8-1 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity & end point authentication 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

8-2 What is network security? Confidentiality: only sender, intended receiver should “understand” message contents m sender encrypts message m receiver decrypts message Authentication: sender, receiver want to confirm identity of each other Message Integrity: sender, receiver want to ensure that any message that is altered (in transit, or afterwards) will be detected Access control and Availability: services accessible and available to authorized users

8-3 Friends and enemies: Alice, Bob, Trudy r well-known in network security world r Bob, Alice want to communicate “securely” r Trudy (intruder) may intercept, delete, insert, modify messages secure sender secure receiver channel data, control messages data Alice Bob Trudy

8-4 Who might Bob, Alice be? r … well, real-life Bobs and Alices! r Web browser/server for electronic transactions (e.g., on-line purchases) r on-line banking client/server r DNS servers r routers exchanging routing table updates r other examples?

8-5 What can a “bad” guy do? m eavesdrop: intercept messages m actively insert messages into connection m impersonation: can fake (spoof) source address in packet (or any field in packet) m hijacking: “take over” ongoing connection by removing sender or receiver, inserting itself in its place, or inserting itself in the middle m denial of service: prevent service from being used by others (e.g., by overloading resources) m …

8-6 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity & end point authentication 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

8-7 The language of cryptography symmetric key crypto: sender, receiver keys identical public-key crypto: encryption key is public, decryption key is private (also called asymmetric key crypto) plaintext ciphertext K A encryption algorithm decryption algorithm Alice’s encryption key Bob’s decryption key K B

8-8 Symmetric key cryptography r symmetric key crypto: Bob and Alice share (know) the same key r standards: DES, AES, RC4, IDEA, … plaintext ciphertext K A-B encryption algorithm decryption algorithm K A-B plaintext message, m K (m) A-B K (m) A-B m = K ( ) A-B

8-9 Public Key Cryptography symmetric key crypto r requires sender and receiver to share a secret key r Q: how to agree on key in first place (particularly if never “met”)? public key cryptography r radically different approach [Diffie- Hellman76, RSA78] r sender, receiver do not share secret key r a public key for encrypting (or verifying) r a private key for decrypting (or signing)

8-10 Public key cryptography plaintext message, m ciphertext encryption algorithm decryption algorithm Bob’s public key plaintext message K (m) B + K B + Bob’s private key K B - m = K ( K (m) ) B + B -

8-11 Public key encryption algorithms need K ( ) and K ( ) such that B B.. given public key K, it should be impossible to compute private key K B B Requirements: 1 2 RSA: Rivest, Shamir, Adleman algorithm + - K (K (m)) = m B B

8-12 RSA: another important property The following property will be very useful later: K ( K (m) ) = m B B - + K ( K (m) ) B B + - = use public key first, followed by private key use private key first, followed by public key Result is the same!

8-13 Session keys r RSA requires exponentiation which is computationally intensive e.g., DES is at least 100 times faster than RSA Session key, K S r Bob and Alice use RSA to exchange a symmetric key K S r Once both have K S, they use symmetric key crypto for confidential communication

8-14 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity & end point authentication 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

8-15 Message Integrity r Allows communicating parties to verify that received messages are authentic. m Content of message has not been altered m Source of message is who/what you think it is m Message has not been replayed m Sequence of messages is maintained r Let’s first consider message digests

8-16 Message Digests r Function H( ) that takes as input an arbitrary length message and outputs a fixed-length string m Note that H( ) is a many- to-1 function m H( ) is often called a “hash function” r Desirable properties of crypto hash function: m Easy to calculate m Irreversible: Can’t determine m from H(m) m Collision resistant: computationally difficult to produce m and m’ such that H(m) = H(m’) m Seemingly random output large message m H: Hash Function H(m)

8-17 Internet checksum: poor crypto hash function Internet checksum has some properties of hash function: ü produces fixed length digest (16-bit sum) of message ü is many-to-one But for a message with given hash value, it is easy to find another message with same hash value. Simple checksum example: I O U B O B 49 4F E F 42 message ASCII format B2 C1 D2 AC 49 4F E F 42 message ASCII format B2 C1 D2 AC different messages but identical checksums! I O U B O B

