Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity and digital signatures 8.4 End-point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL 8.7 Network layer security: IPsec and VPNs 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS Security 8-1

WEP design goals symmetric key crypto confidentiality end host authorization data integrity self-synchronizing: each packet separately encrypted 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) Efficient implementable in hardware or software Security 8-2

Review: symmetric stream ciphers keystream generator key combine each byte of keystream with byte of plaintext to get ciphertext: m(i) = ith unit of message ks(i) = ith unit of keystream c(i) = ith unit of ciphertext c(i) = ks(i)  m(i) ( = exclusive or) m(i) = ks(i)  c(i) WEP uses RC4 Security 8-3

About WEP Encryption algorithm in IEE 802.11 – 1999 Standard (Wired Eqivalent Privacy) User Stream Cipher – RC4 for confidentiality and CRC for integrity WEP is broken and is deprecated form wireless security measuers Network Security

Stream cipher and packet independence recall design goal: each packet separately encrypted if for frame n+1, use keystream from where we left off for frame n, then each frame is not separately encrypted need to know where we left off for packet n WEP approach: initialize keystream with key + new IV for each packet: keystream generator Key+IVpacket keystreampacket Security 8-5

+ WEP Working Seed = IV + Key RC4 takes seed as input and gives Keystream as output IV (Initializing Vector): 24 bits (fixed size) Key : 40 or 104 bits Seed = 16 or 128 bits (IV+ key) RC4 1 + 1 1

WEP encryption (1) sender calculates Integrity Check Value (ICV, four-byte hash/CRC over data each side has 104-bit shared key sender creates 24-bit initialization vector (IV), appends to key: gives 128-bit key sender also appends keyID (in 8-bit field) 128-bit key inputted into pseudo random number generator to get keystream data in frame + ICV is encrypted with RC4: bytes of keystream are XORed with bytes of data & ICV IV & keyID are appended to encrypted data to create payload payload inserted into 802.11 frame encrypted data ICV IV MAC payload Key ID Security 8-7

WEP encryption (2) new IV for each frame Security 8-8

WEP decryption overview encrypted data ICV IV MAC payload Key ID receiver extracts IV inputs IV, shared secret key into pseudo random generator, gets keystream XORs keystream with encrypted data to decrypt data + ICV verifies integrity of data with ICV note: message integrity approach used here is different from MAC (message authentication code) and signatures (using PKI). Security 8-9

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! Security 8-10

WEP authentication Notes: not all APs do it, even if WEP is being used authentication request nonce (128 bytes) nonce encrypted shared key success if decrypted value equals nonce Notes: not all APs do it, even if WEP is being used AP indicates if authentication is necessary in beacon frame done before association Security 8-11

Why WEP is weak No Key Management Key stream MUST NEVER be reused One key for all Key stream MUST NEVER be reused IV space is too small satisfy this since key is always the same (214 is small number) IV is transmitted as clear text No Standard procedure for IV generation Frist few key stream bytes are predictable in RC4 algorithm with weak IV

Breaking 802.11 WEP encryption security hole: 24-bit IV, one IV per frame, -> IV’s eventually reused IV transmitted in plaintext -> IV reuse detected attack: Trudy causes Alice to encrypt known plaintext d1 d2 d3 d4 … Trudy sees: ci = di XOR kiIV Trudy knows ci di, so can compute kiIV Trudy knows encrypting key sequence k1IV k2IV k3IV … Next time IV is used, Trudy can decrypt! Security 8-13

Other Possibilities Bit flip attacks possible since CRC is linear and RC4 is transparent to XOR ICV uses CRC32, ICV is good for noise detection, but extremely poor as a cryptographic hash WEP is weak against Replay Attacks, fragmentation Attacks, known plain text replay attacks etc. Share key authentication is poor – it reveals 128 byte key stream Network Security

Tools Airsnort Aircrack-ng: FMS, COREC, REPLAY attacks Aircrack-ptw: ARP packet based attcatck Aireplay-ng: BIT FLIPPING, REPALY attacks WEPCrack …… And many more

802.11i: improved security numerous (stronger) forms of encryption possible provides key distribution uses authentication server separate from access point Security 8-16

802.11i: four phases of operation AP: access point STA: client station AS: Authentication server wired network 1 Discovery of security capabilities STA and AS mutually authenticate, together generate Master Key (MK). AP serves as “pass through” 2 3 STA derives Pairwise Master Key (PMK) AS derives same PMK, sends to AP 4 STA, AP use PMK to derive Temporal Key (TK) used for message encryption, integrity Security 8-17

EAP: extensible authentication protocol EAP: end-end client (mobile) to authentication server protocol EAP sent over separate “links” mobile-to-AP (EAP over LAN) AP to authentication server (RADIUS over UDP) wired network EAP TLS EAP EAP over LAN (EAPoL) RADIUS IEEE 802.11 UDP/IP Security 8-18

