Last time □ Message Integrity □ Authentication □ Key distribution and

Last time Message Integrity Authentication Key distribution and certification 24-1

This time Firewalls Attacks and countermeasures Security in many layers 24-2

Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Authentication 8.4 Integrity 8.5 Key Distribution and certification 8.6 Access control: firewalls 8.7 Attacks and counter measures 8.8 Security in many layers 24-3

Firewalls Firewall Isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. public Internet administered network firewall 24-4

Firewalls: Why Prevent denial of service attacks: SYN flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections. Prevent illegal modification/access of internal data. 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: application-level packet-filtering 24-5

Packet Filtering Should arriving packet be allowed in? Departing packet let out? Internal network connected to Internet via router firewall Router filters packet-by-packet, decision to forward/drop packet based on: source IP address, destination IP address TCP/UDP source and destination port numbers ICMP message type TCP SYN and ACK bits 24-6

Packet Filtering Example 1: block incoming and outgoing datagrams with IP protocol field 17 or with either source or dest port 23. All incoming and outgoing UDP flows, as well as telnet connections, are blocked. Example 2: Block inbound TCP segments with ACK 0. Prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside. 24-7

Application gateways host-to-gateway telnet session gateway-to-remote host telnet session Filters packets on application data as well as on IP/TCP/UDP fields. Example: allow select internal users to telnet outside. 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. 24-8

Limitations of firewalls and gateways IP spoofing: router can’t know if data “really” comes from claimed source If multiple applications need special treatment, each has own app. gateway. Client software must know how to contact gateway. Filters often use all or nothing policy for UDP. Tradeoff: degree of communication with outside world, level of security Many highly protected sites still suffer from attacks. e.g., must set IP address of proxy in Web browser 24-9

Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Authentication 8.4 Integrity 8.5 Key Distribution and certification 8.6 Access control: firewalls 8.7 Attacks and counter measures 8.8 Security in many layers 24-10

Internet security threats Mapping: before attacking: “case the joint” – find out what services are implemented on network Use ping to determine what hosts have addresses on network Port-scanning: try to establish TCP connection to each port in sequence (see what happens) nmap (http://www.insecure.org/nmap/) mapper: “network exploration and security auditing” Countermeasures? 24-11

Internet security threats Mapping: countermeasures record traffic entering network look for suspicious activity (IP addresses, ports being scanned sequentially) 24-12

Internet security threats Packet sniffing: broadcast media promiscuous NIC reads all packets passing by can read all unencrypted data (e.g. passwords) e.g.: C sniffs B’s packets C A src:B dest:A payload B Countermeasures? 24-13

Internet security threats Packet sniffing: countermeasures all hosts in organization run software that checks periodically if host interface in promiscuous mode (or try to remotely detect this) one host per segment of broadcast media (switched Ethernet at hub) C A src:B dest:A payload B 24-14

Internet security threats IP Spoofing: can generate “raw” IP packets directly from application, putting any value into IP source address field receiver can’t tell if source is spoofed e.g.: C pretends to be B C A src:B dest:A payload B Countermeasures? 24-15

Internet security threats IP Spoofing: ingress filtering routers should not forward outgoing packets with invalid source addresses (e.g., datagram source address not in router’s network) great, but ingress filtering can not be mandated for all networks C A src:B dest:A payload B 24-16

Internet security threats Denial of service (DoS): flood of maliciously generated packets “swamp” receiver Distributed DOS (DDoS): multiple coordinated sources swamp receiver e.g., C and remote host SYN-attack A C A SYN SYN SYN SYN SYN B Countermeasures? SYN SYN 24-17

Internet security threats Denial of service (DoS): countermeasures filter out flooded packets (e.g., SYN) before reaching host: throw out good with bad traceback to source of floods (most likely an innocent, compromised machine) C A SYN SYN SYN SYN SYN B SYN SYN 24-18

Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Authentication 8.4 Integrity 8.5 Key Distribution and certification 8.6 Access control: firewalls 8.7 Attacks and counter measures 8.8 Security in many layers 8.8.1. Secure email 8.8.2. Secure sockets 8.8.3. IPsec 8.8.4. Security in 802.11 Bonus: Secure Instant Messaging 24-19

