Chapter 8 Network Security Chapter goals r understand

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Chapter 8: Network Security Chapter goals: r understand principles of network security: m cryptography and its many uses beyond “confidentiality” m authentication m message integrity r security in practice: m firewalls and intrusion detection systems m security in application, transport, network, link layers 8: Network Security 1

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Message integrity 8. 4 End point authentication 8. 5 Securing e-mail 8. 6 Securing TCP connections: SSL 8. 7 Network layer security: IPsec 8. 8 Securing wireless LANs 8. 9 Operational security: firewalls and IDS 8: Network Security 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 message not altered (in transit, or afterwards) without detection Access and availability: services must be accessible and available to users 8: Network Security 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, add messages Alice data channel secure sender Bob data, control messages secure receiver data Trudy 8: Network Security 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: Network Security 5

There are bad guys (and girls) out there! Q: What can a “bad guy” do? A: a lot! 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 himself in place m denial of service: prevent service from being used by others (e. g. , by overloading resources) more on this later …… 8: Network Security 6

The language of cryptography Alice’s K encryption A key plaintext encryption algorithm Bob’s K decryption B key ciphertext decryption plaintext algorithm symmetric key crypto: sender, receiver keys identical public-key crypto: encryption key public, decryption key secret (private) 8: Network Security 8

Symmetric key cryptography substitution cipher: substituting one thing for another m monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq E. g. : Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc Q: How hard to break this simple cipher? : q brute force (how hard? ) q other? 8: Network Security 9

Symmetric key cryptography KA-B plaintext message, m encryption ciphertext algorithm K (m) A-B decryption plaintext algorithm m=K A-B ( KA-B(m) ) symmetric key crypto: Bob and Alice share know same (symmetric) key: K A-B r e. g. , key is knowing substitution pattern in mono alphabetic substitution cipher r Q: how do Bob and Alice agree on key value? 8: Network Security 10

Symmetric key crypto: 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 11

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 12

Block Cipher loop for n rounds 64 -bit input 8 bits 8 bits T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 8 8 bits 8 bits r one pass through: one input bit affects eight output bits 64 -bit scrambler 64 -bit output r multiple passes: each input bit affects all output bits r block ciphers: DES, 3 DES, AES 8: Network Security 13

Cipher Block Chaining r cipher block: if input block repeated, will produce same cipher text: t=1 … t=17 m(1) = “HTTP/1. 1” block cipher c(1) m(17) = “HTTP/1. 1” block cipher c(17) = “k 329 a. M 02” r cipher block chaining: XOR ith input block, m(i), with previous block of cipher text, c(i-1) m m c(0) transmitted to receiver in clear what happens in “HTTP/1. 1” scenario from above? m(i) c(i-1) + block cipher c(i) 8: Network Security 14

Public key cryptography symmetric key crypto r requires sender, receiver know shared secret key r Q: how to agree on key in first place (particularly if never “met”)? public key cryptography r radically different approach [Diffie. Hellman 76, RSA 78] r sender, receiver do not share secret key r public encryption key known to all r private decryption key known only to receiver 8: Network Security 15

Public key cryptography + Bob’s public B key K K plaintext message, m encryption ciphertext algorithm + K (m) B - Bob’s private B key decryption plaintext algorithm message + m = K B(K (m)) B 8: Network Security 16

Public key encryption algorithms Requirements: 1 2 . . + need K B( ) and K - ( ) such that B - + K (K (m)) = m B B + given public key KB , it should be impossible to compute private key K B RSA: Rivest, Shamir, Adleman algorithm 8: Network Security 17

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

RSA: Encryption, decryption 0. Given (n, e) and (n, d) as computed above 1. To encrypt bit pattern, m, compute e mod n (i. e. , remainder when m e is divided by n) c=m 2. To decrypt received bit pattern, c, compute d m = c d mod n (i. e. , remainder when c is divided by n) Magic d m = (m e mod n) mod n happens! c 8: Network Security 19

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. encrypt: decrypt: letter m me l 12 1524832 c 17 d c 48196857210675091411825223071697 c = me mod n 17 m = cd mod n letter 12 l 8: Network Security 20

RSA: m = (m e mod n) Why is that d mod n Useful number theory result: If p, q prime and n = pq, then: y y mod (p-1)(q-1) x mod n = x mod n e (m mod n) d mod n = med mod n = m ed mod (p-1)(q-1) mod n (using number theory result above) 1 = m mod n (since we chose ed to be divisible by (p-1)(q-1) with remainder 1 ) = m 8: Network Security 21

