Secure Communication Principles and Encryption Techniques

1. Chapter9 SecurityinComputerNetwork Compiled By: Ashish Kr. Jha

2. 2 • PropertiesofSecureCommunication • Confidentiality/Secrecy • ▫ Dataisonlyunderstandabletothecommunicatingparties • Authentication • IntegrityandNon-repudiability • ▫ Evenifsenderandreceiverareabletoauthenticateeachother,theymustensurethatthedatareceivedisnotalteredeithermaliciouslyorbyaccident. • ▫ RefusetoAccept • Accesscontrolandavailability • ▫ Usingfirewall Compiled By: Ashish Kr. Jha

3. 3 NetworkSecurityIssues Compiled By: Ashish Kr. Jha

4. 4 • PrinciplesofCryptography • Techniquesemployedinprotectingintegrity orsecrecyof • electronicmessagesbyconvertingthemintounreadable(ciphertext)form. • Onlytheuseofasecretkeycanconverttheciphertextback • intohumanreadable(cleartext)form. • Cryptographysoftwareand/orhardwaredevicesusemathematicalformulas(algorithms)tochangetextfromoneformtoanother. • Twoprinciples: • ▫ SymmetricKeyCryptography(SecretKey) • ▫ AsymmetricKeyCryptography(PublicKey) Compiled By: Ashish Kr. Jha

5. 5 • SymmetricKeyCryptographyPrinciples • Useasinglesecretkey • Theciphertexthasalmostthesamesizeastheoriginalmessage • Builtonasharedsecretorsomerandomunpredictabledata • Thestrengthmostlydependsonthekeylength • Encryptlargefilesfastandefficiently • Gobymanynames(sessionkey,singlekey,bulkencryption) Compiled By: Ashish Kr. Jha

6. 6 SymmetricKeyCryptographyPrinciples Compiled By: Ashish Kr. Jha

7. 7 • SymmetricKeyCryptographyPrinciples • MonoAlphabetic • PolyAlphabetic • DES,3DES • Rijndael(AESWinner) Compiled By: Ashish Kr. Jha

8. 8 • MonoAlphabetic • CaesarCipher • ▫ Veryoldandsimplesymmetrickeyalgorithm • ▫ Takeeachalphabetin plain textandreplaceitbyletterthatis ‘’’”’‘k’ lettersahead. • Eg.Fork=5=>‘a’ isreplacedby'f’, ‘x’ by ‘c’andsoon • ▫ Veryeasytobreak=>only25valuesofKeys • Monoalphabeticcipher: • ▫ Anylettercanbesubstitutedbyotheraslingaseachletterhas • uniquesubstituteletter • Eg.Plaintext: • Ciphertext: bob.iloveyou.alice nkn.sgktcwky.mgsbc ▫ BetterthanCaesarcipherinthatthereare26!(1026)possiblepairingofletters. Compiled By: Ashish Kr. Jha

9. 9 PolyAlphabeticcipher • Key:C1C2C2 • ▫ Then text “abcde” would be transmitted as “fuvix” Compiled By: Ashish Kr. Jha

10. 10 • DataEncryptionStandard(DES) • DESistheblockcipher— analgorithmthattakesafixed-length • stringofplaintextbitsandtransformsitthroughaseriesof • complicatedoperationsintoanotherciphertextbitstringofthesamelength. • InthecaseofDES,theblocksizeis64bits. • • • DESalsousesakeytocustomizethetransformation,sothatdecryptioncansupposedlyonlybeperformedbythosewhoknowtheparticularkeyusedtoencrypt. • Thekeyostensiblyconsistsof64bits;however,only56oftheseare • actuallyusedbythealgorithm. • Eightbitsareusedsolelyforcheckingparity,andarethereafterdiscarded. • Hencetheeffectivekeylengthis56bits,anditisalwaysquotedas • such. • Every8thbitoftheselectedkeyisdiscarded,thatis,positions8,16,24,32,40,48,56,64areremovedfromthe64bitkeyleavingbehindonlythe56bitkey. Compiled By: Ashish Kr. Jha

