Overview of Network Security

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1 Overview of Network Security from à Computer Networking: A Top Down Approach, 4 th edition. Jim Kurose, Keith Ross AddisonWesley, July Roadmap: What is network security? Principles of cryptography Message integrity End point authentication Securing Securing TCP connections: SSL Network layer security: Ipsec Securing wireless LANs Operational security: firewalls and IDS 82 1

2 Sample Attacks: Network Stack people application session transport network data link physical , Web, NFS RPC TCP IP RF Phishing attacks, usability Sendmail, FTP, NFS bugs, chosenprotocol and versionrollback attacks RPC worms, portmapper exploits SYN flooding, RIP attacks, sequence number prediction IP smurfing and other address spoofing attacks WEP attacks RF fingerprinting, DoS Only as secure as the single weakest layer or interconnection between the layers 3 Network Defenses People End uses Password managers, company policies Systems Blueprints Implementations Protocols and policies Firewalls, intrusion detection, file system encryption TLS, IPsec, access control Building blocks Cryptographic primitives RSA, DSS, SHA all defense mechanisms must work correctly and securely 4 2

3 What is network security? Confidentiality: only sender & intended receiver should see message contents sender encrypts message receiver decrypts message Authentication: sender, receiver want to confirm each other s identity Message integrity: sender, receiver want to ensure that message not altered (in transit, or afterwards) without detection Access and availability: services must be accessible and available to users 85 Friends and enemies: Alice, Bob, Eve wellknown in network security world Bob and Alice want to communicate securely Eve (adversary) may intercept, delete, add messages Alice channel data, control messages Bob data secure sender secure receiver data Eve 86 3

4 Who might Bob, Alice be? well, reallife Bobs and Alices! Web browser/server for electronic transactions (e.g., online purchases) online banking client/server DNS clients & DNS servers routers exchanging routing table updates other examples? 87 There are bad guys out there! Q: What can Eve do? A: a lot! eavesdropping: intercept messages insertion: introduce fake messages into connection impersonation: can fake (spoof) source address in packet (or any field in packet) hijacking: take over ongoing connection by removing sender or receiver, inserting itself in place denialofservice: prevent service from being used by others (e.g., DoS by overloading resources) more on this later 88 4

5 Crypto 89 The language of cryptography K A Alice s encryption key Bob s decryption K B key plaintext encryption algorithm ciphertext decryption algorithm plaintext symmetric key crypto: sender, receiver keys are the same publickey crypto: encryption key public, decryption key secret (private) and unique to each user 810 5

6 Symmetric key cryptography substitution cipher: substituting one thing for another 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? 811 Symmetric key cryptography K AB K AB plaintext message, m encryption algorithm ciphertext K (m) AB decryption algorithm plaintext m = K ( K (m) ) AB AB symmetric key crypto: Bob and Alice share know same (symmetric) key: K AB e.g., key is the permutation in the monoalphabetic cipher Q: how do Bob and Alice agree on the key? 812 6

7 Symmetric key crypto: DES DES: Data Encryption Standard US encryption standard [NIST 1993] 56bit symmetric key, 64bit plaintext input How secure is DES? DES Challenge: 56bitkeyencrypted phrase ( Strong cryptography makes the world a safer place ) decrypted (brute force) in 4 months no known backdoor decryption approach making DES more secure: use three keys sequentially (3DES) on each nblock use cipherblock chaining (hides patterns, prevents block rearrangement/deletion attacks) 813 Symmetric key crypto: DES DES operation initial permutation 16 identical rounds of function application, each using different 48 bits of key final permutation 814 7

8 AES: Advanced Encryption Standard new (Nov. 2001) symmetrickey NIST standard, replacing DES processes data in 128 bit blocks supports 128, 192, or 256 bit keys brute force decryption (trying each key) taking 1 sec on DES, takes 149 trillion years for AES 815 Block Cipher 64bit input loop for n rounds 8bits T 1 8bits 8bits 8bits 8bits 8bits 8bits 8bits T 2 T 3 T 4 T 5 T 6 T 7 T 8 one pass through: one input bit affects eight output bits 8 bits 8 bits 8 bits 8 bits 8 bits 64bit scrambler 64bit output 8 bits 8 bits multiple passes: each input bit afects all output bits block ciphers: DES, 3DES, AES 8 bits 816 8

