Friends and enemies:Alice,Bob,Trudy well-known in network security world Bob,Alice (lovers!)want to communicate "securely" Trudy (intruder)may intercept,delete,add messages Alice Bob channel data,control messages data secure secure data sender receiver Trudy 8:Network Security 8-5 8: Network Security 8-5 Friends and enemies: Alice, Bob, Trudy  well-known in network security world  Bob, Alice (lovers!) want to communicate “securely”  Trudy (intruder) may intercept, delete, add messages secure sender secure receiver channel data, control messages data data Alice Bob Trudy