RSA: another important property The following property will be very useful later: - + B B K (K (m)) + = m = 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: Network Security 22

Message Integrity 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) m e. g. , as in Internet checksum r computationally infeasible to find two different messages, x, y such that H(x) = H(y) m m equivalently: given m = H(x), (x unknown), can not determine x. note: Internet checksum fails this requirement! 8: Network Security 24

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 given message with given hash value, it is easy to find another message with same hash value: message IOU 1 00. 9 9 BOB ASCII format 49 4 F 55 31 30 30 2 E 39 39 42 4 F 42 B 2 C 1 D 2 AC message IOU 9 00. 1 9 BOB ASCII format 49 4 F 55 39 30 30 2 E 31 39 42 4 F 42 B 2 C 1 D 2 AC different messages but identical checksums! 8: Network Security 25

Message Authentication Code (shared secret) s H(. ) (message) m append H(. ) m H(m+s) public Internet H(m+s) m compare H(m+s) s (shared secret) 8: Network Security 26

MACs in practice r MD 5 hash function widely used (RFC 1321) m computes 128 -bit MAC in 4 -step process. m arbitrary 128 -bit string x, appears difficult to construct msg m whose MD 5 hash is equal to x r SHA-1 is also used m US standard [NIST, FIPS PUB 180 -1] m 160 -bit MAC 8: Network Security 27

Digital Signatures cryptographic technique analogous to handwritten signatures. r sender (Bob) digitally signs document, 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: Network Security 28

Digital Signatures simple digital signature for message m: r Bob “signs” m by encrypting with his private key - KB, creating “signed” message, KB(m) Bob’s message, m Dear Alice Oh, how I have missed you. I think of you all the time! …(blah) Bob K B Bob’s private key public key encryption algorithm K B(m) Bob’s message, m, signed (encrypted) with his private key 8: Network Security 29

Digital Signatures (more) - r suppose Alice receives msg m, digital signature K B(m) r Alice verifies m signed by Bob by applying Bob’s + - public key KB to KB(m) then checks KB(KB(m) ) = m. + - r if KB(KB(m) ) = m, 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: ü Alice can take m, and signature KB(m) to court and prove that Bob signed m. 8: Network Security 30

Digital signature Alice verifies signature and integrity of digitally signed message: Bob sends digitally signed message: large message m H: hash function Bob’s private key + - KB encrypted msg digest H(m) digital signature (encrypt) encrypted msg digest KB(H(m)) large message m H: hash function KB(H(m)) Bob’s public key + KB digital signature (decrypt) H(m) equal ? 8: Network Security 31

Public Key Certification public key problem: r When Alice obtains Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s? solution: r trusted certification authority (CA) 8: Network Security 32

Certification Authorities r Certification Authority (CA): binds public key to particular entity, E. r E registers its public key with CA. m m m E provides “proof of identity” to CA. CA creates certificate binding E to its public key. certificate containing E’s public key digitally signed by CA: CA says “This is E’s public key. ” - + K CA(KB ) Bob’s public key Bob’s identifying information + KB digital signature (encrypt) CA private key K- CA + KB certificate for Bob’s public key, signed by CA 8: Network Security 33

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 + KB - + K CA(KB ) digital signature (decrypt) CA public key Bob’s public + key KB + K CA 8: Network Security 34

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 35

Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap 1. 0: Alice says “I am Alice” Failure scenario? ? 8: Network Security 37

Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap 1. 0: Alice says “I am Alice” in a network, Bob can not “see” Alice, so Trudy simply declares herself to be Alice 8: Network Security 38

Authentication: another try Protocol ap 2. 0: Alice says “I am Alice” in an IP packet containing her source IP address Alice’s “I am Alice” IP address Failure scenario? ? 8: Network Security 39

Authentication: another try Protocol ap 2. 0: Alice says “I am Alice” in an IP packet containing her source IP address Alice’s IP address Trudy can create a packet “spoofing” “I am Alice” Alice’s address 8: Network Security 40

Authentication: another try Protocol ap 3. 0: Alice says “I am Alice” and sends her secret password to “prove” it. Alice’s “I’m Alice” IP addr password Alice’s IP addr OK Failure scenario? ? 8: Network Security 41

Authentication: another try Protocol ap 3. 0: Alice says “I am Alice” and sends her secret password to “prove” it. Alice’s “I’m Alice” IP addr password Alice’s IP addr OK playback attack: Trudy records Alice’s packet and later plays it back to Bob Alice’s “I’m Alice” IP addr password 8: Network Security 42