802.1x Authentications Authentication Server Verification Server AP Supplicant EAPoL Start EAP Request/Identity EAP Response/Identity Access Request Valid User - Challenge EAP Challenge Request Access Challenge EAP Challenge Response Access Request Challenge Response Correct Response EAP Success Access Accept The Master Key is shared only between the AS and supplicant The supplicant and AS derive a Pairwise Master Key (PMK) & This is moved form the AS to the authenticator The Master Key is generated between the AS and Supplicant upon a successful 802.1x authentication and bound for the entire session

Summary WPA-Personal TKIP PSK WPA2 – Personal AES-CCMP WPA – Enterprise 802.1X/EAP WPA2 – Enterprise

Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity and digital signatures 8.4 End-point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL 8.7 Network layer security: IPsec and VPNs 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS Security 8-21

Firewalls firewall isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others administered network public Internet trusted “good guys” untrusted “bad guys” firewall Security 8-22

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 three types of firewalls: stateless packet filters stateful packet filters application gateways Security 8-23

Stateless 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 Security 8-24

Stateless packet filtering: example example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23 result: all incoming, outgoing UDP flows and telnet connections are blocked example 2: block inbound TCP segments with ACK=0. result: prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside. Security 8-25

Stateless packet filtering: more examples 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 130.207.244.203, 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 (e.g. 130.207.255.255). Prevent your network from being tracerouted Drop all outgoing ICMP TTL expired traffic Security 8-26

Access Control Lists ACL: table of rules, applied top to bottom to incoming packets: (action, condition) pairs: looks like OpenFlow forwarding (Ch. 4)! action source address dest protocol port flag bit allow 222.22/16 outside of TCP > 1023 80 any ACK UDP 53 --- ---- deny all Security 8-27

Stateful packet filtering stateless packet filter: heavy handed tool admits packets that “make no sense,” e.g., dest port = 80, ACK bit set, even though no TCP connection established: action source address dest protocol port flag bit allow outside of 222.22/16 TCP 80 > 1023 ACK stateful packet filter: track status of every TCP connection track connection setup (SYN), teardown (FIN): determine whether incoming, outgoing packets “makes sense” timeout inactive connections at firewall: no longer admit packets Security 8-28

Stateful packet filtering ACL augmented to indicate need to check connection state table before admitting packet action source address dest proto port flag bit check conxion allow 222.22/16 outside of TCP > 1023 80 any ACK x UDP 53 --- ---- deny all Security 8-29

Application gateways filter packets on application data as well as on IP/TCP/UDP fields. example: allow select internal users to telnet outside application gateway host-to-gateway telnet session router and filter gateway-to-remote host telnet session 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. Security 8-30

Limitations of firewalls, gateways IP spoofing: router can’t know if data “really” comes from claimed source if multiple app’s. need special treatment, each has own app. gateway client software must know how to contact gateway. e.g., must set IP address of proxy in Web browser 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 Security 8-31

Intrusion detection systems packet filtering: operates on TCP/IP headers only no correlation check among sessions IDS: intrusion detection system deep packet inspection: look at packet contents (e.g., check character strings in packet against database of known virus, attack strings) examine correlation among multiple packets port scanning network mapping DoS attack Security 8-32

Examine Correlations Assume intruder behaviour is different from that of legitimate user False positives: Legitimate user identified as intruder False negatives: Intruder not identified

IDS Measurements

IDS Architecture Host-based IDS on multiple computers within organisation or internetwork Host agent collects and analyse audit records on individual hosts LAN monitor agent analyses LAN traffic Host and LAN monitor agents send alerts to central manager Central manager combines data to detect intrusion; may request data form specific hosts Issues Deal with different audit record formats Data transmitted over network by agents needs to be secured Central architecture: single point of failure Complex coordination

Network-Based Intrusion Detection Monitor traffic at selected points on network Analyse traffic to detect intrusion patterns Inline sensors Inserted into network Runs as software on existing switch, router or firewall Can prevent attack as soon as detected Passive sensor Monitors copy of traffic Extra device that receives copy of traffic Minimal impact on performance of traffic

Example of Network Intrusion Detection System

Honeypots Decoy system designed to Lure potential attacker away form critical systems Collect information about the attacker’s activity Encourage the attacker to stay on the long enough for administrators to respond Filled with fabricated information that a legitimate user of the system wouldn’t access Resource that has no production value Incoming communications is most likely a probe, scan or attack Outbound communication suggest that the system has probably been compromised Once intruders are within the network, administrators can observe their behaviour to figure out defences.

Example of Honeypot Deployment

Summary: Intrusion detection systems multiple IDSs: different types of checking at different locations firewall internal network Internet IDS sensors Web server DNS server FTP server demilitarized zone Security 8-40

Network Security (summary) basic techniques…... cryptography (symmetric and public) message integrity end-point authentication …. used in many different security scenarios secure email secure transport (SSL) IP sec 802.11 operational security: firewalls and IDS Security 8-41