11. 11 DataEncryptionStandard(DES) Compiled By: Ashish Kr. Jha

12. 12 • DataEncryptionStandard(DES) • Theoperationofoneoftheseintermediatestagesisillustratedin(b). • Eachstagetakestwo32-bitinputsandproducestwo32-bitoutputs. • Theleftoutputissimplyacopyoftherightinput. • TherightoutputisthebitwiseXORoftheleftinputandafunctionoftherightinputandthekeyforthisstage,Ki • Allthecomplexityliesinthisfunction. • Thefunctionconsistsoffoursteps,carriedoutinsequence. • First,a48-bitnumber,E,isconstructedbyexpandingthe32-bitRi-1 • accordingtoafixedtranspositionandduplicationrule. • Second,EandKiareXORedtogether. • Thisoutputisthenpartitionedintoeightgroupsof6bitseach,eachofwhichisfedintoadifferentS-box.Eachofthe64possibleinputstoanS-boxismappedontoa4-bitoutput. • Finally,these8x4bitsarepassedthroughaP-box.. Compiled By: Ashish Kr. Jha

13. 13 • DataEncryptionStandard(DES) • Ineachofthe16iterations,adifferentkeyisused. • Beforethealgorithmstarts,a56-bittranspositionisappliedtothekey. • Justbeforeeachiteration,thekeyispartitionedintotwo28-bitunits,eachofwhichisrotatedleftbyanumberofbitsdependentontheiterationnumber. • Kiisderivedfromthisrotatedkeybyapplyingyetanother56-bit • transpositiontoit. • Adifferent48-bitsubsetofthe56bitsisextractedandpermutedoneachround. Compiled By: Ashish Kr. Jha

14. 14 DataEncryptionStandard(DES) Compiled By: Ashish Kr. Jha

15. 15 • PublicKeyEncryption • Twokeys: • ▫ publicencryptionkeye • ▫ privatedecryptionkeyd • encryptioneasywheneisknown • decryptionhardwhendisnotknown • decryptioneasywhendisknown • Themostfamousalgorithm:RSA Compiled By: Ashish Kr. Jha

16. 16 PublicKeyEncryption Compiled By: Ashish Kr. Jha

17. 17 RSAAlgorithm Choosetwolargeprimenumbersp,q. (e.g.,1024bitseach) Computen=pq,z=(p-1)(q-1) Choosee(withe<n)thathasnocommonfactors withz.(e,zare relativelyprimeರ). Choosedsuchthated-1isexactlydivisiblebyz. (inotherwords:edmodz=1). Publickeyis(n,e).Privatekeyis(n,d). K+B K- B Compiled By: Ashish Kr. Jha

18. 18 RSAAlgorithm *Given(n,e)and(n,d)ascomputedabove *Toencryptbitpattern,m,compute (i.e.,remainderwhenmeisdividedbyn) c=memodn *Todecryptreceivedbitpattern,c,compute (i.e.,remainderwhencdisdividedbyn) m=cdmodn m=(memodn)dmodn c Magic happens! Compiled By: Ashish Kr. Jha

19. 19 • RSAAlgorithm • EncryptMessage“LOVE”usingRSAAlgorithm • ▫ Letp=5,q=7(Smallernumbersareselectedforsimplicity) • ▫ Thenn=pq=5*7=35 • ▫ Andz=(p-1)(q-1)=4*6=24 • ▫ Selectinge=5(e<nandnocommonfactorwithz)andd=29(suchthat:edmodz=1) • =>PublicKey(n,e)=(35,5) • =>PrivateKey(n,d)=(35,29) Compiled By: Ashish Kr. Jha

20. 20 RSAAlgorithm Encryption(LOVE=>QOVJ) Decryption(QOVJ=>LOVE) Compiled By: Ashish Kr. Jha

21. 21 RSAAlgorithm AnimportantpropertyofRSAAlgorithm: - + + - K(K(m))= K(K(m)) m = B B B B useprivatekeyfirst,followedbypublickey usepublickeyfirst,followedbyprivatekey Resultisthesame! Compiled By: Ashish Kr. Jha

22. 22 • DigitalSignature • Cryptographictechnique • signatures. analogous tohand-written establishingheis • Sender(Bob)digitallysignsdocument, • documentowner/creator. • Verifiable:recipient(Alice)canprovetosomeonethatBob,andnooneelse(includingAlice),musthavesigneddocument Compiled By: Ashish Kr. Jha

23. 23 DigitalSignature Bobsignsmbyencryptingwithhisprivate - - keyKB,creating“signed” message,KB(m) Compiled By: Ashish Kr. Jha

