Friends and enemies: Alice Bob. Trudy Dato Data control, data messages Secure Secure sencer eceiver channe 网 Alice Trudy o well-known in network security world o Bob, Alice (lovers! )want to communicate"securely o Trudy, the intruder"may intercept, delete, add messages 7: Network Security 27: Network Security 2 Friends and enemies: Alice, Bob, Trudy  well-known in network security world  Bob, Alice (lovers!) want to communicate “securely”  Trudy, the “intruder” may intercept, delete, add messages Figure 7.1 goes here