Authentication: yet another try Protocol ap 3. 1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Alice’s encrypted “I’m Alice” IP addr password Alice’s IP addr OK Failure scenario? ? 8: Network Security 43

Authentication: another try Protocol ap 3. 1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Alice’s encrypted “I’m Alice” IP addr password Alice’s IP addr OK record and playback still works! Alice’s encrypted “I’m Alice” IP addr password 8: Network Security 44

Authentication: yet another try Goal: avoid playback attack Nonce: number (R) used only once –in-a-lifetime ap 4. 0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key “I am Alice” R KA-B(R) Failures, drawbacks? Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice! 8: Network Security 45

Authentication: ap 5. 0 ap 4. 0 requires shared symmetric key r can we authenticate using public key techniques? ap 5. 0: use nonce, public key cryptography “I am Alice” R Bob computes + - - K A (R) “send me your public key” + KA KA(KA (R)) = R and knows only Alice could have the private key, that encrypted R such that + K (K (R)) = R A A 8: Network Security 46

ap 5. 0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) I am Alice R K (R) T K (R) A Send me your public key + K T Send me your public key + K A - + m = K (K (m)) A A + K (m) A Trudy gets - + m = K (K (m)) sends T to Alice m T + K (m) T encrypted with Alice’s public key 8: Network Security 47

ap 5. 0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Difficult to detect: q Bob receives everything that Alice sends, and vice versa. (e. g. , so Bob, Alice can meet one week later and recall conversation) q problem is that Trudy receives all messages as well! 8: Network Security 48

Secure e-mail q Alice wants to send confidential e-mail, m, to Bob. KS m KS K (. ) S + . K B( ) K+ B KS(m ) + + KB(KS ) . K S( ) - Internet + KB(KS ) m KS - . K B( ) KB Alice: q q generates random symmetric private key, KS. encrypts message with KS (for efficiency) also encrypts KS with Bob’s public key. sends both KS(m) and KB(KS) to Bob. 8: Network Security 50

Secure e-mail q Alice wants to send confidential e-mail, m, to Bob. KS m KS K (. ) S + . K B( ) K+ B KS(m ) + + KB(KS ) . K S( ) - Internet + KB(KS ) m KS - . K B( ) KB Bob: q uses his private key to decrypt and recover K S q uses KS to decrypt KS(m) to recover m 8: Network Security 51

Secure e-mail (continued) • Alice wants to provide sender authentication message integrity. m H(. ) KA - . K A( ) - - KA(H(m)) + + KA Internet m m + . K A( ) H(m ) compare . H( ) H(m ) • Alice digitally signs message. • sends both message (in the clear) and digital signature. 8: Network Security 52

Secure e-mail (continued) • Alice wants to provide secrecy, sender authentication, message integrity. m . H( ) KA - . K A( ) - KA(H(m)) + . K S( ) m KS KS + . K B( ) K+ B + Internet + KB(KS ) Alice uses three keys: her private key, Bob’s public key, newly created symmetric key 8: Network Security 53

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. A PGP signed message: ---BEGIN PGP SIGNED MESSAGE--Hash: SHA 1 Bob: My husband is out of town tonight. Passionately yours, Alice ---BEGIN PGP SIGNATURE--Version: PGP 5. 0 Charset: noconv yh. HJRHh. GJGhgg/12 Ep. J+lo 8 g. E 4 v. B 3 mq. Jh FEv. ZP 9 t 6 n 7 G 6 m 5 Gw 2 ---END PGP SIGNATURE--- 8: Network Security 54

Secure sockets layer (SSL) r provides transport layer security to any TCP-based application using SSL services. m e. g. , between Web browsers, servers for e-commerce (https) r security services: m server authentication, data encryption, client authentication (optional) Application TCP socket Application TCP SSL sublayer TCP IP IP TCP API SSL socket TCP enhanced with SSL 8: Network Security 56

SSL: three phases TCP SYN 1. Handshake: r Bob establishes TCP connection to Alice r authenticates Alice via CA signed certificate r creates, encrypts (using Alice’s public key), sends master secret key to Alice m nonce exchange not shown ACK P SYN TC TCP ACK SSL hello te ca certifi create Master Secret (MS) KA +(MS) decrypt using KA to get MS 8: Network Security 57