24. 24 DigitalSignature • Suppose Alice receives message m, digital signature KB-(m) • Alice verifies m signed by Bob by applying Bob’s public key KB+ to KB-(m) then checks KB+(KB-(m) ) = m. • If K+B(K-B(m) ) = m, whoever signed m must have used Bob’s private key. • Alice thus verifies that: • ü Bob signed m. (Authentication) • ü No one else signed m. • ü Bob signed m and notm’.(Message Integrity) • Non-repudiation: • Alice can take m, and signature KB(m) to court and prove that Bob signed m. Compiled By: Ashish Kr. Jha

25. 25 • Diffie-HellmanAlgorithm • AKeyExchangeAlgorithm • Exponentialkeyagreement • Allowstwouserstoexchangeasecretkey • Requiresnopriorsecrets • Real-timeoveranun-trustednetwork • Basedonthedifficultyofcomputingdiscretelogarithmsoflargenumbers. • UsedinSSL,SSH,IPSec,Ciscoencryptingrouters,SunsecureRPCandetc. • #AliceandBobwanttoshareasecretkeyforuseinasymmetriccipher,buttheironlymeansofcommunicationisinsecure. Compiled By: Ashish Kr. Jha

26. 26 Diffie-HellmanAlgorithm Compiled By: Ashish Kr. Jha

27. 27 • Diffie-HellmanAlgorithm • First,usersagreeonalargeprimepandanonzerointegerg. • pandgarebothpubliclyavailablenumbers • ▫ pisatleast512bits • Userspickprivatevaluesaandb • Computepublicvalues • ▫ x=gamodp • ▫ y=gbmodp • Publicvaluesxandyareexchanged • Computeshared,privatekey • ▫ ka=yamodp • ▫ kb=xbmodp • Algebraicallyitcanbeshownthatka=kb • Usersnowhaveasymmetricsecretkeytoencrypt Compiled By: Ashish Kr. Jha

28. 28 • Diffie-HellmanAlgorithm • AliceandBobgetpublicnumbers • ▫ P=23,G=9 • AliceandBobcomputepublicvalues • ▫ X=94mod23=6561mod23=6 • ▫ Y=93mod23=729mod23=16 • AliceandBobexchangepublicnumbers • AliceandBobcomputesymmetrickeys • ▫ ka=yamodp=164mod23=9 • ▫ kb=xbmodp=63mod23=9 • AliceandBobnowcantalksecurely! Compiled By: Ashish Kr. Jha

29. 29 • SecuringEmailusingPrettyGoodPrivacy(PGP) • PrettyGoodPrivacy(PGP)isane-mailencryptionschemethathasbecomeade-factostandard,withthousandsofusersallovertheglobe. • Dependingontheversion,thePGPsoftwareusesMD5orSHAforcalculatingthemessagedigest;CAST,Triple-DESorIDEAforsymmetrickeyencryption;andRSAforthepublickeyencryption. • Inaddition,PGPprovidesdatacompression. • WhenPGPisinstalled,thesoftwarecreatesapublickeypairfor • theuser. • Thepublickeycanbepostedontheuser'sWebsiteorplacedinapublickeyserver. • Theprivatekeyisprotectedbytheuseofapassword.Thepasswordhastobeenteredeverytimetheuseraccessestheprivatekey. • PGPgivestheusertheoptionofdigitallysigningthemessage,encryptingthemessage,orbothdigitallysigningandencrypting. Compiled By: Ashish Kr. Jha

30. 30 SecuringEmailusingPrettyGoodPrivacy(PGP) Compiled By: Ashish Kr. Jha

31. 31 • SecureSocketLayer(SSL) • Securesocketslayer(SSL),originallydevelopedbyNetscape,isaprotocoldesignedtoprovidedataencryptionandauthenticationbetweenaWebclientandaWebserver. • WheneverawebsurfervisitsasecuresitethatusesSSLtechnology,itcreatesanencryptedlinkbetweentheirbrowsersessionandthewebserver • Theprotocolbeginswithahandshakephasethatnegotiatesanencryptionalgorithm(e.g.,DESorRSA)andkeys,andauthenticatestheservertotheclient. • Optionally,theclientcanalsobeauthenticatedtotheserver. • Oncethehandshakeiscompleteandthetransmissionofapplicationdatabegins,andalldataisencryptedusingsessionkeysnegotiatedduringthehandshakephase. • SSLiswidelyusedinInternetcommerce,beingimplementedinalmostallpopularbrowsersandWebservers Compiled By: Ashish Kr. Jha

