1. Chapter 40 Network Security (Access Control, Encryption, Firewalls)

2. 2 Secure Networks  Secure network is not an absolute term  Need to define security policy for organization  Network security policy cannot be separated from security policy for attached computers  Costs and benefits of security policies must be assessed

3. 3 Network Security Policy Devising a network security policy can be complex because a rational policy requires an organization to assess the value of information. The policy must apply to information stored in computers as well as to information traversing a network.

4. 4 Aspects of Security  Data integrity  Data availability  Data confidentiality  Privacy

5. 5 Responsibility and Control  Accountability: how an audit trail is kept  Authorization: who is responsible for each item and how is responsibility delegated to others

6. 6 Integrity Mechanisms  Techniques to ensure integrity  Parity bits  Checksums  CRCs  These cannot guarantee data integrity (e.g., against intentional change  Use of message authentication code (MAC) that cannot be broken or forged

7. 7 Access Control and Passwords  Passwords used to control access  Over a network, passwords susceptible to snooping

8. 8 Encryption and Confidentiality  To ensure confidentiality of a transmitted message, use encryption  Secret key or public key schemes encryptiondecryption message m Secret key S

9. 9 Public Key Cryptosystem  Each processor has private key S and public key P  S is kept secret, and cannot be deduced from P  P is made available to all processors  Encryption and decryption with S and P are inverse functions: P(S(m)) = m and S(P(m)) = m

10. 10 Message Digest  Digest function maps arbitrary length message m to fixed length digest d(m)  One-way function: given d(m), can't find m  Collision-free: infeasible to generate m and m' such that d(m) = d(m')

11. 11 Digital Signature  To sign message m, sender computes digest d(m)  Sender computes S(d(m)) and sends along with m  Receiver computes P(S(d(m))) = d(m)  Receiver computes digest of m and compares with result above; if match, signature is verified

12. 12 Digital Signature

13. 13 Internet Firewall  Protect an organization’s computers from internet problems (firewall between two structures to prevent spread of fire)

14. 14 Internet Firewall  All traffic entering the organization passes through the firewall  All traffic leaving the organization passes through the firewall  The firewall implements the security policy and rejects any traffic that doesn’t adhere  The firewall must be immune to security attacks

15. 15 Packet Filtering  Packet filter is embedded in router  Specify which packets can pass through and which should be blocked

16. 16 Using Packet Filters to Create a Firewall  Three components in a firewall  Packet filter for incoming packets  Packet filter for outgoing packets  Secure computer system to run application-layer gateways or proxies

17. 17 Virtual Private Networks  Two approaches to building corporate intranet for an organization with multiple sites:  Private network connections (confidential)  Public internet connections (low cost)  Virtual Private Network  Achieve both confidentiality and low cost  Implemented in software

18. 18 Virtual Private Network  VPN software in router at each site gives appearance of a private network

19. 19 Virtual Private Network  Obtain internet connection for each site  Choose router at each site to run VPN software  Configure VPN software in each router to know about the VPN routers at other sites  VPN software acts as a packet filter; next hop for outgoing datagram is another VPN router  Each outgoing datagram is encrypted

20. 20 Tunneling  Desire to encrypt entire datagram so source and destination addresses are not visible on Internet  How can internet routers do proper forwarding?  Solution: VPN software encrypts entire datagram and places inside another for transmission  Called IP-in-IP tunneling (encapsulation)

21. 21 Tunneling  Datagram from computer x at site 1 to computer y at site 2  Router R 1 on site 1 encrypts, encapsulates in new datagram for transmission to router R 2 on site 2

22. 22 Summary  Security is desirable but must be defined by an organization  Assess value of information and define a security policy  Aspects to consider include privacy and data integrity, availability, and confidentiality

23. 23 Summary (continued)  Mechanisms to provide aspects of security  Encryption: secret and public key cryptosystems  Firewalls: packet filtering  Virtual private networks  Use Internet to transfer data among organization’s sites but ensure that data cannot be read by others