8-18 Message Authentication Code (MAC) m s (shared secret) (message) H(. ) H(m+s) public Internet append m H(m+s) s compare m H(m+s) H(. ) H(m+s) (shared secret)

8-19 Crypto hash functions in practice r MD5 hash function widely used (RFC 1321) m computes 128-bit message digest in 4-step process. m arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x recent (2005) attacks on MD5, may not be collision resistant r SHA-1 is also used m US standard [ NIST, FIPS PUB 180-1] m 160-bit message digest m more robust algorithms such as SHA–224, SHA–256, SHA– 384 and SHA–512

8-20 Digital Signatures objective Like hand-written signatures r sender (Bob) digitally signs document with his own private key, establishing he is document owner/creator. r verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document

8-21 Digital Signatures simple digital signature for message m: r Bob “signs” m by encrypting with his private key K B, creating “signed” message, K B (m) - - Dear Alice Oh, how I have missed you. I think of you all the time! …(blah blah blah) Bob Bob’s message, m public key encryption algorithm Bob’s private key K B - Bob’s message, m, signed (encrypted) with his private key K B - (m)

8-22 Digital Signatures (more) r suppose Alice receives msg m, digital signature K B (m) r Alice verifies m by using Bob’s public key K B to check K B (K B (m) ) = m. r if verified, whoever signed m must have used Bob’s private key Alice thus verifies that: ü Bob signed m. ü No one else signed m. ü Bob signed m and not m’. non-repudiation: Alice can take m, and signature K B (m) to court and prove that Bob signed m or he let someone else use his private key. -

8-23 large message m H: hash function H(m) digital signature (encrypt) Bob’s private key K B - + Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message: K B (H(m)) - encrypted msg digest K B (H(m)) - encrypted msg digest large message m H: hash function H(m) digital signature (decrypt) H(m) Bob’s public key K B + equal ? Digital signature

8-24 Public Key Certification public key problem: r When Alice obtains Bob’s public key (from web site, , diskette), how does she know it is Bob’s public key, not Trudy’s? solution: r trusted certification authority (CA)

8-25 Certification Authorities r Certification Authority (CA): binds public key to a particular entity, E. r E registers its public key with CA. m E provides “proof of identity” to CA. m CA creates certificate binding E to its public key. m certificate containing E’s public key digitally signed by CA: CA says “This is E’s public key.” Bob’s public key K B + Bob’s identifying information digital signature (encrypt) CA private key K CA - K B + certificate for Bob’s public key, signed by CA - K CA (K ) B +

8-26 Certification Authorities r when Alice wants Bob’s public key: m gets Bob’s certificate (Bob or elsewhere). m apply CA’s public key to Bob’s certificate, get Bob’s public key Bob’s public key K B + digital signature (decrypt) CA public key K CA + K B + - K (K ) B + Chain of authorities, root certificate

8-27 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity & end point authentication 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

8-28 Authentication: challenge-response Goal: avoid playback attack Nonce: number (R) used only once in a lifetime To prove that Alice is “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!

8-29 Authentication using nonce and public key crypto ? “I am Alice” R Bob computes K (R) A - “send me your public key” K A + (K (R)) = R A - K A + and knows only Alice could have the private key to encrypt R such that (K (R)) = R A - K A +

8-30 Security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) I am Alice R T K (R) - Send me your public key T K + A K (R) - Send me your public key A K + T K (m) + T m = K (K (m)) + T - Trudy gets sends m to Alice encrypted with Alice’s public key A K (m) + A m = K (K (m)) + A - R

8-31 Security hole (cont.)  Difficult to detect - Bob receives everything that Alice sends, and vice versa. (e.g., so Bob, Alice can meet later and recall conversation)  problem is that Trudy receives all messages as well  public key needs to come from a trustworthy source, such as, a public key certificate

8-32 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity & end point authentication 8.4 Securing TCP connections: SSL 8.5 Network layer security: IPsec 8.6 Securing wireless LANs 8.7 Operational security: firewalls and IDS