32. 32 SecureSocketLayer(SSL) Compiled By: Ashish Kr. Jha

33. 33 • SecureSocketLayerFeatures • SSLserverauthentication: • ▫ SSL-enabledbrowsermaintainlistsoftrustedCAಬsalongwiththeirpublickeys. • ▫ Browserrequestsservercertificate,issuedbytrustedCA. • ▫ Serversenditscertificate. • ▫ BrowserusesCAಬspublickeytoextractserverಬspublickeyfromcertificate. • EncryptedSSLsession: • ▫ Browsergeneratessymmetricsessionkey,encryptsitwith • serverಬspublickey,sendsencryptedkeytoserver. • ▫ Usingprivatekey,serverdecryptssessionkey. • ▫ Browser,serverknowsessionkey • ▫ AlldatasentintoTCPsocket(byclientorserver)encryptedwith • sessionkey. • SSLclientauthentication: • ▫ Analogoustoserverauthentication Compiled By: Ashish Kr. Jha

34. 34 NetworkLayerSecurity-VPN • Avirtualprivatenetwork(VPN)isaprivate networkthat interconnectsremote(andoftengeographically separate) • networksthroughprimarilypubliccommunicationinfrastructuressuchastheInternet. • Itenablesacomputertosendandreceivedataacrosssharedorpublicnetworksasifitisdirectlyconnectedtotheprivatenetwork. • AVPNiscreatedbyestablishingavirtualpoint-to-pointconnectionthroughtheuseofeitherdedicatedconnection,orvirtualtunnelingprotocols,ortrafficencryptions. • ThroughVPN,youcanaccessyourprivatenetworkoverinternet. • VPNsprovidesecuritythroughtunnelingprotocolsandsecurityproceduressuchasencryption.Forexample,aVPNcouldbeusedtosecurelyconnectthebranchofficesofanorganizationtoaheadofficenetworkthroughthepublicInternet. Compiled By: Ashish Kr. Jha

35. 35 VirtualPrivateNetwork • HosttoGateway/remote-accessVPNsand • GatewaytoGateway/SitetoSiteVPN Compiled By: Ashish Kr. Jha

36. 36 RemoteAccessVPN Compiled By: Ashish Kr. Jha

37. 37 SitetoSiteVPN Compiled By: Ashish Kr. Jha

38. 38 • VPN • VPNsallowemployeestosecurelyaccesstheircompany's • intranetwhiletravelingoutsidetheoffice. • Similarly,VPNssecurelyconnectgeographicallydisparate • officesofanorganization,creatingonecohesivenetwork. • VPNtechnologyisalsousedbyInternetuserstoconnecttoproxyserversforthepurposeofprotectingpersonalidentityandlocation. Compiled By: Ashish Kr. Jha

39. 39 • IPSecurity(IPSec) • InternetProtocolSecurity (IPsec) isaprotocolsuitefor by a securingInternetauthenticatingand Protocolencrypting (IP) communications each IPpacketof • communicationsession. • IPsecusescryptographicsecurityservicestoprotectcommunicationsoverInternetProtocol(IP)networks. • IPseccanbeusedinprotectingdataflowsbetweenapairof hosts(host-to-host), betweenapairofsecuritygateways • (network-to-network),orbetweenasecuritygatewayandahost(network-to-host). • Twoprincipalprotocols • ▫ AuthenticationHeader(AH)protocoland • ▫ EncapsulationSecurityPayload(ESP)protocol Compiled By: Ashish Kr. Jha

40. 40 • IPSecurity(IPSec) • Whenasourcehostsendssecuredatagramstoadestinationhost,itdoessowitheithertheAHprotocolorwiththeESPprotocol. • TheAHprotocolprovidessourceauthenticationanddata • integritybutdoesnotprovidesecrecy. • TheESPprotocolprovidesdataintegrityandsecrecy.Providingmoreservices,theESPprotocolisnaturallymorecomplicatedandrequiresmoreprocessingthantheAHprotocol. Compiled By: Ashish Kr. Jha

41. 41 • IPSecelements • IPSeccontainsthefollowingelements: • EncapsulatingSecurityPayload(ESP):Providesconfidentiality,authentication,andintegrity. • AuthenticationHeader(AH):Providesauthenticationand • integrity. • InternetKeyExchange(IKE):ProvideskeymanagementandSecurityAssociation(SA)management. Compiled By: Ashish Kr. Jha

