Cryptanalysis of Some Lightweight Symmetric Ciphers

Abstract

I de seneste ar er efterspørgslen efter systemer til letvægtskryptering steget, da mange anvendelser involverer RFID og sensornetværk, som af natur har meget lav ydeevne, og derfor er ude af stand til at udfre sædvanlige kryptografiske operationer. For at imødekomme dette problem, har det kryptografiske fællesskab udviklet adskillige byggeblokke til brug i letvægtskryptering, som indebærer strømcifre, blokcifre og, som det seneste, hashfunktioner. Ud af disse letvægtsalgoritmer, får især blokcifret PRESENT megen opmærksomhed fra det kryptografiske fællesskab, og er for nylig blevet vedtaget af ISO som en international standard i letvægtskryptografi.Denne afhandling har til formal at analysere og evaluere sikkerheden af nogle af de senest foreslåede symmetriske metoder til letvægtskryptering, med fokus på cifre som designmæssigt ligner PRESENT, navnlig PRESENT selv, samt blokcifret PRINTcipher. Vi giver en metode til at estimere sandsynligheden af differential- og linere approksimationer, for karakteristikker af lav vægt, pa kryptosystemer som designmæssigt ligner PRESENT, såvel som systemer der tillader differential- og linere karakteristikker af lav Hamming vægt. Vi undersger virkningen af nøgleskema-planlægningen på fordelingen af linere approksimationer for en variant af PRESENT med identiske rundengler. Vi foreslår et nyt angreb med navnet Invariant underrum-angreb, som specifikt blev anvendt pa letvægtscifret PRINTcipher. Desuden giver vi adskillige angreb pa et nyt strømciffer kaldet A2U2.In recent years, the need for lightweight encryption systems has been increasing as many applications use RFID and sensor networks which have a very low computational power and thus incapable of performing standard cryptographic operations. In response to this problem, the cryptographic community designed a number of lightweight cryptographic primitives that varies from stream ciphers, block ciphers and recently to hash functions.Out of these many lightweight primitives, the block cipher PRESENT gets a lot of attention from the cryptographic community and it has been recently adopted by ISO as one of the international standards in lightweight cryptography.This thesis aims at analyzing and evaluating the security of some the recently proposed lightweight symmetric ciphers with a focus on PRESENT-like ciphers, namely, the block cipher PRESENT and the block cipher PRINTcipher.We provide an approach to estimate the probability of differential and linear approximations with low-weight differential and linear characteristics on PRESENT-like ciphers as well as ciphers allowing low hamming weight differential and linear characteristics. We study the effect of key scheduling in the distribution of linear approximations on a variant of PRESENT with identical round keys. We propose a new attack named the Invariant Subspace Attack that was specifically mounted against the lightweight block cipher PRINTcipher. Furthermore, we mount several attacks on a recently proposed stream cipher called A2U2