponent - Inverse Exponent
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