Block Ciphers And Cryptanalysis

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1、BlockCiphersAndCryptanalysisFauzanMirzafauzan@dcs.rhbnc.ac.ukDepartmentofMathematicsRoyalHollowayUniversityofLondonAbstractThisreportgivesabasicintroductiontoblockcipherdesignandanalysis.Theconceptsanddesignprinciplesofblockciphersareexplained,particularly

2、theclassofblockciphersknownasFeistelciphers.SomemodernblockciphercryptanalysismethodsaredemonstratedbyapplyingthemtovariantsofaweakFeistelciphercalledSimplifiedTEA(STEA),whichisbasedontheTinyEncryptionAlgorithm(TEA).1IntroductionThisreportgivesabasicintrodu

3、ctiontoblockcipherdesignandanalysis(intendedtobeunderstand-ablebyanyonewithsomeknowledgeofdiscretemathematics).Wewillexaminetheconceptofblockciphers,howtheyobtaintheirsecurity,andtheprinciplesinvolvedintheirdesign.Apopularclassofblockciphers,knownasFeistel

4、ciphers,willbedescribedindetail.Thisreportalsodescribesanddemonstratessomemodernblockciphercryptanalysismethodsbyshowinghowtheymaybeap-pliedtovariantsofaspecially-designedweakblockcipher,whichislooselymodelledontheTinyEncryptionAlgorithm(TEA).Theweakervari

5、antsofTEAarecalledSimplifiedTEA(STEA).Section2isabasicintroductiontoblockcipherdesign,summarisingthetypesofattacksthatastrongciphershouldbeabletoresist,describingtheconceptsofconfusionanddiffusion,andex-plainingtheprincipleofFeistelciphers.Insection3,theTEA

6、blockcipherisdescribedanditsonlyknownweaknessisexplained.Insection4,theSTEAblockcipherisintroduced.VariousmethodsofattackingSTEA,includingaveryefficientknown-plaintextattackandsomegeneralblockciphercryptanalysismethodsaredescribedinsections5–10.Finally,ackn

7、owledgementsareinsection11.PreliminariesThisreportassumesthefollowingnotationforsomebinaryoperations.Exclusive-ORTheoperationofadditionofn-tuplesoverthefieldF2(alsoknownasexclusive-or)isdenotedbyxy.nIntegerAdditionTheoperationofintegeradditionmodulo2isdeno

8、tedbyx
y(wherex;y2Z2n).Thevalueofnshouldbeclearfromthecontext.IntegerSubtractionTheoperationofintegersubtractionmodulo2nisdenotedbyxy(wherex;y2Z2n).Thevalueofnshouldbeclearfromthecontext.Also,notethatxyx
y.