Chapter 8 Network Security A note on the

Chapter 8 Network Security A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in Power. Point form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: q If you use these slides (e. g. , in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) q If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright 1996 -2004 J. F Kurose and K. W. Ross, All Rights Reserved Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. 8: Network Security 8 -1

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 m key distribution r security in practice: m firewalls m security in application, transport, network, link layers 8: Network Security 8 -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 8: Network Security 8 -3

What is network security? Confidentiality: only sender, intended receiver should “understand” message contents m sender encrypts message m receiver decrypts message 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 Authentication: sender, receiver want to confirm identity of each other 8: Network Security 8 -4

Friends and enemies: Alice, Bob, Trudy r well-known in network security world r Bob, Alice (lovers!) 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 8 -5

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 8 -6

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) 8: Network Security 8 -7

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: Network Security 8 -8

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 -9

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 8 -10

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 8 -11

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 8 -12

Symmetric key crypto: DES operation initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation 8: Network Security 8 -13

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 8 -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 8 -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 8 -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, Adelson algorithm 8: Network Security 8 -17

Use of Public Key crypto systems 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 8 -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: Network Security 8 -19

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 8 -20

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 8 -21

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 8 -22

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 8 -23

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 8 -24

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 8 -25

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 8 -26

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 8 -27

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 8 -28

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 8 -29

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 8 -30

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 8 -31

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Authentication 8. 4 Message 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: Network Security 8 -32

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 8 -33

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 8 -34

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 8 -35

Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length, easyto-compute digital “fingerprint” r apply hash function H to m, get fixed size message digest, H(m). large message m H: Hash Function H(m) Hash function properties: r many-to-1 r produces fixed-size msg digest (fingerprint) r given message digest x, computationally infeasible to find m such that x = H(m) 8: Network Security 8 -36

Digital signature = signed message digest 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 8 -37

Hash Function Algorithms r MD 5 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 MD 5 hash is equal to x. r SHA-1 is also used. m US standard [NIST, FIPS PUB 180 -1] m 160 -bit message digest 8: Network Security 8 -38

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: Network Security 8 -39

Trusted Intermediaries Symmetric key problem: Public key problem: r How do two entities r When Alice obtains establish shared secret key over network? Solution: r trusted key distribution center (KDC) acting as intermediary between entities 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 8 -40

Key Distribution Center (KDC) r Alice, Bob need shared symmetric key. r KDC: server shares different secret key with each registered user (many users) r Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC KA-KDC KP-KDC KB-KDC KA-KDC KX-KDC KY-KDC KB-KDC KZ-KDC 8: Network Security 8 -41

Certification Authorities r Certification authority (CA): binds public key to particular entity, E. r E (person, router) 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” 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 8 -42

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 8: Network Security 8 -43

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: Network Security 8 -44

Firewalls firewall isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. public Internet administered network Trusted Zone firewall Untrusted Zone 8: Network Security 8 -45

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) two types of firewalls: m Application gateway m packet-filtering (stateless, stateful) 8: Network Security 8 -46

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 8 -47

Stateless Packet Filtering r Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. m All incoming and 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 8 -48

Access Control List ACL: table of rules, applied top to bottom to incoming packets: (action, condition) 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 Network Security

Stateful Packet Filter 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 allow source address dest address outside of 222. 22/16 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): determine whether incoming, outgoing packets “makes sense” timeout inactive connections at firewall: no longer admit packets Network Security

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 8 -51

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 8 -52

Intrusion Detection Systems r Multiple IDSs: different types of checking at different locations firewall internal network IDS sensors Web DNS server FTP server Internet demilitarized zone 8: Network Security 8 -53

Other security technologies r Web Application Firewall r Content Filters r Network Layer Sandbox r Security Information and Event Monitoring (SIEM) r… 8: Network Security 8 -54

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: Network Security 8 -55

Attack Methodology r Bad Guys Cyber Kill-Chain m An industry approved stages of a cyberattack. Source: Lockheed Martin 8: Network Security 8 -56

Reconnaissance (Recon) r Recon, footprinting or information gathering is the first stage of attack. r It generally includes the following steps: m Determining the network range m Identifying active machines m Finding open ports and access points (Mapping) m OS fingerprinting m Fingerprinting services and identify vulnerabilities (Scanning) 8: Network Security 8 -57

Internet security threats Mapping: m Port-scanning: try to establish TCP connection to each port in sequence (see what happens) m nmap (http: //www. insecure. org/nmap/) mapper: “network exploration and security auditing” Countermeasures? 8: Network Security 8 -58

Scanning r After identifying the services, run a vulnerability scanner to identify the weaknesses 8: Network Security 8 -59

Weaponization, Delivery and Exploit r Find relevant exploit codes to attack the target 8: Network Security 8 -60

Installation, C 2 and AOO r After gaining access m The attacker can install further tools to gain more access (i. e. , privilege esclations) m Set up a command control channel (C 2) m And laterally move into other systems to gain sensitive information 8: Network Security 8 -61

Countermeasures r Keep software updated r Use malware protection r Use intrusion detection system, firewalls and special DDo. S protection tools 8: Network Security 8 -62

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 8: Network Security 8 -63

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 8 -64

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 8 -65

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 8 -66

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 8 -67

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 8 -68

Secure sockets layer (SSL) r transport layer security to any TCPbased app using SSL services. r used between Web browsers, servers for e-commerce (shttp). r security services: m m m server authentication data encryption client authentication (optional) r server authentication: m SSL-enabled browser includes public keys for trusted CAs. m Browser requests server certificate, issued by trusted CA. m Browser uses CA’s public key to extract server’s public key from certificate. r check your browser’s security menu to see its trusted CAs. 8: Network Security 8 -69

