密码编码学与网络安全(第五版)答案.docx
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Chapter1:
Introduction 5
Chapter2:
ClassicalEncryptionTechniques 7
Chapter3:
BlockCiphersandtheDateEncryptionStandard 13
Chapter4:
FiniteFields 21
Chapter5:
AdvancedEncryptionStandard 28
Chapter6:
MoreonSymmetricCiphers 33
Chapter7:
ConfidentialityUsingSymmetricEncryption 38
Chapter8:
IntroductiontoNumberTheory 42
Chapter9:
Public-KeyCryptographyandRSA 46
Chapter10:
KeyManagement;OtherPublic-KeyCryptosystems 55
Chapter11:
MessageAuthenticationandHashFunctions 59
Chapter12:
HashandMACAlgorithms 62
Chapter13:
DigitalSignaturesandAuthenticationProtocols 66
Chapter14:
AuthenticationApplications 71
Chapter15:
ElectronicMailSecurity 73
Chapter16:
IPSecurity 76
Chapter17:
WebSecurity 80
Chapter18:
Intruders 83
Chapter19:
MaliciousSoftware 87
Chapter20:
Firewalls 89
AnswerstoQuestions
1.1 TheOSISecurityArchitectureisaframeworkthatprovidesasystematicwayofdefiningtherequirementsforsecurityandcharacterizingtheapproachestosatisfyingthoserequirements.Thedocumentdefinessecurityattacks,mechanisms,andservices,andtherelationshipsamongthesecategories.
1.2 Passiveattackshavetodowitheavesdroppingon,ormonitoring,transmissions.Electronicmail,filetransfers,andclient/serverexchangesareexamplesoftransmissionsthatcanbemonitored.Activeattacksincludethemodificationoftransmitteddataandattemptstogainunauthorizedaccesstocomputersystems.
1.3 Passiveattacks:
releaseofmessagecontentsandtrafficanalysis.Activeattacks:
masquerade,replay,modificationofmessages,anddenialofservice.
1.4 Authentication:
Theassurancethatthecommunicatingentityistheonethatitclaimstobe.
Accesscontrol:
Thepreventionofunauthorizeduseofaresource（i.e.,thisservicecontrolswhocanhaveaccesstoaresource,underwhatconditionsaccesscanoccur,andwhatthoseaccessingtheresourceareallowedtodo）.
Dataconfidentiality:
Theprotectionofdatafromunauthorizeddisclosure.
Dataintegrity:
Theassurancethatdatareceivedareexactlyassentbyanauthorizedentity（i.e.,containnomodification,insertion,deletion,orreplay）.
Nonrepudiation:
Providesprotectionagainstdenialbyoneoftheentitiesinvolvedinacommunicationofhavingparticipatedinallorpartofthecommunication.
Availabilityservice:
Thepropertyofasystemorasystemresourcebeingaccessibleandusableupondemandbyanauthorizedsystementity,accordingtoperformancespecificationsforthesystem（i.e.,asystemisavailableifitprovidesservicesaccordingtothesystemdesignwheneverusersrequestthem）.
1.5 SeeTable1.3.
AnswerstoProblems
1.1
Releaseofmessagecontents
Trafficanalysis
Masquerade
Replay
Modificationofmessages
Denialofservice
Peerentityauthentication
Y
Dataoriginauthentication
Y
Accesscontrol
Y
Confidentiality
Y
Trafficflowconfidentiality
Y
Dataintegrity
Y
Y
Non-repudiation
Y
Availability
Y
1.2
Releaseofmessagecontents
Trafficanalysis
Masquerade
Replay
Modificationofmessages
Denialofservice
Encipherment
Y
Digitalsignature
Y
Y
Y
Accesscontrol
Y
Y
Y
Y
Y
Dataintegrity
Y
Y
Authenticationexchange
Y
Y
Y
Y
Trafficpadding
Y
Routingcontrol
Y
Y
Y
Notarization
Y
Y
Y
Chapter2
ClassicalEncryptionTechniquesr
AnswerstoQuestions
2.1 Plaintext,encryptionalgorithm,secretkey,ciphertext,decryptionalgorithm.
2.2 Permutationandsubstitution.
2.3 Onekeyforsymmetricciphers,twokeysforasymmetricciphers.
2.4 Astreamcipherisonethatencryptsadigitaldatastreamonebitoronebyteatatime.Ablockcipherisoneinwhichablockofplaintextistreatedasawholeandusedtoproduceaciphertextblockofequallength.
2.5 Cryptanalysisandbruteforce.
2.6 Ciphertextonly.Onepossibleattackunderthesecircumstancesisthebrute-forceapproachoftryingallpossiblekeys.Ifthekeyspaceisverylarge,thisbecomesimpractical.Thus,theopponentmustrelyonananalysisoftheciphertextitself,generallyapplyingvariousstatisticalteststoit.Knownplaintext.Theanalystmaybeabletocaptureoneormoreplaintextmessagesaswellastheirencryptions.Withthisknowledge,theanalystmaybeabletodeducethekeyonthebasisofthewayinwhichtheknownplaintextistransformed.Chosenplaintext.Iftheanalystisabletochoosethemessagestoencrypt,theanalystmaydeliberatelypickpatternsthatcanbeexpectedtorevealthestructureofthekey.