9 Cipher Block Chaining ECB: electronic codebook: if input block repeated, will produce same cipher text: CBC: cipher block chaining: XOR ith input block, m(i), with previous block of cipher text, c(i1) c(0) = IV transmitted to receiver in clear what happens in HTTP/ 1.1 scenario from above? t=1 m(1) = HTTP/1.1 block cipher t=17 m(17) = HTTP/1.1 block cipher c(i1) m(i) block cipher c(i) c(1) c(17) = k329am02 = k329am Public key cryptography symmetric key crypto requires sender, receiver to have the same (shared) secret key Q: how to agree on key in first place (particularly if never met )? public key cryptography radically different approach [Diffie Hellman76, RSA78] sender, receiver do not preshare secret key public encryption key known to all private decryption key known only to receiver 818 9

10 Public key cryptography K B Bob s public key K Bob s private B key plaintext message, m encryption algorithm ciphertext K (m) B decryption algorithm plaintext message m = K (K (m)) B B 819 Public key encryption algorithms Requirements: need K ( ) and K ( ) such that B B K (K (m)) = m B B given public key K B, it should be impossible to compute private key K B RSA: Rivest, Shamir, Adleman algorithm

11 RSA: Choosing keys 1. Choose two large prime numbers p, q. (e.g., 1024 bits each) 2. Compute n = pq, z = (p1)(q1) 3. Choose e (with e<n) that has no common factors with z. (e, z are relatively prime ). 4. Choose d such that ed1 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 821 RSA: Encryption, decryption 0. Given (n,e) and (n,d) as computed above 1. To encrypt message m compute c = m e e mod n (i.e., remainder when m is divided by n) 2. To decrypt ciphertext c compute m = c d mod n (i.e., remainder when c d is divided by n) Magic happens! m = (m e d mod n) mod n c

12 RSA example: Bob chooses p=5, q=7. Then n=35, z=24. e=5 (so e, z relatively prime). d=29 (so ed1 divisible by z). encrypt: letter m m e c = m e mod n l decrypt: c c d m = c d mod n letter l 823 RSA: Why it works? Useful number theory result: If p,q prime and n = pq, then: y y mod (p1)(q1) x mod n = x mod n e (m mod n) d ed mod n = m mod n ed mod (p1)(q1) = m mod n (using number theory result above) 1 = m mod n = m (since we chose ed to be divisible by (p1)(q1) with remainder 1 )

13 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! 825 Data Integrity

14 Message Integrity Bob receives msg from Alice, wants to ensure: message originally came from Alice message not changed since sent by Alice Cryptographic Hash: takes input m, produces fixed length value, H(m) e.g., as in Internet checksum computationally infeasible to find two different messages, x, y such that H(x) = H(y) equivalently: given m = H(x), (x unknown), can not determine x. note: Internet checksum fails this requirement! 827 Internet checksum: poor crypto hash function Internet checksum has some properties of hash function: produces fixed length digest (16bit sum) of message is manytoone But given message with given hash value, it is easy to find another message with same hash value: message ASCII format message ASCII format I O U B O B 49 4F E F 42 I O U B O B 49 4F E F 42 B2 C1 D2 AC different messages but identical checksums! B2 C1 D2 AC

15 Message Authentication Code (message) m H(.) append H(ms) m H(ms) public Internet (shared secret) s m H(ms) H(.) H(ms) compare s (shared secret) 829 MACs in practice MD5 hash function was widely used till about 2009 (RFC 1321) computes 128bit MAC in 4step process. arbitrary 128bit string x, appears difficult to construct msg m whose MD5 hash is equal to x recent (2005) attacks on MD5 SHA1 an SHA2 are used today US standards: FIPS PUB 1801 and , 224, 256, 384, 512bit output 512, 1024 input block size BOTH MD5 and SHA1 ARE CONSIDERED INSECURE! 830 USE SHA2 instead! 15

