分组密码和数据加密标准.ppt

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1、第3章分组密码与数据加密标准ModernBlockCiphersmodernblockciphersoneofthemostwidelyusedtypesofcryptographicalgorithmsprovidesecrecy/authenticationservicesfocusonDES(DataEncryptionStandard)toillustrateblockcipherdesignprinciplesBlockvsStreamCiphersblockciphersprocessmessages

2、inblocks,eachofwhichisthenen/decryptedlikeasubstitutiononverybigcharacters64-bitsormorestreamciphersprocessmessagesabitorbyteatatimewhenen/decryptingmanycurrentciphersareblockciphersbroaderrangeofapplicationsBlockCipherPrinciplesmostsymmetricblockciphersareba

3、sedonaFeistelCipherStructureneededsincemustbeabletodecryptciphertexttorecovermessagesefficientlyblockcipherslooklikeanextremelylargesubstitutionwouldneedtableof264entriesfora64-bitblockinsteadcreatefromsmallerbuildingblocksusingideaofaproductcipherIdealBlockC

4、ipherClaudeShannonandSubstitution-PermutationCiphersClaudeShannonintroducedideaofsubstitution-permutation(S-P)networksin1949paperformbasisofmodernblockciphersS-Pnetsarebasedonthetwoprimitivecryptographicoperationsseenbefore:substitution(S-box)permutation(P-bo

5、x)provideconfusion&diffusionofmessage&keyConfusionandDiffusioncipherneedstocompletelyobscurestatisticalpropertiesoforiginalmessageaone-timepaddoesthismorepracticallyShannonsuggestedcombiningS&Pelementstoobtain:diffusion–dissipatesstatisticalstructureofplainte

6、xtoverbulkofciphertextconfusion–makesrelationshipbetweenciphertextandkeyascomplexaspossibleFeistelCipherStructureFeisteldevisedFeistelcipherbasedonconceptofinvertibleproductcipherpartitionsinputblockintotwohalvesprocessthroughmultipleroundswhichperformasubsti

7、tutiononleftdatahalfbasedonroundfunctionofrighthalf&subkeythenhavepermutationswappinghalvesimplementsShannon’sS-PnetconceptFeistelCipherStructureFeistelCipherDesignElementsblocksizekeysizenumberofroundssubkeygenerationalgorithmroundfunctionfastsoftwareen/decr

8、yptioneaseofanalysisFeistelCipherDecryptionDataEncryptionStandard(DES)mostwidelyusedblockcipherinworldadoptedin1977byNBS(nowNIST)asFIPSPUB46encrypts64-bitdatausing56-bitkeyhaswidespreadus