8-33 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity & end point authentication 8.4 Securing TCP connections: SSL 8.5 Network layer security: IPsec 8.6 Securing wireless LANs 8.7 Operational security: firewalls and IDS

8-34 Secure sockets layer r provides transport layer security to any TCP-based application m e.g., between Web browsers, servers for e-commerce (https) m two variants: SSL 3.0, TLS 1.2 r security services: m server authentication, confidentiality, integrity, client authentication (optional) TCP IP TCP enhanced with SSL TCP socket Application TCP IP TCP API SSL/TLS Application secure socket

8-35 SSL: handshake r Bob (client) establishes TCP connection to Alice (server) r Bob authenticates Alice from her CA signed certificate r Bob creates, encrypts (using Alice’s public key), sends pre-master secret to Alice (nonces not shown in messages) SSL hello certificate K A + (PMS) TCP SYN TCP SYNACK TCP ACK decrypt using K A - to get PMS create Pre-master secret (PMS)

8-36 SSL: key derivations r Master secret generated from pre-master secret, server and client nonces r Alice, Bob use shared master secret to generate 4 symmetric keys: m E B : Bob->Alice data encryption key m E A : Alice->Bob data encryption key m M B : Bob->Alice MAC key m M A : Alice->Bob MAC key r encryption and MAC algorithms negotiable between Bob, Alice

Actually two “finished” messages (encrypted), before transfer of application data, to detect any tampering of handshake messages : 1.client sends a MAC of all handshake messages 2.server sends a MAC of all handshake messages More detail

8-38 SSL record protocol r SSL record protocol provides transport service to SSL Handshake protocol, application data, and other protocols (e.g., Alert protocol for ending session) m Type is for demultiplexing r Application data undergo m Fragmentation into blocks m Compression ( Optionally ) m Computing MAC ( Message Authentication Code ) using entire record + MAC key + value of sequence counter m Encryption Type, Version, and Length fields are left in the clear

8-39 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity & end point authentication 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

8: Network Security SSL (8/09) 8-40 What is confidentiality at the network-layer? Between two network entities: r Sending entity encrypts the payloads of datagrams. Payload could be: m TCP segment, UDP segment, ICMP message, OSPF message, and so on. r All data sent from one network entity to the other would be hidden: m Web pages, , P2P file transfers, TCP SYN packets, and so on. m That is, “blanket coverage”.

8: Network Security SSL (8/09) 8-41 IP header IPsec header Secure payload IP header IPsec header Secure payload IP header IPsec header Secure payload IP header payload IP header payload headquarters branch office salesperson in hotel Public Internet laptop w/ IPsec Router w/ IPv4 and IPsec Router w/ IPv4 and IPsec Virtual Private Network (VPN)

8: Network Security SSL (8/09) 8-42 IPsec services r Origin authentication r Data integrity r Replay attack prevention r Confidentiality r Two protocols providing different service models: m authentication header (AH) protocol m encapsulation security payload (ESP) protocol

8: Network Security SSL (8/09) 8-43 IPsec Transport Mode r IPsec datagrams emitted and received by two end systems. r Protects upper level protocols of these end systems IPsec

8: Network Security SSL (8/09) 8-44 IPsec – tunneling mode (1) r End routers are IPsec aware. Hosts need not be. IPsec

8: Network Security SSL (8/09) 8-45 IPsec – tunneling mode (2) r Also tunneling mode. IPsec

8: Network Security SSL (8/09) 8-46 Two protocols r Authentication Header (AH) protocol m provides source authentication & data integrity but not confidentiality m 51 in protocol field of IP header r Encapsulation Security Protocol (ESP) m provides source authentication, data integrity, and confidentiality m 50 in protocol field of IP header

8: Network Security SSL (8/09) 8-47 Four combinations are possible Transport mode with AH Transport mode with ESP Tunnel mode with AH Tunnel mode with ESP Most common and most important

8: Network Security SSL (8/09) 8-48 Security association r Before sending data, a logical connection is established from sending entity to receiving entity, called security association (SA) m SAs are unidirectional r Both sending and receiving entities maintain state information for the SA m Recall that TCP endpoints also maintain state information. m IP is connectionless; IPsec is connection-oriented r How many SAs in VPN for headquarters, branch office, and n traveling salesperson?