SSL: three phases 2. Key Derivation: r Alice, Bob use shared secret (MS) to generate 4 keys: m m EB: Bob->Alice data encryption key EA: Alice->Bob data encryption key MB: Bob->Alice MAC key MA: Alice->Bob MAC key r encryption and MAC algorithms negotiable between Bob, Alice r why 4 keys? 8: Network Security 58

SSL: three phases 3. Data transfer TCP byte stream block n bytes together b 1 b 2 b 3 … bn d . MB H( ) d H(d) . H( ) d SSL record format Type Ver Len SSL seq. # encrypt d, MAC, seq. # H(d) d EB compute MAC H(d) unencrypted using EB 8: Network Security 59

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Message integrity 8. 4 End point authentication 8. 5 Securing e-mail 8. 6 Securing TCP connections: SSL 8. 7 Network layer security: IPsec 8. 9 Operational security: firewalls and IDS 8: Network Security 60

IPsec: Network Layer Security r network-layer secrecy: 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 principal protocols: m authentication header (AH) protocol m encapsulation security payload (ESP) protocol m 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 8: Network Security 61

Authentication Header (AH) and Encapsulation Security Payload (ESP) Protocols AH protocol r provides source authentication, data integrity, no confidentiality r AH header inserted between IP header, data field. IP header AH header ESP Protocol: r provides source authentication, data integrity, and confidentiality r Data encrypted r More complex than AH data (e. g. , TCP, UDP segment) IP datagram with an AH header 8: Network Security 62

Key management r Tow approaches: manual and automated r Manual: system admins manually configure hosts with crypto algorithms and secret keys r Automated: crypto algorithms and keys for each SA are obtained automatically, on demand for each SA. m Internet Key Exchange protocol (IKE) m Uses public key cryptography to distribute keys 8: Network Security 63

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Message integrity 8. 4 End point authentication 8. 5 Securing e-mail 8. 6 Securing TCP connections: SSL 8. 7 Network layer security: IPsec 8. 9 Operational security: firewalls and IDS 8: Network Security 64

Firewalls firewall isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. public Internet administered network firewall 8: Network Security 65

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 8: Network Security 66

Stateless packet filtering Should arriving packet be allowed in? Departing packet let out? r internal network connected to Internet via router firewall r router filters packet-by-packet, decision to forward/drop packet based on: m m source IP address, destination IP address TCP/UDP source and destination port numbers ICMP message type TCP SYN and ACK bits 8: Network Security 67

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 68

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 Drop all incoming UDP packets - except DNS and router broadcasts. the available bandwidth. Prevent your network from being used for a smurf Do. S attack. Drop all ICMP packets going to a “broadcast” address (eg 130. 207. 255). Prevent your network from being tracerouted Drop all outgoing ICMP TTL expired traffic 8: Network Security 69

Access Control Lists r ACL: table of rules, applied top to bottom to incoming packets: (action, condition) pairs action source address dest address protocol source port dest port allow 222. 22/16 outside of 222. 22/16 TCP > 1023 80 allow outside of 222. 22/16 TCP 80 > 1023 ACK allow 222. 22/16 UDP > 1023 53 --- allow outside of 222. 22/16 UDP 53 > 1023 ---- deny all all all 222. 22/16 outside of 222. 22/16 flag bit any 8: Network Security 70

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: source address dest address outside of 222. 22/16 action allow protocol source port dest port flag bit TCP 80 > 1023 ACK r stateful packet filter: track status of every TCP connection m m track connection setup (SYN), teardown (FIN): can determine whether incoming, outgoing packets “makes sense” timeout inactive connections at firewall: no longer admit packets 8: Network Security 71

Stateful packet filtering r ACL augmented to indicate need to check connection state table before admitting packet action source address dest address proto source port dest port allow 222. 22/16 outside of 222. 22/16 TCP > 1023 80 allow outside of 222. 22/16 TCP 80 > 1023 ACK allow 222. 22/16 UDP > 1023 53 --- allow outside of 222. 22/16 deny all 222. 22/16 outside of 222. 22/16 flag bit check conxion any UDP 53 > 1023 ---- all all x x all 8: Network Security 72

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 application gateway-to-remote host telnet session 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 Security 73

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 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. e. g. , must set IP address of proxy in Web browser 8: Network Security 74

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 • Do. S attack 8: Network Security 75

Intrusion detection systems r multiple IDSs: different types of checking at different locations application gateway firewall Internet internal network IDS sensors Web server FTP server DNS server demilitarized zone 8: Network Security 76

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 email m secure transport (SSL) m IP sec m 802. 11 Operational Security: firewalls and IDS 8: Network Security 77