密码编码学与网络安全第五版答案.docx
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密码编码学与网络安全第五版答案.docx
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密码编码学与网络安全第五版答案
Chapter1:
Introduction5
Chapter2:
ClassicalEncryptionTechniques7
Chapter3:
BlockCiphersandtheDateEncryptionStandard13
Chapter4:
FiniteFields21
Chapter5:
AdvancedEncryptionStandard28
Chapter6:
MoreonSymmetricCiphers33
Chapter7:
ConfidentialityUsingSymmetricEncryption38
Chapter8:
IntroductiontoNumberTheory42
Chapter9:
Public-KeyCryptographyandRSA46
Chapter10:
KeyManagement;OtherPublic-KeyCryptosystems55
Chapter11:
MessageAuthenticationandHashFunctions59
Chapter12:
HashandMACAlgorithms62
Chapter13:
DigitalSignaturesandAuthenticationProtocols66
Chapter14:
AuthenticationApplications71
Chapter15:
ElectronicMailSecurity73
Chapter16:
IPSecurity76
Chapter17:
WebSecurity80
Chapter18:
Intruders83
Chapter19:
MaliciousSoftware87
Chapter20:
Firewalls89
AnswerstoQuestions
1.1TheOSISecurityArchitectureisaframeworkthatprovidesasystematicwayofdefiningtherequirementsforsecurityandcharacterizingtheapproachestosatisfyingthoserequirements.Thedocumentdefinessecurityattacks,mechanisms,andservices,andtherelationshipsamongthesecategories.
1.2Passiveattackshavetodowitheavesdroppingon,ormonitoring,transmissions.Electronicmail,filetransfers,andclient/serverexchangesareexamplesoftransmissionsthatcanbemonitored.Activeattacksincludethemodificationoftransmitteddataandattemptstogainunauthorizedaccesstocomputersystems.
1.3Passiveattacks:
releaseofmessagecontentsandtrafficanalysis.Activeattacks:
masquerade,replay,modificationofmessages,anddenialofservice.
1.4Authentication:
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).
1.5SeeTable1.3.
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
2.1Plaintext,encryptionalgorithm,secretkey,ciphertext,decryptionalgorithm.
2.2Permutationandsubstitution.
2.3Onekeyforsymmetricciphers,twokeysforasymmetricciphers.
2.4Astreamcipherisonethatencryptsadigitaldatastreamonebitoronebyteatatime.Ablockcipherisoneinwhichablockofplaintextistreatedasawholeandusedtoproduceaciphertextblockofequallength.
2.5Cryptanalysisandbruteforce.
2.6Ciphertextonly.Onepossibleattackunderthesecircumstancesisthebrute-forceapproachoftryingallpossiblekeys.Ifthekeyspaceisverylarge,thisbecomesimpractical.Thus,theopponentmustrelyonananalysisoftheciphertextitself,generallyapplyingvariousstatisticalteststoit.Knownplaintext.Theanalystmaybeabletocaptureoneormoreplaintextmessagesaswellastheirencryptions.Withthisknowledge,theanalystmaybeabletodeducethekeyonthebasisofthewayinwhichtheknownplaintextistransformed.Chosenplaintext.Iftheanalystisabletochoosethemessagestoencrypt,theanalystmaydeliberatelypickpatternsthatcanbeexpectedtorevealthestructureofthekey.
2.7Anencryptionschemeisunconditionallysecureiftheciphertextgeneratedbytheschemedoesnotcontainenoughinformationtodetermineuniquelythecorrespondingplaintext,nomatterhowmuchciphertextisavailable.Anencryptionschemeissaidtobecomputationallysecureif:
(1)thecostofbreakingthecipherexceedsthevalueoftheencryptedinformation,and
(2)thetimerequiredtobreakthecipherexceedstheusefullifetimeoftheinformation.
2.8TheCaesarcipherinvolvesreplacingeachletterofthealphabetwiththeletterstandingkplacesfurtherdownthealphabet,forkintherange1through25.
2.9Amonoalphabeticsubstitutionciphermapsaplaintextalphabettoaciphertextalphabet,sothateachletteroftheplaintextalphabetmapstoasingleuniqueletteroftheciphertextalphabet.
2.10ThePlayfairalgorithmisbasedontheuseofa5⨯5matrixoflettersconstructedusingakeyword.Plaintextisencryptedtwolettersatatimeusingthismatrix.
2.11Apolyalphabeticsubstitutioncipherusesaseparatemonoalphabeticsubstitutioncipherforeachsuccessiveletterofplaintext,dependingonakey.
2.121.Thereisthepracticalproblemofmakinglargequantitiesofrandomkeys.Anyheavilyusedsystemmightrequiremillionsofrandomcharactersonaregularbasis.Supplyingtrulyrandomcharactersinthisvolumeisasignificanttask.
2.Evenmoredauntingistheproblemofkeydistributionandprotection.Foreverymessagetobesent,akeyofequallengthisneededbybothsenderandreceiver.Thus,amammothkeydistributionproblemexists.
2.13Atranspositioncipherinvolvesapermutationoftheplaintextletters.
2.14Steganographyinvolvesconcealingtheexistenceofamessage.
AnswerstoProblems
2.1a.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.
2.2Thereare12allowablevaluesofa(1,3,5,7,9,11,15,17,19,21,23,25).Thereare26allowablevaluesofb,from0through25).ThusthetotalnumberofdistinctaffineCaesarciphersis12´26=312.
2.3Assumethatthemostfrequentplaintextletteriseandthesecondmostfrequentletterist.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.
2.4AgoodglassintheBishop'shostelintheDevil'sseat—twenty-onedegreesandthirteenminutes—northeastandbynorth—mainbranchseventhlimbeastside—shootfromthelefteyeofthedeath'shead—abeelinefromthetreethroughtheshotfiftyfeetout.(fromTheGoldBug,byEdgarAllanPoe)
2.5a.ThefirstlettertcorrespondstoA,thesecondletterhcorrespondstoB,eisC,sisD,andsoon.Secondandsubsequentoccurrencesofaletterinthekeysentenceareignored.Theresult
ciphertext:
SIDKHKDMAFHCRKIABIESHIMCKDLFEAILA
plaintext:
basilisktoleviathanblakeiscontact
b.Itisamonalphabeticcipherandsoeasilybreakable.
c.Thelastsentencemaynotcontainallthelettersofthealphabet.Ifthefirstsentenceisused,thesecondandsubsequentsentencesmayalsobeuseduntilall26lettersareencountered.
2.6Thecipherreferstothewordsinthepageofabook.Thefirstentry,534,referstopage534.Thesecondentry,C2,referstocolumntwo.Theremainingnumbersarewordsinthatcolumn.ThenamesDOUGLASandBIRLSTONEaresimplywordsthatdonotappearonthatpage.Elementary!
(fromTheValleyofFear,bySirArthurConanDoyle)
2.7a.
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