8: Network Security SSL (8/09) / /24 SA Internet Headquarters Branch Office R1 R2 Example SA from R1 to R2 R1 stores SA state info r 32-bit identifier for SA: Security Parameter Index (SPI) r the origin interface of the SA ( ) r destination interface of the SA ( ) r type of encryption to be used (for example, 3DES with CBC) r encryption key r type of integrity check (for example, HMAC with MD5) r authentication key

8: Network Security SSL (8/09) 8-50 Security Association Database (SAD) r Endpoint holds state of its SAs in a SAD r With n salespersons, 2 + 2n SAs in R1’s SAD r When sending IPsec datagram, R1 accesses SAD to determine how to process datagram. r When IPsec datagram arrives to R2, R2 examines SPI in IPsec datagram, indexes SAD with SPI, and processes datagram accordingly.

8: Network Security SSL (8/09) 8-51 IPsec datagram – tunnel mode, ESP / /24 SA Internet Headquarters Branch Office R1 R2 new IP header ESP hdr original IP hdr Original IP datagram payload ESP trl ESP MAC encrypted “enchilada” authenticated padding pad length next header SPI Seq #

8: Network Security SSL (8/09) 8-52 Inside the enchilada: r ESP header: m SPI, so receiving entity knows what to do m Sequence number, to thwart replay attacks r ESP trailer: Padding for block ciphers m next header identifies protocol in payload field r Use shared keys to encrypt and create ESP MAC field new IP header ESP hdr original IP hdr Original IP datagram payload ESP trl ESP MAC encrypted “enchilada” authenticated padding pad length next header SPI Seq #

8: Network Security SSL (8/09) 8-53 Summary: IPsec services r Suppose Trudy sits somewhere between R1 and R2. She doesn’t know the keys. m Will Trudy be able to see contents of original datagram? How about source, destination IP addresses, transport protocol, application port? m Flip bits without detection? m Masquerade as R1 using R1’s IP address? m Replay a datagram?

8: Network Security SSL (8/09) 8-54 Athentication methods r PSK (pre-shared key) r PKI (public key infrastructure) m pubic/private keys and certificates

8: Network Security SSL (8/09) 8-55 IKE: Key Management in IPsec r In previous examples, we manually established IPsec SAs in IPsec endpoints: Example SA SPI: Source IP: Dest IP: Protocol: ESP Encryption algorithm: 3DES-cbc HMAC algorithm: MD5 Encryption key: 0x7aeaca… HMAC key: 0xc0291f … r Such manual keying is impractical for large VPN with, say, hundreds of people. r Instead use IPsec IKE (Internet Key Exchange)

8: Network Security SSL (8/09) 8-56 Two Phases of IKE r Phase I: Establish bi-directional IKE SA m IKE SA is different from IPsec SA, also called ISAKMP security association m Each endpoint has private and public keys m Use Diffie-Hellman algorithm to establish a shared secret (Mutual authentication is not done in phase I)

8: Network Security SSL (8/09) 8-57 Diffie-Hellman algorithm r Publicly known large prime number p and number g that is prime root mod p  Alice and Bob independently choose private keys,  and , respectively and exchange their public keys  public key of Alice is g  mod p  public key of Bob is g  mod p r Shared secret known only to Alice and Bob is g  mod p = (g  mod p)  mod p = (g  mod p)  mod p r Note that in phase I, public keys of Alice and Bob have not been authenticated

8: Network Security SSL (8/09) 8-58 Two Phases of IKE (cont.) r Phase I: Establish bi-directional IKE SA r Phase II: The endpoints reveal their identities to each other and mutually authenticate using public key certificates m Identities are not revealed to passive sniffers since messages are sent over encrypted IKE SA channel between the endpoints

8: Network Security SSL (8/09) 8-59 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity & end point authentication 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

8: Network Security SSL (8/09) 8-60 IEEE security r first attempt at Wi Fi security: Wired Equivalent Privacy (WEP) m Many weaknesses m Pre-shared key only for authentication r Most recent standard i (WPA2)

