Presentation on theme: "8: Network Security8-1 Chapter 8 Network Security A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students,"— Presentation transcript:
8: Network Security8-1 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 PowerPoint 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: 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!) 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 J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach, 4 th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007.
8: Network Security8-2 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 Security8-3 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 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 Security8-4 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
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8: Network Security8-9 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 secure sender secure receiver channel data, control messages data Alice Bob Trudy
8: Network Security8-10 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 Security8-11 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 Security8-12 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 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 Security8-13 The language of cryptography symmetric key crypto: sender, receiver keys identical public-key crypto: encryption key public, decryption key secret (private) plaintext ciphertext K A encryption algorithm decryption algorithm Alice’s encryption key Bob’s decryption key K B
8: Network Security8-14 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 Security8-15 Symmetric key cryptography symmetric key crypto: Bob and Alice share know same (symmetric) key: K r e.g., key is knowing substitution pattern in mono alphabetic substitution cipher r Q: how do Bob and Alice agree on key value? plaintext ciphertext K A-B encryption algorithm decryption algorithm A-B K plaintext message, m K (m) A-B K (m) A-B m = K ( ) A-B
8: Network Security8-16 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 Security8-17 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 Security8-18 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
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8: Network Security8-22 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 Security8-23 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 Security8-24 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- Hellman76, RSA78] 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 Security8-25 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: Network Security8-26 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: Network Security8-27 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
8: Network Security8-28 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 Security8-29 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 Security8-30 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 Security8-31 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: Network Security8-32 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 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 Security8-33 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 equivalently: given m = H(x), (x unknown), can not determine x. m note: Internet checksum fails this requirement!
8: Network Security8-34 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: I O U B O B 49 4F E F 42 message ASCII format B2 C1 D2 AC I O U B O B 49 4F E F 42 message ASCII format B2 C1 D2 AC different messages but identical checksums!
8: Network Security8-35 Message Authentication Code 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: Network Security8-36 MACs in practice r MD5 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 MD5 hash is equal to x recent (2005) attacks on MD5 r SHA-1 is also used m US standard [ NIST, FIPS PUB 180-1] m 160-bit MAC
8: Network Security8-37 Digital Signatures cryptographic technique analogous to hand- written 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 Security8-38 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: Network Security8-39 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 K B to K B (m) then checks K B (K B (m) ) = m. r if K B (K B (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: Alice can take m, and signature K B (m) to court and prove that Bob signed m. -
8: Network Security8-40 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 = signed MAC
8: Network Security8-41 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: Network Security8-42 Certification Authorities r Certification Authority (CA): binds public key to 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: Network Security8-43 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 +
8: Network Security8-44 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 Security8-45 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 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 Security8-46 Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice” Failure scenario?? “I am Alice”
8: Network Security8-47 Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice” in a network, Bob can not “see” Alice, so Trudy simply declares herself to be Alice “I am Alice”
8: Network Security8-48 Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Failure scenario?? “I am Alice” Alice’s IP address
8: Network Security8-49 Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Trudy can create a packet “spoofing” Alice’s address “I am Alice” Alice’s IP address
8: Network Security8-50 Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. Failure scenario?? “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr
8: Network Security8-51 Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. playback attack: Trudy records Alice’s packet and later plays it back to Bob “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr “I’m Alice” Alice’s IP addr Alice’s password
8: Network Security8-52 Authentication: yet another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Failure scenario?? “I’m Alice” Alice’s IP addr encrypted password OK Alice’s IP addr
8: Network Security8-53 Authentication: another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. record and playback still works! “I’m Alice” Alice’s IP addr encrypted password OK Alice’s IP addr “I’m Alice” Alice’s IP addr encrypted password
8: Network Security8-54 Authentication: yet another try Goal: avoid playback attack Failures, drawbacks? Nonce: number (R) used only once –in-a-lifetime ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key “I am Alice” R K (R) A-B Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!
8: Network Security8-55 Authentication: ap5.0 ap4.0 requires shared symmetric key r can we authenticate using public key techniques? ap5.0: use nonce, public key cryptography “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, that encrypted R such that (K (R)) = R A - K A +
8: Network Security8-56 ap5.0: 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: Network Security8-57 ap5.0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Difficult to detect: Bob receives everything that Alice sends, and vice versa. (e.g., so Bob, Alice can meet one week later and recall conversation) problem is that Trudy receives all messages as well!
