Network Security - Lecture 22

Network SecurityLecture 22Presented by: Dr. Munam Ali Shah Part – 2 (e): Incorporating security in other parts of the networkSummary of the Previous LectureIn previous lecture we continued our discussion on Confidentiality using symmetric encryptionWe talked about Master Key/Session KeyWe also talked about Key storage, key hierarchy, key renewal and lifetime of a session keyWe also explored the issues with centralized and decentralized key distribution.Summary of the previous lectureA key distribution scenarioOutlines of today’s lectureSome discussion on Decentralized Key ControlMessage Authentication Mechanism Message encryptionMAC HashObjectivesYou would be able to present an understanding of the confidentiality and message authentication mechanism.You would be able demonstrate knowledge about different functions and protocols used for message authenticationDecentralized Key ControlDecentralized Key ControlFor n end system, [n(n-1)]/2 master keys are required.message send using master key are short, crypt analysis is difficult, session are used for limited timeControlling key usageCan define different types of key on the basis of usageData encryption key: for general communicationPIN-encryption key: for PIN transferFile encrypting key: for file transferNeeds a control in systems that limit the ways in which the key is usedSimple plan: attached 8 bit tag with each 64 bit keyOne bit indicate whether the key is session or masterOne bit indicate whether the key is used for encryptionOne bit indicate whether the key is used for decryptionRemaining bits are spare for future useA key distribution scenarioLet us assume that user A wishes to establish a logical connection with B and requires a one-time session key to protect the data transmitted over the connection. A has a master key, Ka, known only to itself and the KDC; similarly, B shares the master key Kb with the KDC. The steps occurred are given in the figure (in next slide)A key distribution scenarioMan-in-the middle attackMessage AuthenticationConfidentiality and AuthenticationSo far we have talked about confidentiality onlyClassical ciphersBlock ciphersStream ciphersAuthentication is the second most important goal of cryptographyProvided by authentication functionsDigital signatures provide authentication as well as non-repudiationAuthentication FunctionsTwo levels of message authentication mechanismLower level: Authentication functionHigher level: Authentication protocolAuthentication functions have 3 classesMessage encryptionMessage Authentication Code (MAC)Hash functionMessage EncryptionIn a way, message encryption can provide authenticationBut not reliableSmall changes in ciphertext may not be detected Done in two waysSymmetric (private key) encryptionAsymmetric (public key) encryptionMessage EncryptionSymmetric encryption: confidentiality and authenticationMessage EncryptionPublic-key encryption: confidentialityMessage EncryptionPublic-key encryption: authentication and non-repudiationMessage EncryptionPublic-key encryption: confidentiality, authentication and non-repudiationMessage Authentication Code (MAC)MAC = C(K,M)M: Input messageC: MAC functionK: Shared secret keyMessage + MAC are sent to the intended recipientRecipient calculates MAC’ = C(K,M’)If MAC = MAC’ then accept else rejectProperties of MACMAC function need not be reversible (in contrast to decryption function)MAC input: arbitrary lengthMAC output: fixed length (typically much smaller than message length)MAC is many-to-one functionMessage Authentication CodeAuthenticationMessage Authentication CodeAuthentication and confidentiality; authentication tied to plaintextMessage Authentication CodeAuthentication and confidentiality; authentication tied to ciphertextHash FunctionA variation of MACDoes not need a keyh = H(M)h is called hash code/hash value/message digestRequirements of Hash FunctionArbitrary length inputFixed length outputH(x) is easy to computeGiven h, computationally hard to find x such that H(x) = h (called onewayness)Given x, computationally hard to find y ≠ x such that H(x) = H(y) (called weak collision resistance)Comp hard to find a pair x,y such that H(x) = H(y) (called strong collision resistance)Hash FunctionConfidentiality and authenticationHash FunctionAuthenticationHash FunctionAuthentication and non-repudiationHash FunctionConfidentiality, authentication and non-repudiationSummaryIn today’s we explored the limitations of the centralized key distribution and have explored key distribution in a decentralized fashion. Message Authentication Mechanism Message encryptionMAC HashNext lecture topicsWe will talk about authentication through digital signaturesThe End