8: Network Security SSL (8/09) 8-61 WEP weaknesses r RC4 stream encryption, based on XOR of plaintext and key, is subject to “known plaintext attack” m Append 24-bit initialization vector (IV) to 40- bit (or 104-bit) secret to generate key for each frame m There are only 2 24 unique keys m 24-bit IV, one per frame, is transmitted in plaintext -> reuse occurs quickly r other weaknesses, e.g., no message integrity protection

8: Network Security SSL (8/09) i (WPA2) r Provides AES block encryption and message integrity r Allows key distribution in addition to pre- shared key m use of an authentication server separate from access point, in addition to local authentication by access point

8: Network Security SSL (8/09) 8-63 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)

8: Network Security SSL (8/09) 8-64 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 serves as a relay 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 i: four phases of operation

8: Network Security SSL (8/09) 8-65 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity & end point authentication 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

8: Network Security SSL (8/09) 8-66 Firewalls isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others (i.e., access control) firewall

8: Network Security SSL (8/09) 8-67 Three types of firewalls r stateless packet filters r stateful packet filters r application gateways

8: Network Security SSL (8/09) 8-68 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?

8: Network Security SSL (8/09) 8-69 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.

8: Network Security SSL (8/09) 8-70 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 except those with destination IP , 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 ). Prevent your network from being tracerouted Drop all outgoing ICMP TTL expired traffic Stateless packet filtering: more examples

8: Network Security SSL (8/09) 8-71 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

8: Network Security SSL (8/09) 8-72 Stateful packet filtering r stateless packet filter: memoryless 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 “make sense” m timeout inactive connections at firewall: no longer admit packets

8: Network Security SSL (8/09) 8-73 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

8: Network Security SSL (8/09) 8-74 Application gateways r Make policy decisions based on application or application data r Example: allow select internal users to telnet outside. r common examples: mail server, web cache 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 filters all telnet connections not originating from gateway.

8: Network Security SSL (8/09) 8-75 Limitations of firewalls and gateways r If multiple app’s need special treatment, each needs its own gateway r client software must know how to contact gateway. m e.g., must set IP address of proxy in Web browser r IP spoofing: router can’t know if data “really” come from claimed source 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

8: Network Security SSL (8/09) 8-76 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 viruses, attack strings) m examine correlation among multiple packets to detect port scanning network mapping DoS attack

8: Network Security SSL (8/09) 8-77 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

8: Network Security SSL (8/09) 8-78 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

8: Network Security SSL (8/09) 8-79 End of Chapter 8

8: Network Security SSL (8/09) 8-80 Unused slides

8: Network Security SSL (8/09) 8-81 Chapter 8: Network Security Chapter goals: r understand principles of network security: m cryptography and its many uses beyond “confidentiality” m message integrity m authentication r security in practice: m security in application, transport, network, link layers m firewalls and intrusion detection systems

8: Network Security SSL (8/09) 8-82 A certificate contains: r Serial number (unique to issuer) r info about certificate owner, including algorithm and key value itself (not shown) r info about certificate issuer r valid dates r digital signature by issuer

8: Network Security SSL (8/09) 8-83 IPsec datagram Focus for now on tunnel mode with ESP new IP header ESP hdr original IP hdr Original IP datagram payload ESP trl ESP MAC encrypted “enchilada” authenticated padding pad length next header SPI Seq #

8: Network Security SSL (8/09) 8-84 Firewall: objectives 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

8: Network Security SSL (8/09) 8-85 IEEE security r War-driving: drove around Bay area with laptop and card [Shipley 2001] m more than 9000 accessible from public roadways m 85% use no encryption/authentication m packet-sniffing and various attacks easy r Securing m encryption, authentication m first attempt at Wi Fi security: Wired Equivalent Privacy (WEP): has weaknesses m recent standard i (WPA2)

8: Network Security SSL (8/09) 8-86 Wired Equivalent Privacy (WEP): r authentication as in protocol ap4.0 m host requests authentication from access point m access point sends 128 bit nonce m host encrypts nonce using shared symmetric key m access point decrypts nonce, authenticates host r no key distribution mechanism r authentication: knowing the shared key is enough