2.7 Anencryptionschemeisunconditionallysecureiftheciphertextgeneratedbytheschemedoesnotcontainenoughinformationtodetermineuniquelythecorrespondingplaintext,nomatterhowmuchciphertextisavailable.Anencryptionschemeissaidtobecomputationallysecureif:
（1）thecostofbreakingthecipherexceedsthevalueoftheencryptedinformation,and
（2）thetimerequiredtobreakthecipherexceedstheusefullifetimeoftheinformation.
2.8 TheCaesarcipherinvolvesreplacingeachletterofthealphabetwiththeletterstandingkplacesfurtherdownthealphabet,forkintherange1through25.
2.9 Amonoalphabeticsubstitutionciphermapsaplaintextalphabettoaciphertextalphabet,sothateachletteroftheplaintextalphabetmapstoasingleuniqueletteroftheciphertextalphabet.
2.10 ThePlayfairalgorithmisbasedontheuseofa5´5matrixoflettersconstructedusingakeyword.Plaintextisencryptedtwolettersatatimeusingthismatrix.
2.11 Apolyalphabeticsubstitutioncipherusesaseparatemonoalphabeticsubstitutioncipherforeachsuccessiveletterofplaintext,dependingonakey.
2.12 1.Thereisthepracticalproblemofmakinglargequantitiesofrandomkeys.Anyheavilyusedsystemmightrequiremillionsofrandomcharactersonaregularbasis.Supplyingtrulyrandomcharactersinthisvolumeisasignificanttask.
2.Evenmoredauntingistheproblemofkeydistributionandprotection.Foreverymessagetobesent,akeyofequallengthisneededbybothsenderandreceiver.Thus,amammothkeydistributionproblemexists.
2.13 Atranspositioncipherinvolvesapermutationoftheplaintextletters.
2.14 Steganographyinvolvesconcealingtheexistenceofamessage.
AnswerstoProblems
2.1 a. No.Achangeinthevalueofbshiftstherelationshipbetweenplaintextlettersandciphertextletterstotheleftorrightuniformly,sothatifthemappingisone-to-oneitremainsone-to-one.
b. 2,4,6,8,10,12,13,14,16,18,20,22,24.Anyvalueofalargerthan25isequivalenttoamod26.
c. Thevaluesofaand26musthavenocommonpositiveintegerfactorotherthan1.Thisisequivalenttosayingthataand26arerelativelyprime,orthatthegreatestcommondivisorofaand26is1.Toseethis,firstnotethatE（a,p）=E（a,q）（0≤p≤q<26）ifandonlyifa（p–q）isdivisibleby26.1.Supposethataand26arerelativelyprime.Then,a（p–q）isnotdivisibleby26,becausethereisnowaytoreducethefractiona/26and（p–q）islessthan26.2.Supposethataand26haveacommonfactork>1.ThenE（a,p）=E（a,q）,ifq=p+m/k≠p.
2.2 Thereare12allowablevaluesofa（1,3,5,7,9,11,15,17,19,21,23,25）.Thereare26allowablevaluesofb,from0through25）.ThusthetotalnumberofdistinctaffineCaesarciphersis12´26=312.
2.3 Assumethatthemostfrequentplaintextletteriseandthesecondmostfrequentletterist.Notethatthenumericalvaluesaree=4;B=1;t=19;U=20.Thenwehavethefollowingequations:
1=（4a+b）mod26
20=（19a+b）mod26
Thus,19=15amod26.Bytrialanderror,wesolve:
a=3.
Then1=（12+b）mod26.Byobservation,b=15.
2.4 AgoodglassintheBishop'shostelintheDevil'sseat—twenty-onedegreesandthirteenminutes—northeastandbynorth—mainbranchseventhlimbeastside—shootfromthelefteyeofthedeath'shead—abeelinefromthetreethroughtheshotfiftyfeetout.（fromTheGoldBug,byEdgarAllanPoe）
2.5 a. ThefirstlettertcorrespondstoA,thesecondletterhcorrespondstoB,eisC,sisD,andsoon.Secondandsubsequentoccurrencesofaletterinthekeysentenceareignored.Theresult
ciphertext:
SIDKHKDMAFHCRKIABIESHIMCKDLFEAILA
plaintext:
basilisktoleviathanblakeiscontact
b. Itisamonalphabeticcipherandsoeasilybreakable.
c. Thelastsentencemaynotcontainallthelettersofthealphabet.Ifthefirstsentenceisused,thesecondandsubsequentsentencesmayalsobeuseduntilall26lettersareencountered.
2.6 Thecipherreferstothewordsinthepageofabook.Thefirstentry,534,referstopage534.Thesecondentry,C2,referstocolumntwo.Theremainingnumbersarewordsinthatcolumn.ThenamesDOUGLASandBIRLSTONEaresimplywordsthatdonotappearonthatpage.Elementary!
（fromTheValleyofFear,bySirArthurConanDoyle）
2.7 a.
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