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Generic Round-Function-Recovery Attacks for Feistel Networks over Small Domains

  • F. Betül Durak
  • Serge Vaudenay
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 10892)

Abstract

Feistel Networks (FN) are now being used massively to encrypt credit card numbers through format-preserving encryption. In our work, we focus on FN with two branches, entirely unknown round functions, modular additions (or other group operations), and when the domain size of a branch (called Open image in new window ) is small. We investigate round-function-recovery attacks.

The best known attack so far is an improvement of Meet-In-The-Middle (MITM) attack by Isobe and Shibutani from ASIACRYPT 2013 with optimal data complexity Open image in new window and time complexity Open image in new window , where Open image in new window is the round number in FN. We construct an algorithm with a surprisingly better complexity when Open image in new window is too low, based on partial exhaustive search. When the data complexity varies from the optimal to the one of a codebook attack Open image in new window , our time complexity can reach Open image in new window . It crosses the complexity of the improved MITM for Open image in new window .

We also estimate the lowest secure number of rounds depending on Open image in new window and the security goal. We show that the format-preserving-encryption schemes FF1 and FF3 standardized by NIST and ANSI cannot offer 128-bit security (as they are supposed to) for Open image in new window and Open image in new window , respectively (the NIST standard only requires Open image in new window ), and we improve the results by Durak and Vaudenay from CRYPTO 2017.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Ecole Polytechnique Fédérale de Lausanne (EPFL)LausanneSwitzerland

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