8: Network Security SSL (8/09) 8-87 WEP data encryption r Host/AP share 40 bit symmetric key (semi- permanent) r Host appends 24-bit initialization vector (IV) to create 64-bit key—a new IV for each frame r 64 bit key used to generate (by RC4) a stream of keys, k i (IV) r k i (IV) used to encrypt ith byte, d i, in frame: c i = d i XOR k i (IV) r Each frame contains its IV (in the clear) and encrypted data

8: Network Security SSL (8/09) WEP encryption Sender-side WEP encryption

8: Network Security SSL (8/09) 8-89 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 r plus other weaknesses (e.g., no message integrity protection)

8: Network Security SSL (8/09) 8-90 R1 converts original datagram into IPsec datagram r Appends to back of original datagram (which includes original header fields) an “ESP trailer” field. r Encrypts result using algorithm & key specified by SA. r Appends to front of this encrypted quantity the “ESP header, creating “enchilada”. r Creates authentication MAC over the whole enchilada, using algorithm and key specified in SA; r Appends MAC to back of enchilada, forming payload; r Creates brand new IP header, with all the classic IPv4 header fields, which it appends before payload.

8: Network Security SSL (8/09) 8-91 IPsec sequence numbers r For new SA, sender initializes seq. # to 0 r Each time datagram is sent on SA: m Sender increments seq # counter m Places value in seq # field r Goal: m Prevent attacker from sniffing and replaying a packet Receipt of duplicate, authenticated IP packets may disrupt service r Method: m Destination checks for duplicates m But doesn’t keep track of ALL received packets; instead uses a window

8: Network Security SSL (8/09) 8-92 Security Policy Database (SPD) r Policy: For a given datagram, sending entity needs to know if it should use IPsec. r Needs also to know which SA to use m May use: source and destination IP address; protocol number. r Info in SPD indicates “what” to do with arriving datagram; r Info in the SAD indicates “how” to do it.

8: Network Security SSL (8/09) IKE: PSK and PKI r Authentication (proof who you are) with either m pre-shared secret (PSK) or m with PKI (pubic/private keys and certificates). r With PSK, both sides start with secret: m then run IKE to authenticate each other and to generate IPsec SAs (one in each direction), including encryption and authentication keys r With PKI, both sides start with public/private key pair and certificate. m run IKE to authenticate each other and obtain IPsec SAs (one in each direction). m Similar with handshake in SSL.

8: Network Security SSL (8/09) 8-94 Symmetric key crypto DES: Data Encryption Standard r U.S. encryption standard [NIST 1993] r 64 bit plaintext input, 56-bit symmetric key m cipher-block chaining for sequence of 64-bit data units XOR encrypted nth data unit with n+1 st data unit; the result is then encrypted m Triple-DES (3DES) use three keys sequentially -- output of one key operation as input of next key operation Other symmetric key systems: RC4, IDEA, …, AES—recent (Nov. 2001) NIST standard to succeed DES, 128-bit data, 128, 192, or 256 bit keys

8: Network Security SSL (8/09) 8-95 Message integrity & authenticity Bob receives msg from Alice, wants to ensure: r message originally came from Alice r message not changed since sent by Alice Cryptographic Hash: r takes input m, produces fixed length value, H(m), called message digest m e.g., as in Internet checksum r computationally infeasible to find two different messages, x, y such that H(x) = H(y) m Equivalently: given x = H(m), cannot find m’ such that H(m’)=x m note: Internet checksum fails this requirement

8: Network Security SSL (8/09) 8-96 IPsec: Security Association r For both AH and ESP, source, destination handshake: m create network-layer unidirectional logical channel called a security association (SA) m source and dest share a symmetric key Internet Key Exchange protocol (RFC 2409) allows the use of signatures, public keys, or a pre-shared key r Uniquely determined by: m security protocol (AH or ESP) m source IP address m 32-bit connection ID (Security Parameter Index)

