On the Validity of the Φ-Hiding Assumption in Cryptographic Protocols

  • Authors:
  • Christian Schridde;Bernd Freisleben

  • Affiliations:
  • Department of Mathematics and Computer Science, University of Marburg, Marburg, Germany D-35032;Department of Mathematics and Computer Science, University of Marburg, Marburg, Germany D-35032

  • Venue:
  • ASIACRYPT '08 Proceedings of the 14th International Conference on the Theory and Application of Cryptology and Information Security: Advances in Cryptology
  • Year:
  • 2008

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Abstract

Most cryptographic protocols, in particular asymmetricprotocols, are based on assumptions about the computationalcomplexity of mathematical problems. The Φ -Hidingassumption is such an assumption. It states that if p 1 and p 2 are small primes exactlyone of which divides φ (N ), where N is a number whose factorization is unknown and φ isEuler's totient function, then there is no polynomial-timealgorithm to distinguish which of the primes p 1 and p 2 dividesφ (N ) with a probability significantly greaterthan 1/2. In this paper, it will be shown that theΦ -Hiding assumption is not valid when applied to amodulus N = PQ 2e , whereP ,Q 2 are primes, e 0 is aninteger and P hides the prime in question. This indicatesthat cryptographic protocols using such moduli and relying on theΦ -Hiding assumption must be handled with care.