8: Network Security8-58 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 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 Security8-59 Secure Alice: generates random symmetric private key, K S. encrypts message with K S (for efficiency) also encrypts K S with Bob’s public key. sends both K S (m) and K B (K S ) to Bob. Alice wants to send confidential , m, to Bob. K S ( ). K B ( ) K S (m ) K B (K S ) + m KSKS KSKS KBKB + Internet K S ( ). K B ( ). - KBKB - KSKS m K S (m ) K B (K S ) +
8: Network Security8-60 Secure Bob: uses his private key to decrypt and recover K S uses K S to decrypt K S (m) to recover m Alice wants to send confidential , m, to Bob. K S ( ). K B ( ) K S (m ) K B (K S ) + m KSKS KSKS KBKB + Internet K S ( ). K B ( ). - KBKB - KSKS m K S (m ) K B (K S ) +
8: Network Security8-61 Secure (continued) Alice wants to provide sender authentication message integrity. Alice digitally signs message. sends both message (in the clear) and digital signature. H( ). K A ( ) H(m ) K A (H(m)) - m KAKA - Internet m K A ( ). + KAKA + K A (H(m)) - m H( ). H(m ) compare
8: Network Security8-62 Secure (continued) Alice wants to provide secrecy, sender authentication, message integrity. Alice uses three keys: her private key, Bob’s public key, newly created symmetric key H( ). K A ( ). - + K A (H(m)) - m KAKA - m K S ( ). K B ( ). + + K B (K S ) + KSKS KBKB + Internet KSKS
8: Network Security8-63 Pretty good privacy (PGP) r Internet 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. ---BEGIN PGP SIGNED MESSAGE--- Hash: SHA1 Bob:My husband is out of town tonight.Passionately yours, Alice ---BEGIN PGP SIGNATURE--- Version: PGP 5.0 Charset: noconv yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJ hFEvZP9t6n7G6m5Gw2 ---END PGP SIGNATURE--- A PGP signed message:
8: Network Security8-64 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 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 Security8-65 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 (shttp) r security services: m server authentication, data encryption, client authentication (optional) TCP IP TCP enhanced with SSL TCP socket Application TCP IP TCP API SSL sublayer Application SSL socket
8: Network Security8-66 SSL: three phases 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 SSL hello certificate K A + (MS) TCP SYN TCP SYNACK TCP ACK decrypt using K A - to get MS create Master Secret (MS)
8: Network Security8-67 SSL: three phases 2. Key Derivation: r Alice, Bob use shared secret (MS) to generate 4 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 r why 4 keys?
8: Network Security8-68 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
8: Network Security8-69 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 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 Security8-70 IPsec: Network Layer Security r network-layer secrecy: m 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 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 Security8-71 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 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) IP headerdata (e.g., TCP, UDP segment) AH header
8: Network Security8-72 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. IP header TCP/UDP segment ESP header ESP trailer ESP authent. encrypted authenticated
8: Network Security8-73 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 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 Security8-74 IEEE security r war-driving: drive around Bay area, see what networks available? 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 security: Wired Equivalent Privacy (WEP): a failure m current attempt: i
8: Network Security8-75 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 Security8-76 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 r 64 bit key used to generate 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 IV and encrypted bytes, c i sent in frame
8: Network Security8-78 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!
8: Network Security i: improved security r numerous (stronger) forms of encryption possible r provides key distribution r uses authentication server separate from access point
8: Network Security8-80 AP: access point AS: Authentication server wired network STA: client station 1 Discovery of security capabilities 3 STA and AS mutually authenticate, together generate Master Key (MK). AP servers as “pass through” 2 3 STA derives Pairwise Master Key (PMK) AS derives same PMK, sends to AP 4 STA, AP use PMK to derive Temporal Key (TK) used for message encryption, integrity i: four phases of operation
8: Network Security8-81 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 Security8-82 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 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 Security8-83 Firewalls isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. firewall administered network public Internet firewall
8: Network Security8-84 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 Security8-85 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 Security8-86 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 Security8-87 Policy Firewall Setting No outside Web access. Drop all outgoing packets to any IP address, port 80 No incoming TCP connections, except those for institution’s public Web server only. Drop all incoming TCP SYN packets to any IP except , port 80 Prevent Web-radios from eating up the available bandwidth. Drop all incoming UDP packets - except DNS and router broadcasts. Prevent your network from being used for a smurf DoS attack. Drop all ICMP packets going to a “broadcast” address (eg ). Prevent your network from being tracerouted Drop all outgoing ICMP TTL expired traffic Stateless packet filtering: more examples
8: Network Security8-88 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 Security8-89 Stateful packet filtering r stateless packet filter: heavy handed tool m admits packets that “make no sense,” e.g., dest port = 80, ACK bit set, even though no TCP connection established: action source address dest address protocol source port dest port flag bit allowoutside of /16 TCP80> 1023ACK r stateful packet filter: track status of every TCP connection m track connection setup (SYN), teardown (FIN): can determine whether incoming, outgoing packets “makes sense” m timeout inactive connections at firewall: no longer admit packets
8: Network Security8-90 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 Security8-91 Application gateways r filters packets on application data as well as on IP/TCP/UDP fields. r example: allow select internal users to telnet outside. host-to-gateway telnet session gateway-to-remote host telnet session application gateway router and filter 1. require all telnet users to telnet through gateway. 2. for authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections 3. router filter blocks all telnet connections not originating from gateway.
8: Network Security8-92 Limitations of firewalls and gateways r IP spoofing: router can’t know if data “really” comes from claimed source r if multiple app’s. need special treatment, each has own app. gateway. r client software must know how to contact gateway. m e.g., must set IP address of proxy in Web browser r filters often use all or nothing policy for UDP. r tradeoff: degree of communication with outside world, level of security r many highly protected sites still suffer from attacks.
8: Network Security8-93 Intrusion detection systems r packet filtering: m operates on TCP/IP headers only m no correlation check among sessions r IDS: intrusion detection system m deep packet inspection: look at packet contents (e.g., check character strings in packet against database of known virus, attack strings) m examine correlation among multiple packets port scanning network mapping DoS attack
8: Network Security8-94 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 Security8-95 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