8: Network Security SSL (8/09) 8-97 Authentication Header (AH) Protocol r provides source authentication, data integrity, no confidentiality r AH header inserted between IP header, data field. r IP header protocol field: 51 r intermediate routers process datagrams as usual AH header includes: r connection identifier r authentication data: MAC calculated over original IP datagram, AH header fields and symmetric key (except IP TTL field) r next header field: specifies type of data (e.g., TCP, UDP, ICMP) r sequence number IP headerdata (e.g., TCP, UDP segment) AH header

8: Network Security SSL (8/09) 8-98 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 in IP header. IP header TCP/UDP segment ESP header ESP trailer ESP authent. encrypted authenticated

8: Network Security SSL (8/09) 8-99 Many more details r We have decribed the IPsec transport mode for security association between hosts for IPv4 r IPsec tunnel model is for security association between routers r IPsec for IPv6

8: Network Security SSL (8/09) Symmetric key cryptography substitution cipher: substituting one thing for another m monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc E.g.: Q: How hard to break this simple cipher?:  brute force (how hard?)  other?

8: Network Security SSL (8/09) Symmetric key crypto: DES DES: Data Encryption Standard r US encryption standard [NIST 1993] r 56-bit symmetric key, 64-bit plaintext input r How secure is DES? m DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months m no known “backdoor” decryption approach r making DES more secure: m use three keys sequentially (3-DES) on each datum m use cipher-block chaining

8: Network Security SSL (8/09) Symmetric key crypto: DES initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation DES operation

8: Network Security SSL (8/09) AES: Advanced Encryption Standard r new (Nov. 2001) symmetric-key NIST standard, replacing DES r processes data in 128 bit blocks r 128, 192, or 256 bit keys r brute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AES

8: Network Security SSL (8/09) Block Cipher r one pass through: one input bit affects eight output bits 64-bit input T1T1 8bits 64-bit scrambler 64-bit output loop for n rounds T2T2 T3T3 T4T4 T6T6 T5T5 T7T7 T8T8 r multiple passes: each input bit afects all output bits r block ciphers: DES, 3DES, AES

8: Network Security SSL (8/09) Cipher Block Chaining r cipher block: if input block repeated, will produce same cipher text: t=1 m(1) = “HTTP/1.1” block cipher c(1) = “k329aM02” … r cipher block chaining: XOR ith input block, m(i), with previous block of cipher text, c(i-1) m c(0) transmitted to receiver in clear m what happens in “HTTP/1.1” scenario from above? + m(i) c(i) t=17 m(17) = “HTTP/1.1” block cipher c(17) = “k329aM02” block cipher c(i-1)

8: Network Security SSL (8/09) RSA: Choosing keys 1. Choose two large prime numbers p, q. (e.g., 1024 bits each) 2. Compute n = pq, z = (p-1)(q-1) 3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”). 4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ). 5. Public key is (n,e). Private key is (n,d). K B + K B -

8: Network Security SSL (8/09) RSA: Encryption, decryption 0. Given (n,e) and (n,d) as computed above 1. To encrypt bit pattern, m, compute c = m mod n e (i.e., remainder when m is divided by n) e 2. To decrypt received bit pattern, c, compute m = c mod n d (i.e., remainder when c is divided by n) d m = (m mod n) e mod n d Magic happens! c

8: Network Security SSL (8/09) RSA example: Bob chooses p=5, q=7. Then n=35, z=24. e=5 (so e, z relatively prime). d=29 (so ed-1 exactly divisible by z. letter m m e c = m mod n e l c m = c mod n d c d letter l encrypt: decrypt:

8: Network Security SSL (8/09) RSA: Why is that m = (m mod n) e mod n d (m mod n) e mod n = m mod n d ed Useful number theory result: If p,q prime and n = pq, then: x mod n = x mod n yy mod (p-1)(q-1) = m mod n ed mod (p-1)(q-1) = m mod n 1 = m (using number theory result above) (since we chose ed to be divisible by (p-1)(q-1) with remainder 1 )

8: Network Security SSL (8/09) SSL: three phases 3. Data transfer H( ). MBMB b 1 b 2 b 3 … b n d dH(d) d H( ). EBEB TCP byte stream block n bytes together compute MAC encrypt d, MAC, SSL seq. # SSL seq. # dH(d) Type Ver Len SSL record format encrypted using E B unencrypted