42. 42 • IPSecSecurityFeatures • IPSecisthemostsecuremethodcommerciallyavailableforconnectingnetworksites.IPSecwasdesignedtoprovidethefollowingsecurityfeatureswhentransferringpacketsacrossnetworks: • Authentication:Verifiesthatthepacketreceivedisactuallyfromtheclaimedsender. • Integrity:Ensuresthatthecontentsofthepacketdidnot • changeintransit. • Confidentiality:Concealsthemessagecontentthroughencryption. Compiled By: Ashish Kr. Jha

43. 43 WirelessSecurityusingWEP • The802.11standardprescribesadataprotocolcalledWEP(WiredEquivalent link-levelsecurityPrivacy),whichis • designedtomakethesecurityofawirelessLANasgoodas • thatofawiredLAN. • When802.11securityisenabled,eachstationhasasecretkeysharedwiththebasestation. • Howthekeysaredistributedisnotspecifiedbythestandard. • WEPalgorithmisusedto(WEPfeatures): ▫ Protectwirelesscommunicationfromeavesdropping (secretly listeningtotheprivateconversationofotherswithouttheirconsent) ▫ Preventunauthorizedaccesstowirelessnetwork • GoalsofWEP • 1.AccessControl 2.DataIntegrity 3.Confidentiality Compiled By: Ashish Kr. Jha

44. 44 • WEP • WEPreliesonasecretkeywhichissharedbetweenthe • senderandthereceiver. • ▫ Sender:Mobilestation(eg:laptopwithwirelessethernet) • ▫ Receiver:AccessPoint(eg:basestation) • SecretKeyisusedtoencryptpacketsbeforetheyaretransmitted • Integritycheckisusedtoensurepacketarenotmodifiedin • transit. • WEPencryptionusesastreamcipherbasedontheRon'sCode4(RC4)algorithm.RC4wasdesignedbyRonaldRivestandkeptsecretuntilitleakedoutandwaspostedtotheInternetin1994. • InWEP,RC4generatesakeystreamthatisXORedwiththeplaintexttoformtheciphertext. Compiled By: Ashish Kr. Jha

45. 45 WEP Compiled By: Ashish Kr. Jha

46. 46 • WEP • WEPusesRC4encryption algorithm known 64or asಯstream cipherರtoprotectconfidentialityofitsdata • usestheRC4streamcipher, • consistingof: usinga 128-bitkey • ▫ A24-bitInitializationVector(IV) • ▫ A40or104-bitsecretkey • TheSecretKeyissharedamongthestationsandeverytime • newIVisselected • InitializationVector(IV): • ▫ Usedtoavoidencryptingtwociphertextwithsamekeystream • ▫ UsedtoproducedifferentRC4keyforeachpacket Compiled By: Ashish Kr. Jha

47. 47 • Firewall • Afirewallisacombinationofhardwareandsoftwarethatisolatesanorganization'sinternalnetworkfromtheInternetatlarge,allowingspecificconnectionstopassandblockingothers. • Organizationsemployfirewallsforoneormoreofthefollowingreasons: • ▫ Topreventintrudersfrominterferingwiththedaily • operationoftheinternalnetwork. • denialofserviceattack,SYNFINAttack • ▫ Topreventintrudersfromdeletingormodifyinginformationstoredwithintheinternalnetwork. • ▫ Topreventintrudersfromobtainingsecretinformation. Compiled By: Ashish Kr. Jha

48. 48 • FirewallTypes • PacketFiltering • ApplicationLevelGateway Compiled By: Ashish Kr. Jha

49. 49 • PacketFiltering • Workatthenetworklayer. • InternalnetworkconnectedtoInternetviarouterfirewall. • Routerfilterspacket-by-packetandcomparestoasetof • criteriabeforeitisforwarded. • FilteringBasedon • ▫ Source/DestinationIPaddress. • ▫ TCPorUDPsourceanddestinationport. ▫ ICMPmessagetype. ▫ ConnectioninitializationdatagramsusingtheTCPACKbit Shouldarrivingpacketbeallowedin?Departingpacketletout? Compiled By: Ashish Kr. Jha

50. 50 • PacketFiltering • Example1:blockincomingandoutgoingdatagramswithIPprotocolfield=17andwitheithersourceordestport=23. • ▫ AllincomingandoutgoingUDPflowsandtelnet • connectionsareblocked. • Example2:BlockinboundTCPsegmentswithACK=0. • ▫ PreventsexternalclientsfrommakingTCPconnectionswithinternalclients,butallowsinternalclientstoconnecttooutside. Compiled By: Ashish Kr. Jha