操作系统老师chapter

1SecurityChapter99.1Thesecurityenvironment9.2Basicsofcryptography9.3Userauthentication9.4Attacksfrominsidethesystem9.5Attacksfromoutsidethesystem9.6Protectionmechanisms9.7Trustedsystems2TheSecurityEnvironment

ThreatsSecuritygoalsandthreats3入侵者通常的入侵者分类非专业用户的随意浏览内部人员的窥视为获取利益而尝试商业或军事间谍4数据意外遗失导致的原因常有天灾fires,floods,wars软硬件错误CPUmalfunction,baddisk,programbugs人为错误dataentry,wrongtapemounted5BasicsofCryptographyRelationshipbetweentheplaintextandtheciphertext6单字母替换eachletterreplacedbydifferentletterGiventheencryptionkey,

easytofinddecryptionkey秘密秘钥加密亦称对称秘钥加密秘密秘钥加密7公钥加密体系Alluserspickapublickey/privatekeypairpublishthepublickeyprivatekeynotpublishedPublickeyistheencryptionkeyprivatekeyisthedecryptionkey8单向函数Functionsuchthatgivenformulaforf(x)easytoevaluatey=f(x)Butgivenycomputationallyinfeasibletofindx9数字签名Computingasignatureblock(MD5,SHA)Whatthereceivergets接受方将计算得到x与E(D(x))=x比较，一致则说明无篡改(b)10用户验证用户登录的方法基于三个方面的考虑:SomethingtheuserknowsSomethingtheuserhasSomethingtheuserisThisisdonebeforeusercanusethesystem11AuthenticationUsingPasswords(a)Asuccessfullogin(b)Loginrejectedafternameentered(c)Loginrejectedafternameandpasswordtyped12AuthenticationUsingPasswordsHowacrackerbrokeintoLBLaU.S.Dept.ofEnergyresearchlab13AuthenticationUsingPasswordsTheuseofsalttodefeatputationofencryptedpasswordsSaltPassword,,,,14一次性Password单向函数y=f(x)

Pi-1=f(Pi)取n=4:P1=f(f(f(f(x)))),P2=f(f(f(x))),P3=f(f(x)),P4=f(x)Server:初始时P0=f(P1),i=1User:userID

Server,Server:i=1userUser:P1

Server,server比较f(P1)与P0匹配则允许登录，并用P1覆盖P0，i=215挑战－响应验证User选择某种算法（如x2),server随机发送给user一个参数，如7，此时用户输入正确的49，从而不断变化password增强：user事先选择密钥k放置在user和server，登录时，server发随机数r给user，user计算f(r,k)的值y并发给server，server也计算f(r,k)的值是否与收到的y匹配16AuthenticationUsingaPhysicalObjectMagneticcardsmagneticstripecardschipcards:storedvaluecards,smartcards17AuthenticationUsingBiometricsAdeviceformeasuringfingerlength.18对策LimitingtimeswhensomeonecanloginAutomaticcallbackatnumberprespecifiedLimitednumberoflogintriesAdatabaseofallloginsSimpleloginname/passwordasatrapsecuritypersonnelnotifiedwhenattackerbites19OperatingSystemSecurity

