1. Network Security7-1 Today r Collect Ch6 HW r Assign Ch7 HW m Ch7 #2,3,4,5,7,9,10,12 m Due Wednesday Nov 19 r Continue with Chapter 7 (Security)

2. Network Security7-2 Chapter 7 roadmap 7.1 What is network security? 7.2 Principles of cryptography 7.3 Authentication 7.4 Integrity 7.5 Key Distribution and certification 7.6 Access control: firewalls 7.7 Attacks and counter measures 7.8 Security in many layers

3. Network Security7-3 Firewalls isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. firewall

4. Network Security7-4 Firewalls: Why prevent denial of service attacks: m SYN flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections. prevent illegal modification/access of internal data. m e.g., attacker replaces CIA’s homepage with something else allow only authorized access to inside network (set of authenticated users/hosts) two types of firewalls: m application-level m packet-filtering

5. Network Security7-5 Packet Filtering r internal network connected to Internet via router firewall r router filters packet-by-packet, decision to forward/drop packet based on: m source IP address, destination IP address m TCP/UDP source and destination port numbers m ICMP message type m TCP SYN and ACK bits Should arriving packet be allowed in? Departing packet let out?

6. Network Security7-6 Packet Filtering r Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. m All incoming and outgoing UDP flows and telnet connections are blocked. r Example 2: Block inbound TCP segments with ACK=0. m Prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside.

7. Network Security7-7 Application gateways r Filters packets on application data as well as on IP/TCP/UDP fields. r Example: allow select internal users to telnet outside. host-to-gateway telnet session gateway-to-remote host telnet session application gateway router and filter 1. Require all telnet users to telnet through gateway. 2. For authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections 3. Router filter blocks all telnet connections not originating from gateway.

8. Network Security7-8 Limitations of firewalls and gateways r IP spoofing: router can’t know if data “really” comes from claimed source r if multiple app’s. need special treatment, each has own app. gateway. r client software must know how to contact gateway. m e.g., must set IP address of proxy in Web browser r filters often use all or nothing policy for UDP. r tradeoff: degree of communication with outside world, level of security r many highly protected sites still suffer from attacks.

9. Network Security7-9 Chapter 7 roadmap 7.1 What is network security? 7.2 Principles of cryptography 7.3 Authentication 7.4 Integrity 7.5 Key Distribution and certification 7.6 Access control: firewalls 7.7 Attacks and counter measures 7.8 Security in many layers

10. Network Security7-10 Internet security threats Mapping: m before attacking: “case the joint” – find out what services are implemented on network  Use ping to determine what hosts have addresses on network m Port-scanning: try to establish TCP connection to each port in sequence (see what happens) m nmap (http://www.insecure.org/nmap/) mapper: “network exploration and security auditing” Countermeasures?

11. Network Security7-11 Internet security threats Mapping: countermeasures m record traffic entering network m look for suspicious activity (IP addresses, pots being scanned sequentially)

12. Network Security7-12 Internet security threats Packet sniffing: m broadcast media m promiscuous NIC reads all packets passing by m can read all unencrypted data (e.g. passwords) m e.g.: C sniffs B’s packets A B C src:B dest:A payload Countermeasures?

13. Network Security7-13 Internet security threats Packet sniffing: countermeasures m all hosts in organization run software that checks periodically if host interface in promiscuous mode. m one host per segment of broadcast media (switched Ethernet at hub) A B C src:B dest:A payload

14. Network Security7-14 Internet security threats IP Spoofing: m can generate “raw” IP packets directly from application, putting any value into IP source address field m receiver can’t tell if source is spoofed m e.g.: C pretends to be B A B C src:B dest:A payload Countermeasures?

15. Network Security7-15 Internet security threats IP Spoofing: ingress filtering m routers should not forward outgoing packets with invalid source addresses (e.g., datagram source address not in router’s network) m great, but ingress filtering can not be mandated for all networks A B C src:B dest:A payload

16. Network Security7-16 Internet security threats Denial of service (DOS): m flood of maliciously generated packets “swamp” receiver m Distributed DOS (DDOS): multiple coordinated sources swamp receiver m e.g., C and remote host SYN-attack A A B C SYN Countermeasures?