SSL and TCP/IP Application SSL TCP IP normal application TCP IP application with SSL r SSL provides application programming interface (API) to applications r C and Java SSL libraries/classes readily available 8: Network Security 8 -70

Could do something like PGP KA m . H( ) - . KA( ) - KA(H(m)) + . KS( ) m KS KS + . KB( ) + KB + Internet + KB(KS ) r but want to send byte streams & interactive data r want set of secret keys for entire connection r want certificate exchange as part of protocol: handshake phase 8: Network Security 8 -71

Network Layer confidentiality between two network entities: r sending entity encrypts datagram payload, payload could be: m TCP or UDP segment, ICMP message, OSPF message …. r all data sent from one entity to other would be hidden: m web pages, e-mail, P 2 P file transfers, TCP SYN packets … r “blanket coverage” 8: Network Security 8 -72

Virtual Private Networks (VPN) motivation: r institutions often want private networks for security. • costly: separate routers, links, DNS infrastructure. r VPN: institution’s inter-office traffic is sent over public Internet instead m encrypted before entering public Internet m logically separate from other traffic 8: Network Security 8 -73

Virtual Private Network (VPN)… IPs r he ec ader IP heade ad ylo laptop w/ IPsec Secure payload router w/ IPv 4 and IPsec IP er ad he pa IPsec header salesperson in hotel e cur Se load pay router w/ IPv 4 and IPsec IP header sec IP der ea IP r h e ad he Secur e payloa d public Internet ad ylo he IP ad er pa headquarters branch office 8: Network Security 8 -74

IPsec modes IPsec r Tunnel Mode IPsec § Host Mode 8: Network Security 8 -75

Two IPsec Protocols r Authentication Header (AH) protocol m provides source authentication & data integrity but not confidentiality r Encapsulation Security Protocol (ESP) m provides source authentication, data integrity, and confidentiality m more widely used than AH 8: Network Security 8 -76

Four Combinations Host mode with AH Host mode with ESP Tunnel mode with AH Tunnel mode with ESP most common and most important 8: Network Security 8 -77

Network Security (summary) Basic techniques…. . . m cryptography (symmetric and public) m authentication m message integrity m key distribution …. used in many different security scenarios m secure email m secure transport (SSL) m IP sec m 802. 11 8: Network Security 8 -78

Appendix 8: Network Security 8 -79

Key Distribution Center (KDC) Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other? KDC generates R 1 KA-KDC(A, B) Alice knows R 1 KA-KDC(R 1, KB-KDC(A, R 1) ) KB-KDC(A, R 1) Bob knows to use R 1 to communicate with Alice and Bob communicate: using R 1 as session key for shared symmetric encryption 8: Network Security 8 -80

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 digital signature (decrypt) CA public key Bob’s public + key KB + K CA 8: Network Security 8 -81

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 D 2 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 D 2 42 B 2 C 1 D 2 AC different messages but identical checksums! 8: Network Security 8 -82

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). + KB - KB 8: Network Security 8 -83

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 8 -84

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 8 -85

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 8 -86

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 8 -87

Internet security threats Mapping: countermeasures m record traffic entering network m look for suspicious activity (IP addresses, pots being scanned sequentially) m Many SIEM tools identify these types of threats 8: Network Security 8 -88

Internet security threats Packet sniffing: m broadcast media m promiscuous NIC reads all packets passing by m can read all unencrypted data (e. g. passwords) m e. g. : C sniffs B’s packets C A src: B dest: A payload B Countermeasures? 8: Network Security 8 -89

Internet security threats Packet sniffing: countermeasures m all hosts in organization run software that checks periodically if host interface in promiscuous mode. m one host per segment of broadcast media (switched Ethernet at hub) C A src: B dest: A payload B 8: Network Security 8 -90

Internet security threats IP Spoofing: m can generate “raw” IP packets directly from application, putting any value into IP source address field m receiver can’t tell if source is spoofed m e. g. : C pretends to be B C A src: B dest: A Countermeasures? payload B 8: Network Security 8 -91

Internet security threats IP Spoofing: ingress filtering m routers should not forward outgoing packets with invalid source addresses (e. g. , datagram source address not in router’s network) m great, but ingress filtering can not be mandated for all networks C A src: B dest: A payload B 8: Network Security 8 -92

Internet security threats Denial of service (DOS): m flood of maliciously generated packets “swamp” receiver m Distributed DOS (DDOS): multiple coordinated sources swamp receiver m e. g. , C and remote host SYN-attack A C A SYN SYN SYN B Countermeasures? SYN 8: Network Security 8 -93

Internet security threats Denial of service (DOS): countermeasures m filter out flooded packets (e. g. , SYN) before reaching host: throw out good with bad m traceback to source of floods (most likely an innocent, compromised machine) C A SYN SYN SYN B SYN 8: Network Security 8 -94

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 principle 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 8 -95

Authentication Header (AH) Protocol r provides source authentication, data integrity, no confidentiality r AH header inserted between IP header, data field. r protocol field: 51 r intermediate routers process datagrams as usual IP header AH header includes: r connection identifier r authentication data: source- signed message digest calculated over original IP datagram. r next header field: specifies type of data (e. g. , TCP, UDP, ICMP) data (e. g. , TCP, UDP segment) 8: Network Security 8 -96

ESP Protocol r provides secrecy, host r ESP authentication, data field is similar to AH integrity. authentication field. r data, ESP trailer encrypted. r Protocol = 50. r next header field is in ESP trailer. authenticated encrypted IP header ESP TCP/UDP segment header trailer ESP authent. 8: Network Security 8 -97