1 WEP Design Goals r Symmetric key crypto m Confidentiality m Station authorization m Data integrity r Self synchronizing: each packet separately encrypted m Given encrypted packet and key, can decrypt; can continue to decrypt packets when preceding packet was lost Unlike Cipher Block Chaining (CBC) in block ciphers r Efficient m Can be implemented in hardware or software

2 Review: Symmetric Stream Ciphers r Combine each byte of keystream with byte of plaintext to get ciphertext r m(i) = i th unit of message r ks(i) = i th unit of keystream r c(i) = i th unit of ciphertext r c(i) = ks(i)  m(i) (  = exclusive or) r m(i) = ks(i)  c(i) r WEP uses RC4 keystream generator key keystream

3 Stream cipher and packet independence r Recall design goal: each packet separately encrypted r If for frame n+1, use keystream from where we left off for frame n, then each frame is not separately encrypted m Need to know where we left off for packet n r WEP approach: initialize keystream with key + new IV for each packet: keystream generator Key+IV packet keystream packet

4 WEP encryption (1) r Sender calculates Integrity Check Value (ICV) over data m four-byte hash/CRC for data integrity r Each side has 104-bit shared key r Sender creates 24-bit initialization vector (IV), appends to key: gives 128-bit key r Sender also appends keyID (in 8-bit field) r 128-bit key inputted into pseudo random number generator to get keystream r data in frame + ICV is encrypted with RC4: m Bytes of keystream are XORed with bytes of data & ICV m IV & keyID are appended to encrypted data to create payload m Payload inserted into frame encrypted dataICVIV MAC payload Key ID

5 WEP encryption (2) New IV for each frame

6 WEP decryption overview r Receiver extracts IV r Inputs IV and shared secret key into pseudo random generator, gets keystream r XORs keystream with encrypted data to decrypt data + ICV r Verifies integrity of data with ICV m Note that message integrity approach used here is different from the MAC (message authentication code) and signatures (using PKI). encrypted dataICVIV MAC payload Key ID

7 End-point authentication w/ nonce Nonce: number (R) used only once –in-a-lifetime How: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key “I am Alice” R K (R) A-B Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!

8 WEP Authentication AP authentication request nonce (128 bytes) nonce encrypted shared key success if decrypted value equals nonce Not all APs do it, even if WEP is being used. AP indicates if authentication is necessary in beacon frame. Done before association.

Breaking WEP encryption security hole: r 24-bit IV, one IV per frame, -> IV’s eventually reused r IV transmitted in plaintext -> IV reuse detected r attack: m Trudy causes Alice to encrypt known plaintext d 1 d 2 d 3 d 4 … m Trudy sees: c i = d i XOR k i IV m Trudy knows c i d i, so can compute k i IV m Trudy knows encrypting key sequence k 1 IV k 2 IV k 3 IV … m Next time IV is used, Trudy can decrypt!

802.11i: improved security r numerous (stronger) forms of encryption possible r provides key distribution r uses authentication server separate from access point

AP: access point AS: Authentication server wired network STA: client station 1 Discovery of security capabilities 3 STA and AS mutually authenticate, together generate Master Key (MK). AP servers as “pass through” 2 3 STA derives Pairwise Master Key (PMK) AS derives same PMK, sends to AP STA, AP use PMK to derive Temporal Key (TK) used for message encryption, integrity i: four phases of operation

wired network EAP TLS EAP EAP over LAN (EAPoL) IEEE RADIUS UDP/IP EAP: extensible authentication protocol r EAP: end-end client (mobile) to authentication server protocol r EAP sent over separate “links” m mobile-to-AP (EAP over LAN) m AP to authentication server (RADIUS over UDP)

Lecture 23 Firewalls CPE 401/601 Computer Network Systems slides are modified from Jim Kurose & Keith Ross All material copyright J.F Kurose and K.W. Ross, All Rights Reserved

Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity 8.4 Securing 8.5 Securing TCP connections: SSL 8.6 Network layer security: IPsec 8.7 Securing wireless LANs 8.8 Operational security: firewalls and IDS

Firewalls isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. firewall administered network public Internet firewall

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) three types of firewalls: m stateless packet filters m stateful packet filters m application gateways

Stateless 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?

Stateless packet filtering: example r example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. m all incoming, 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.

Policy Firewall Setting No outside Web access. Drop all outgoing packets to any IP address, port 80 No incoming TCP connections, except those for institution’s public Web server only. Drop all incoming TCP SYN packets to any IP except , port 80 Prevent Web-radios from eating up the available bandwidth. Drop all incoming UDP packets - except DNS and router broadcasts. Prevent your network from being used for a smurf DoS attack. Drop all ICMP packets going to a “broadcast” address (eg ). Prevent your network from being tracerouted Drop all outgoing ICMP TTL expired traffic Stateless packet filtering: more examples

action source address dest address protocol source port dest port flag bit allow222.22/16 outside of /16 TCP> any allowoutside of /16 TCP80> 1023ACK allow222.22/16 outside of /16 UDP> allowoutside of /16 UDP53> denyall Access Control Lists r ACL: table of rules, applied top to bottom to incoming packets: (action, condition) pairs

Stateful packet filtering r stateless packet filter: heavy handed tool m admits packets that “make no sense,” e.g., dest port = 80, ACK bit set, even though no TCP connection established: action source address dest address protocol source port dest port flag bit allowoutside of /16 TCP80> 1023ACK r stateful packet filter: track status of every TCP connection m track connection setup (SYN), teardown (FIN): can determine whether incoming, outgoing packets “makes sense” m timeout inactive connections at firewall: no longer admit packets

action source address dest address proto source port dest port flag bit check conxion allow222.22/16 outside of /16 TCP> any allowoutside of /16 TCP80> 1023ACK x allow222.22/16 outside of /16 UDP> allowoutside of /16 UDP53> x denyall Stateful packet filtering r ACL augmented to indicate need to check connection state table before admitting packet

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.

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.

Intrusion detection systems r packet filtering: m operates on TCP/IP headers only m no correlation check among sessions r IDS: intrusion detection system m deep packet inspection: look at packet contents (e.g., check character strings in packet against database of known virus, attack strings) m examine correlation among multiple packets port scanning network mapping DoS attack

Web server FTP server DNS server application gateway Internet demilitarized zone internal network firewall IDS sensors Intrusion detection systems r multiple IDSs: different types of checking at different locations

Network Security (summary) Basic techniques…... m cryptography (symmetric and public) m message integrity m end-point authentication …. used in many different security scenarios m secure m secure transport (SSL) m IP sec m Operational Security: firewalls and IDS