Chapter 2 (E) – Confidentiality Using Symmetric EncryptionConfidentiality using Symmetric EncryptionSlide 3Traffic AnalysisPlacement of EncryptionSlide 6Key DistributionSlide 8Key Distribution ScenarioKey Distribution IssuesSummaryChapter 2 (E) – Confidentiality UsingSymmetric EncryptionConfidentiality using Symmetric Encryption•traditionally symmetric encryption is used to provide message confidentiality•consider typical scenario–workstations on LANs access other workstations & servers on LAN–LANs interconnected using switches/routers–with external lines or radio/satellite links•consider attacks and placement in this scenario–snooping from another workstation–use dial-in to LAN or server to snoop–use external router link to enter & snoop–monitor and/or modify traffic one external linksConfidentiality using Symmetric Encryption•have two major placement alternatives•link encryption–encryption occurs independently on every link–implies must decrypt traffic between links–requires many devices, but paired keys•end-to-end encryption–encryption occurs between original source and final destination–need devices at each end with shared keysTraffic Analysis•when using end-to-end encryption must leave headers in clear–so network can correctly route information•hence although contents protected, traffic pattern flows are not•ideally want both at once–end-to-end protects data contents over entire path and provides authentication–link protects traffic flows from monitoringPlacement of Encryption•can place encryption function at various layers in OSI Reference Model–link encryption occurs at layers 1 or 2–end-to-end can occur at layers 3, 4, 6, 7– as move higher less information is encrypted but it is more secure though more complex with more entities and keysTraffic Analysis•is monitoring of communications flows between parties–useful both in military & commercial spheres–can also be used to create a covert channel•link encryption obscures header details–but overall traffic volumes in networks and at end-points is still visible•traffic padding can further obscure flows–but at cost of continuous trafficKey Distribution•symmetric schemes require both parties to share a common secret key•issue is how to securely distribute this key•often secure system failure due to a break in the key distribution schemeKey Distribution•given parties A and B have various key distribution alternatives:1. A can select key and physically deliver to B2. third party can select & deliver key to A & B3. if A & B have communicated previously can use previous key to encrypt a new key4. if A & B have secure communications with a third party C, C can relay key between A & BKey Distribution ScenarioKey Distribution Issues•hierarchies of KDC’s required for large networks, but must trust each other•session key lifetimes should be limited for greater security•use of automatic key distribution on behalf of users, but must trust system•use of decentralized key distribution•controlling purposes keys are used forSummary•have considered:–use of symmetric encryption to protect confidentiality–need for good key distribution–use of trusted third party