cse497b-lecture-5-cryptography.pdf

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1、Lecture5-CryptographyCSE497b-Spring2007IntroductionComputerandNetworkSecurityProfessorJaegerwww.cse.psu.edu/~tjaeger/cse497b-s07/CSE497bIntroductiontoComputerandNetworkSecurity-Spring2007-ProfessorsJaegerAhistoricalmoment...Theenigmamachinewasusedtosecurecommunicationofgermanmilitarythroughoutthese

2、condworldwar......anditchangedthecourseofhumanhistory.CSE497bIntroductiontoComputerandNetworkSecurity-Spring2007-ProfessorJaegerPage2Intuition•Cryptographyistheart(andsometimesscience)ofsecretwriting–Lesswellknowisthatitisalsousedtoguaranteeotherproperties,e.g.,authenticityofdata–Thisisanenormously

3、deepandimportantfield–However,muchofourtrustinthesesystemsisbasedonfaith(particularlyinefficientsecretkeyalgorithms)•Cryptographerscreateciphers-Cryptography•Cryptanalystbreakciphers-CryptanalysisThehistoryofcryptographyisanarmsracebetweencryptographersandcryptanalysts.CSE497bIntroductiontoComputeran

4、dNetworkSecurity-Spring2007-ProfessorJaegerPageCryptosystemAcryptosystemisa5-tupleconsistingof(E,D,M,K,C)Where,EisanencryptionalgorithmDisandecryptionalgorithmMisthesetofplaintextsKisthesetofkeysCisthesetofciphertextsE:M×K→CD:C×K→MCSE497bIntroductiontoComputerandNetworkSecurity-Spring2007-Professor

5、JaegerPage4Whatisakey?•Akeyisaninputtoacryptographicalgorithmusedtoobtainconfidentiality,integrity,authenticityorotherpropertyoversomedata.–Thesecurityofthecryptosystemoftendependsonkeepingthekeysecrettosomesetofparties.–Thekeyspaceisthesetofallpossiblekeys–Entropyisameasureofthevarianceinkeys•typic

6、allymeasuredinbits•Keysareoftenstoredinsomesecureplace:–passwords,ondiskkeyrings,...–TPM,secureco-processor,smartcards,...•...andsometimesnot,e.g.,certificatesCSE497bIntroductiontoComputerandNetworkSecurity-Spring2007-ProfessorJaegerPage5TranspositionCiphers•Scramblesthesymbolstoproduceoutput•Thekey

7、isthepermutationofsymbolsBULBULEECSE497bIntroductiontoComputerandNetworkSecurity-Spring2007-ProfessorJaegerPageSubstitutionCiphers•Substitutesonesymbolforanother(codebook)•ThekeyisthepermutationBB/AALL/NNUU