TrojanHorsesFreeprogrammadeavailabletounsuspectinguserActuallycontainscodetodoharmPlacealteredversionofutilityprogramonvictim'scomputertrickuserintorunningthatprogram20LoginSpoofing(a)Correctloginscreen(b)Phonyloginscreen21LogicBombsCompanyprogrammerwritesprogrampotentialtodoharmOKaslongashe/sheenterspassworddailyffprogrammerfired,nopasswordandbombexplodes22TrapDoors(a)Normalcode.(b)Codewithatrapdoorinserted23BufferOverflow(a)Situationwhenmainprogramisrunning(b)AfterprogramAcalled(c)Bufferoverflowshowningray24GenericSecurityAttacksTypicalattacksRequestmemory,diskspace,tapesandjustreadTryillegalsystemcallsStartaloginandhitDEL,RUBOUT,orBREAKTrymodifyingcomplexOSstructuresTrytodospecifiedDONOTsConvinceasystemprogrammertoaddatrapdoorBegadmin'ssec’ytohelpapooruserwhoforgotpassword25DesignPrinciplesforSecuritySystemdesignshouldbepublicDefaultshouldbenotaccessCheckforcurrentauthorityGiveeachprocessleastprivilegepossibleProtectionmechanismshouldbesimpleuniforminlowestlayersofsystemSchemeshouldbepsychologicallyacceptableAnd…keepitsimple26NetworkSecurityExternalthreatcodetransmittedtotargetmachinecodeexecutedthere,doingdamageGoalsofviruswriterquicklyspreadingvirusdifficulttodetecthardtogetridofVirus=programcanreproduceitselfattachitscodetoanotherprogramadditionally,doharm27VirusDamageScenariosBlackmailDenialofserviceaslongasvirusrunsPermanentlydamagehardwareTargetacompetitor'scomputerdoharmespionageIntra-corporatedirtytrickssabotageanothercorporateofficer'sfiles28常见的病毒共事者病毒可执行程序病毒内存驻留病毒引导扇区病毒设备驱动程序病毒宏病毒29HowVirusesWork(1)ViruswritteninassemblylanguageInsertedintoanotherprogramusetoolcalleda“dropper”Virusdormantuntilprogramexecutedtheninfectsotherprogramseventuallyexecutesits“payload”30HowVirusesWork(2)RecursiveprocedurethatfindsexecutablefilesonaUNIXsystemViruscouldinfectthemall31HowVirusesWork(3)AnexecutableprogramWithavirusatthefrontWiththevirusattheendWithavirusspreadoverfreespacewithinprogram32HowVirusesWork(4)Aftervirushascapturedinterrupt,trapvectorsAfterOShasretakenprinterinterruptvectorAftervirushasnoticedlossofprinterinterruptvectorandrecapturedit33HowVirusesSpreadVirusplacedwherelikelytobecopiedWhencopiedinfectsprogramsonharddrive,floppymaytrytospreadoverLANAttachtoinnocentlookingemailwhenitruns,usemailinglisttoreplicate34AntivirusandAnti-AntivirusTechniques(a)Aprogram(b)Infectedprogram(c)Compressedinfectedprogram(d)Encryptedvirus(e)Compressedviruswithencryptedcompressioncode35AntivirusandAnti-AntivirusTechniquesIntegritycheckersBehavioralcheckersVirusavoidancegoodOSinstallonlyshrink-wrappedsoftwareuseantivirussoftwaredonotclickonattachmentstoemailfrequentbackupsRecoveryfromvirusattackhaltcomputer,rebootfromsafedisk,runantivirus36TheInternetWormConsistedoftwoprogramsbootstraptouploadwormthewormitselfWormfirsthiditsexistenceNextreplicateditselfonnewmachines37MobileCode(1)Sandboxing(a)Memorydividedinto1-MBsandboxes(b)Onewayofcheckinganinstructionforvalidity38MobileCode(2)AppletscanbeinterpretedbyaWebbrowser39MobileCode(3)Howcodesigningworks40ProtectionMechanisms

ProtectionDomains(1)Examplesofthreeprotectiondomains41ProtectionDomains(2)Aprotectionmatrix42ProtectionDomains(3)Aprotectionmatrixwithdomainsasobjects43AccessControlLists(1)Useofaccesscontrollistsofmanagefileaccess44AccessControlLists(2)Twoaccesscontrollists45Capabilities(1)Eachprocesshasacapabilitylist46Cryptographically-protectedcapabilityGenericRightsCopycapabilityCopyobjectRemovecapabilityDestroyobjectCapabilities(2)ServerObjectRightsf(Objects,Rights,Check)47TrustedSystems

TrustedComputingBase

Areferencemonitor48FormalModelsofSecureSystems(a)Anauthorizedstate(b)Anunauthorizedstate49MultilevelSecurity(1)TheBell-LaPadulamultilevelsecuritymodel50MultilevelSecurity(2)TheBibaModelPrinciplesto