分组密码的工作模式

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1、第6章分组密码的工作模式MultipleEncryption&3DESclearareplacementforDESwasneededtheoreticalattacksthatcanbreakitdemonstratedexhaustivekeysearchattacksAESisanewcipheralternativepriortothisalternativewastousemultipleencryptionwithDESimplementationsTripleDES(3DES)isthe

2、chosenformWhynotDouble-DES?coulduse2DESencryptsoneachblockC=E(K2,E(K1,P))P=D(K1,D(K2,C))issueofreductiontosinglestage(约化为单次加密),andhave“meet-in-the-middle”attackworkswheneveruseaciphertwicesinceX=E(K1,P)=D(K2,C),attackbyencryptingPwithallkeysandstore,the

3、ndecryptCwithkeysandmatchXvaluecanshowtakesO(256)stepsRequireknow…Triple-DESwithTwo-Keyshencemustuse3encryptionswouldseemtoneed3distinctkeysbutcanuse2keyswithE-D-EsequenceC=EK1(DK2(EK1(P)))nbencrypt&decryptequivalentinsecurityifK1=K2thencanworkwithsingl

4、eDESstandardizedinANSIX9.17&ISO8732nocurrentknownpracticalattacksO(2112)穷举攻击/1052差分密码分析Triple-DESwithThree-Keysalthougharenopracticalattacksontwo-keyTriple-DEShavesomeindicationscanuseTriple-DESwithThree-KeystoavoideventheseC=EK3(DK2(EK1(P)))hasbeenadop

5、tedbysomeInternetapplications,egPGP,S/MIMEModesofOperationblockciphersencryptfixedsizeblockseg.DESencrypts64-bitblockswith56-bitkeyneedsomewaytoen/decryptarbitraryamountsofdatainpractiseNIST(SP800-38A)ModesofUsedefines5possiblemodesdefinedforAES&DEShave

6、blockandstreammodestocoverawidevarietyofapplicationscanbeusedwithanyblockcipherElectronicCodebookBook(ECB)messageisbrokenintoindependentblockswhichareencryptedeachblockisavaluewhichissubstituted,likeacodebook,hencenameeachblockisencodedindependentlyofth

7、eotherblocksCi=DESK1(Pi)uses:securetransmissionofsinglevaluesElectronicCodebookBook(ECB)AdvantagesandLimitationsofECBmessagerepetitionsmayshowinciphertextifalignedwithmessageblockparticularlywithdatasuchgraphicsorwithmessagesthatchangeverylittle,whichbe

8、comeacode-bookanalysisproblemweaknessisduetotheencryptedmessageblocksbeingindependentmainuseissendingafewblocksofdataCipherBlockChaining(CBC)messageisbrokenintoblockslinkedtogetherinencryptionoperationeachpreviouscipherblocksisch