(Network security (chapter 18

(Network security (chapter 18

اسلاید 1: William Stallings Data and Computer CommunicationsChapter 18Network Security

اسلاید 2: Security RequirementsConfidentialityIntegrityAvailability

اسلاید 3: Passive AttacksEavesdropping on transmissionsTo obtain informationRelease of message contentsOutsider learns content of transmissionTraffic analysisBy monitoring frequency and length of messages, even encrypted, nature of communication may be guessedDifficult to detectCan be prevented

اسلاید 4: Active AttacksMasqueradePretending to be a different entityReplayModification of messagesDenial of serviceEasy to detectDetection may lead to deterrentHard to prevent

اسلاید 5: Security Threats

اسلاید 6: Conventional Encryption

اسلاید 7: IngredientsPlain textEncryption algorithmSecret keyCipher textDecryption algorithm

اسلاید 8: Requirements for SecurityStrong encryption algorithmEven if known, should not be able to decrypt or work out keyEven if a number of cipher texts are available together with plain texts of themSender and receiver must obtain secret key securelyOnce key is known, all communication using this key is readable

اسلاید 9: Attacking EncryptionCrypt analysisRelay on nature of algorithm plus some knowledge of general characteristics of plain textAttempt to deduce plain text or keyBrute forceTry every possible key until plain text is achieved

اسلاید 10: AlgorithmsBlock cipherProcess plain text in fixed block sizes producing block of cipher text of equal sizeData encryption standard (DES)Triple DES (TDES)

اسلاید 11: Data Encryption StandardUS standard64 bit plain text blocks56 bit key

اسلاید 12: DES Encryption Algorithm

اسلاید 13: DES Single Iteration

اسلاید 14: Strength of DESDeclared insecure in 1998Electronic Frontier FoundationDES Cracker machineDES now worthlessAlternatives include TDEA

اسلاید 15: Triple DEAANSI X9.17 (1985)Incorporated in DEA standard 1999Uses 3 keys and 3 executions of DEA algorithmEffective key length 168 bit

اسلاید 16: Location of Encryption Devices

اسلاید 17: Link EncryptionEach communication link equipped at both endsAll traffic secureHigh level of securityRequires lots of encryption devicesMessage must be decrypted at each switch to read address (virtual circuit number)Security vulnerable at switchesParticularly on public switched network

اسلاید 18: End to End EncryptionEncryption done at ends of systemData in encrypted form crosses network unalteredDestination shares key with source to decryptHost can only encrypt user dataOtherwise switching nodes could not read header or route packetTraffic pattern not secureUse both link and end to end

اسلاید 19: Key DistributionKey selected by A and delivered to BThird party selects key and delivers to A and BUse old key to encrypt and transmit new key from A to BUse old key to transmit new key from third party to A and B

اسلاید 20: Automatic Key Distribution (diag)

اسلاید 21: Automatic Key DistributionSession KeyUsed for duration of one logical connectionDestroyed at end of sessionUsed for user dataPermanent keyUsed for distribution of keysKey distribution centerDetermines which systems may communicate Provides one session key for that connectionFront end processorPerforms end to end encryptionObtains keys for host

اسلاید 22: Traffic PaddingProduce cipher text continuouslyIf no plain text to encode, send random dataMake traffic analysis impossible

اسلاید 23: Message AuthenticationProtection against active attacksFalsification of dataEavesdroppingMessage is authentic if it is genuine and comes from the alleged sourceAuthentication allows receiver to verify that message is authenticMessage has not alteredMessage is from authentic sourceMessage timeline

اسلاید 24: Authentication Using EncryptionAssumes sender and receiver are only entities that know keyMessage includes:error detection code sequence numbertime stamp

اسلاید 25: Authentication Without EncryptionAuthentication tag generated and appended to each messageMessage not encryptedUseful for:Messages broadcast to multiple destinationsHave one destination responsible for authenticationOne side heavily loadedEncryption adds to workloadCan authenticate random messagesPrograms authenticated without encryption can be executed without decoding

اسلاید 26: Message Authentication CodeGenerate authentication code based on shared key and messageCommon key shared between A and BIf only sender and receiver know key and code matches:Receiver assured message has not alteredReceiver assured message is from alleged senderIf message has sequence number, receiver assured of proper sequence

اسلاید 27: Message Authentication Using Message Authentication Code

اسلاید 28: One Way Hash FunctionAccepts variable size message and produces fixed size tag (message digest)Advantages of authentication without encryptionEncryption is slowEncryption hardware expensiveEncryption hardware optimized to large dataAlgorithms covered by patentsAlgorithms subject to export controls (from USA)

اسلاید 29: Using One Way Hash

اسلاید 30: Secure Hash FunctionsHash function must have following properties:Can be applied to any size data blockProduce fixed length outputEasy to computeNot feasible to reverseNot feasible to find two message that give the same hash

اسلاید 31: SHA-1Secure Hash Algorithm 1Input message less than 264 bitsProcessed in 512 bit blocksOutput 160 bit digest

اسلاید 32: Public Key EncryptionBased on mathematical algorithmsAsymmetricUse two separate keysIngredientsPlain textEncryption algorithmPublic and private keyCipher textDecryption algorithm

اسلاید 33: Public Key Encryption (diag)

اسلاید 34: Public Key Encryption - OperationOne key made publicUsed for encryptionOther kept privateUsed for decryptionInfeasible to determine decryption key given encryption key and algorithmEither key can be used for encryption, the other for decryption

اسلاید 35: StepsUser generates pair of keysUser places one key in public domainTo send a message to user, encrypt using public keyUser decrypts using private key

اسلاید 36: Digital SignatureSender encrypts message with their private keyReceiver can decrypt using sneders public keyThis authenticates sender, who is only person who has the matching keyDoes not give privacy of dataDecrypt key is public

اسلاید 37: RSA Algorithm

اسلاید 38: RSA Example

اسلاید 39: IPv4 and IPv6 SecurityIPSecSecure branch office connectivity over InternetSecure remote access over InternetExtranet and intranet connectivityEnhanced electronic commerce security

اسلاید 40: IPSec ScopeAuthentication headerEncapsulated security payloadKey exchangeRFC 2401,2402,2406,2408

اسلاید 41: Security AssociationOne way relationship between sender and receiverFor two way, two associations are requiredThree SA identification parametersSecurity parameter indexIP destination addressSecurity protocol identifier

اسلاید 42: SA ParametersSequence number counterSequence counter overflowAnti-reply windowsAH informationESP informationLifetime of this associationIPSec protocol modeTunnel, transport or wildcardPath MTU

اسلاید 43: Transport and Tunnel ModesTransport modeProtection for upper layer protocolsExtends to payload of IP packetEnd to end between hostsTunnel modeProtection for IP packetEntire packet treated as payload for outer IP “packet”No routers examine inner packetMay have different source and destination addressMay be implemented at firewall

اسلاید 44: Authentication Header

اسلاید 45: Encapsulating Security PayloadESPConfidentiality services

اسلاید 46: ESP Packet

اسلاید 47: Scope of ESP

اسلاید 48: Key ManagementManualAutomaticISAKMP/OakleyOakley key determination protocolInternet security association and key management protocol

اسلاید 49: Required ReadingStallings chapter 18