1. SECURITY

2. Security Threats, Policies, and Mechanisms There are four types of security threats to consider 1. Interception 2 Interruption 3. Modification 4. Fabrication

3. Interception - an unauthorized party has gained access to a service or data Interruption - attempts to make a service inaccessible to other parties Modification - unauthorized changing of data or tampering with a service Fabrication - additional data or activity are generated that would normally not exist.

4. security mechanisms Important security mechanisms are: 1. Encryption 2. Authentication 3. Authorization 4. Auditing

5. Encryption - transforms data into something an attacker cannot understand Authentication - used to verify the claimed identity of a user, client, server, host, or other entity Authorization - Permission may be granted to read records, to modify certain fields in a record, or to add or remove a record Auditing tools - used to trace which clients accessed what, and which way.

6. SECURE CHANNELS Authentication

7. Authentication Based on a Shared Secret Key Bob subsequently sends a challenge RB to Alice, shown as message 2. Such a challenge could take the form of a random number. Alice is required to encrypt the challenge with the secret key KA,B that she shares with Bob, and return the encrypted challenge to Bob. This response is shown as message 3 in Fig. 9-12 containing KA,B(RB)·

9. The protocol proceeds as First, Alice sends her identity to Bob (message 1), indicating that she wants to set up a communication channel between the two. Bob subsequently sends a challenge RB to Alice, shown as message 2. Alice is required to encrypt the challenge with the secret key KA,B that she shares with Bob, and return the encrypted challenge to Bob. This response is shown as message 3 in Fig. 9-12 containing KA,B(RB) When Bob receives the response KA,B(RB) to his challenge RB, he can decrypt the message using the shared key again to see if it contains RB· If so, he then knows that Alice is on the other side

12. Authentication Using a Key Distribution Center

15. Message Integrity and Confidentiality Digital Signatures

17. Secure Group Communication Confidential Group Communication First, consider the problem of protecting communication between a group of N users against eavesdropping. To ensure confidentiality, a simple scheme is to let all group members share the same secret key, which is used to encrypt and decrypt all messages transmitted between group members. Because the secret key in this scheme is shared by all members, it is necessary that all members are trusted to indeed keep the key a secret. An alternative solution is to use a separate shared secret key between each pair of group members. As soon as one member turns out to be leaking information, the others can simply stop sending messages to that member, but still use the keys they were using to communicate with each other. However, instead of having to maintain one key, it is now necessary to maintain N(N - 1)/2 keys, which may be a difficult problem by itself.

18. Using a public-key cryptosystem can improve matters. In that case, each member has its own (public key, private key) pair, in which the public key can be used by all members for sending confidential messages. In this case, a total of N key pairs are needed. If one member ceases to be trustworthy, it is simply removed

19. Secure Replicated Servers

20. Example: Kerberos

22. ACCESS CONTROL General Issues in Access Control

23. Access Control Matrix

24. Protection Domains

25. Firewalls