International Association for Cryptologic Research

International Association
for Cryptologic Research

IACR News item: 23 February 2016

Eike Kiltz, Daniel Masny, Jiaxin Pan
ePrint Report ePrint Report
We perform a concrete security treatment of digital signature schemes obtained from canonical identification schemes via the Fiat-Shamir transform. If the identification scheme is rerandomizable and satisfies the weakest possible security notion (key-recoverability), then the implied signature scheme is unforgeability against chosen-message attacks in the multi-user setting in the random oracle model. The reduction loses a factor of roughly Qh, the number of hash queries. Previous security reductions incorporated an additional multiplicative loss of N, the number of users in the system. As an important application of our framework, we obtain a concrete security treatment for Schnorr signatures.

Our analysis is done in small steps via intermediate security notions, and all our implications have relatively simple proofs. Furthermore, for each step we show the optimality of the given reduction via a meta-reduction.
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