Secure e-mail Alice wants to send confidential e-mail, m, to Bob. KS m KS . ES ( ) . EB ( ) EB ES(m ) EB(KS ) ES(m ) Internet . DS( ) EB(KS ) m KS . DB( ) DB Alice: Generates random symmetric private key, KS. Encrypts message with KS (for efficiency and size reasons) Also encrypts KS with Bob’s public key. Sends both ES(m) and EB(KS) to Bob. 24-20

Secure e-mail Alice wants to send confidential e-mail, m, to Bob. KS m KS . ES ( ) . EB ( ) ES(m ) EB(KS ) ES(m ) Internet . DS( ) EB(KS ) EB m KS . DB( ) DB Bob: Uses his private key to decrypt and recover KS Uses KS to decrypt ES(m) to recover m 24-21

Secure e-mail (continued) Alice wants to provide sender authentication and message integrity. m . H( ) SA VA . SA(H(m)) SA( ) m . SA(H(m)) Internet H(m ) VA( ) m compare . H( ) H(m ) Alice digitally signs message. Sends both message (in the clear) and digital signature. 24-22

Secure e-mail (continued) Alice wants to provide secrecy, sender authentication, and message integrity. m . H( ) SA . SA ( ) SA(H(m)) EB(KS ), ES(m, SA(H(m))) ES( ) m KS . KS . EB( ) Internet EB(KS ) EB Alice uses three keys: her private signing key, Bob’s public encryption key, newly created symmetric key 24-23

Pretty good privacy (PGP) Internet e-mail encryption scheme, de-facto standard. Uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described. Provides secrecy, sender authentication, integrity. Inventor, Phil Zimmerman, was target of 3-year federal investigation. A PGP signed message: ---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 lo8gE4vB3mqJhFEvZP9t6n7G 6m5Gw2 ---END PGP SIGNATURE--- 24-24

Secure sockets layer (SSL) Transport layer security to any TCP-based app using SSL services. Used between Web browsers, servers for e-commerce (https). Security services: server authentication data encryption client authentication (optional) Server authentication: SSL-enabled browser includes public keys for trusted CAs. Server presents a certificate signed by a CA. Browser uses CA’s public key to verify server’s public key. Check your browser’s security menu to see its trusted CAs. 24-25

SSL (continued) Encrypted SSL session: Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server. Using private key, server decrypts session key. Browser, server know session key SSL: basis of IETF Transport Layer Security (TLS). SSL can be used for non-Web applications, e.g., IMAP. Client authentication can be done with client certificates. All data sent into TCP socket (by client or server) encrypted with session key. 24-26

IPsec: Network Layer Security Network-layer secrecy: sending host encrypts the data in IP datagram TCP and UDP segments, ICMP messages, etc. Network-layer authentication destination host can authenticate source IP address Two principal protocols: authentication header (AH) protocol encapsulation security payload (ESP) protocol For both AH and ESP, source, destination handshake: create network-layer logical channel called a security association (SA) Each SA unidirectional. Uniquely determined by: security protocol (AH or ESP) source IP address 32-bit connection ID 24-27

Authentication Header (AH) Protocol AH header includes: authentication, data connection identifier integrity, no authentication data: confidentiality source-signed message AH header inserted digest calculated over between IP header, data original IP datagram. field. next header field: specifies IP header protocol field: type of data (e.g., TCP, 51 UDP, ICMP) Intermediate routers process datagrams as usual Provides source IP header AH header data (e.g., TCP, UDP segment) 24-28

ESP Protocol Provides secrecy, host authentication, data integrity. Data, ESP trailer encrypted. Next header field is in ESP trailer. ESP authentication field is similar to AH authentication field. Protocol 50. authenticated encrypted IP header ESP ESP TCP/UDP segment header trailer ESP authent. 24-29

Recap Firewalls Attacks and countermeasures Security in many layers PGP SSL IPSec 24-30

Next time Security in many layers WEP OTR Final review 24-31