17. Network Security7-17 Internet security threats Denial of service (DOS): countermeasures m filter out flooded packets (e.g., SYN) before reaching host: throw out good with bad m traceback to source of floods (most likely an innocent, compromised machine) A B C SYN

18. Network Security7-18 Chapter 7 roadmap 7.1 What is network security? 7.2 Principles of cryptography 7.3 Authentication 7.4 Integrity 7.5 Key Distribution and certification 7.6 Access control: firewalls 7.7 Attacks and counter measures 7.8 Security in many layers 7.8.1. Secure email 7.8.2. Secure sockets 7.8.3. IPsec 8.8.4. 802.11 WEP

19. Network Security7-19 Secure e-mail Alice:  generates random symmetric private key, K S.  encrypts message with K S (for efficiency)  also encrypts K S with Bob’s public key.  sends both K S (m) and K B (K S ) to Bob.  Alice wants to send confidential e-mail, m, to Bob. K S ( ). K B ( ). + + - K S (m ) K B (K S ) + m KSKS KSKS KBKB + Internet K S ( ). K B ( ). - KBKB - KSKS m K S (m ) K B (K S ) +

20. Network Security7-20 Secure e-mail Bob:  uses his private key to decrypt and recover K S  uses K S to decrypt K S (m) to recover m  Alice wants to send confidential e-mail, m, to Bob. K S ( ). K B ( ). + + - K S (m ) K B (K S ) + m KSKS KSKS KBKB + Internet K S ( ). K B ( ). - KBKB - KSKS m K S (m ) K B (K S ) +

21. Network Security7-21 Secure e-mail (continued) Alice wants to provide sender authentication message integrity. Alice digitally signs message. sends both message (in the clear) and digital signature. H( ). K A ( ). - + - H(m ) K A (H(m)) - m KAKA - Internet m K A ( ). + KAKA + K A (H(m)) - m H( ). H(m ) compare

22. Network Security7-22 Secure e-mail (continued) Alice wants to provide secrecy, sender authentication, message integrity. Alice uses three keys: her private key, Bob’s public key, newly created symmetric key H( ). K A ( ). - + K A (H(m)) - m KAKA - m K S ( ). K B ( ). + + K B (K S ) + KSKS KBKB + Internet KSKS

23. Network Security7-23 Pretty good privacy (PGP) r Internet e-mail encryption scheme, de-facto standard. r uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described. r provides secrecy, sender authentication, integrity. r inventor, Phil Zimmerman, was target of 3-year federal investigation. ---BEGIN PGP SIGNED MESSAGE--- Hash: SHA1 Bob:My husband is out of town tonight.Passionately yours, Alice ---BEGIN PGP SIGNATURE--- Version: PGP 5.0 Charset: noconv yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJ hFEvZP9t6n7G6m5Gw2 ---END PGP SIGNATURE--- A PGP signed message:

24. Network Security7-24 Secure sockets layer (SSL) r transport layer security to any TCP- based app using SSL services. r used between Web browsers, servers for e-commerce (https). r security services: m server authentication m data encryption m client authentication (optional) r server authentication: m SSL-enabled browser includes public keys for trusted CAs. m Browser requests server certificate, issued by trusted CA. m Browser uses CA’s public key to extract server’s public key from certificate. r check your browser’s security menu to see its trusted CAs.

25. Network Security7-25 SSL (continued) Encrypted SSL session: r Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server. r Using private key, server decrypts session key. r Browser, server know session key m All data sent into TCP socket (by client or server) encrypted with session key. r SSL: basis of IETF Transport Layer Security (TLS). r SSL can be used for non-Web applications, e.g., IMAP. r Client authentication can be done with client certificates.

26. Network Security7-26 IPsec: Network Layer Security r Network-layer secrecy: m sending host encrypts the data in IP datagram m TCP and UDP segments; ICMP and SNMP messages. r Network-layer authentication m destination host can authenticate source IP address r Two principle protocols: m authentication header (AH) protocol m encapsulation security payload (ESP) protocol r For both AH and ESP, source, destination handshake: m create network-layer logical channel called a security association (SA) r Each SA unidirectional. r Uniquely determined by: m security protocol (AH or ESP) m source IP address m 32-bit connection ID

27. Network Security7-27 Authentication Header (AH) Protocol r provides source authentication, data integrity, no confidentiality r AH header inserted between IP header, data field. r protocol field: 51 r intermediate routers process datagrams as usual AH header includes: r connection identifier r authentication data: source- signed message digest calculated over original IP datagram. r next header field: specifies type of data (e.g., TCP, UDP, ICMP) IP headerdata (e.g., TCP, UDP segment) AH header

28. Network Security7-28 ESP Protocol r provides secrecy, host authentication, data integrity. r data, ESP trailer encrypted. r next header field is in ESP trailer. r ESP authentication field is similar to AH authentication field. r Protocol = 50. IP header TCP/UDP segment ESP header ESP trailer ESP authent. encrypted authenticated