16 Digital Signatures cryptographic techniques distantly analogous to handwritten signatures. signer (Bob) digitally signs document, establishing he is the document s owner/creator. verifiable, nonforgeable: verifier (Alice) can prove to someone (also herself) that Bob, and no one else (including Alice), must have signed document 831 Digital Signatures simple digital signature for message m: Bob signs m by encrypting with his private key K B, creating signed message, K B (m) Bob s message, m Dear Alice Oh, how I have missed you. I think of you all the time! (blah blah blah) Bob K Bob s private B key public key encryption algorithm K B (m) Bob s message, m, signed (encrypted) with his private key

17 Digital Signatures (more) suppose Alice receives msg m, digital signature K B (m) 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. if K B (K B (m) ) = m, whoever signed m must have used Bob s private key. Alice thus verifies that: Bob signed m. No one else signed m. Bob signed m and not m. nonrepudiation: ü Alice can take m, and signature K B (m) to court and prove that Bob signed m. 833 Digital signature = signed MAC Bob sends digitally signed message: large message m Bob s private key H: hash function K B H(m) digital signature (encrypt) encrypted msg digest K B (H(m)) Alice verifies signature and integrity of digitally signed message: large message m H: hash function H(m) Bob s public key equal? K B encrypted msg digest K B (H(m)) digital signature (decrypt) H(m)

18 Public Key Certification public key problem: When Alice obtains Bob s public key (from web site, , diskette), how does she know it is Bob s public key, not Eve s? solution: trusted certification authority (CA) 835 Certification Authorities Certification Authority (CA): binds public key to particular entity, E. E registers its public key with CA. 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 Bob s digital CA (K ) B public signature K B key K B (encrypt) Bob s identifying information CA private key K CA certificate for Bob s public key, signed by CA

19 Certification Authorities when Alice wants Bob s public key: gets Bob s certificate (Bob or elsewhere). apply CA s public key to Bob s certificate, get Bob s public key K B K CA (K ) B digital signature (decrypt) K B Bob s public key CA public key K CA 837 A certificate contains: Serial number (unique to issuer) info about certificate owner, including algorithm and key value itself (not shown) info about certificate issuer valid dates digital signature by issuer

20 Authentication 839 Authentication Goal: Bob wants Alice to prove her identity to him Protocol ap1.0: Alice says I am Alice I am Alice Failure scenario??

21 Authentication Goal: Bob wants Alice to prove her identity to him Protocol ap1.0: Alice says I am Alice I am Alice in a network, Bob can not see Alice, so Eve simply declares herself to be Alice 841 Authentication: another try Protocol ap2.0: Alice says I am Alice in an IP packet containing her source IP address Alice s IP address I am Alice Failure scenario??

22 Authentication: another try Protocol ap2.0: Alice says I am Alice in an IP packet containing her source IP address Alice s IP address I am Alice Eve can create a packet spoofing Alice s address 843 Authentication: another try Protocol ap3.0: Alice says I am Alice and sends her secret password to prove it. Alice s IP addr Alice s I m Alice password Alice s IP addr OK Failure scenario??

23 Authentication: another try Protocol ap3.0: Alice says I am Alice and sends her secret password to prove it. Alice s IP addr Alice s I m Alice password Alice s IP addr OK playback attack: Eve records Alice s packet and later plays it back to Bob Alice s IP addr Alice s I m Alice password 845 Authentication: yet another try Protocol ap3.1: 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??

24 Authentication: another try Protocol ap3.1: 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 847 Authentication: yet another try Goal: avoid playback attack Nonce: number (R) used only once inalifetime ap4.0: to prove Alice live, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key I am Alice R Failures, drawbacks? K AB (R) Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!

25 Authentication: ap5.0 ap4.0 requires shared symmetric key can we authenticate using public key techniques? ap5.0: use nonce, public key cryptography R I am Alice K A (R) send me your public key K A Bob computes K A (K (R)) = R A and knows only Alice could have the private key, that encrypted R such that K A (K (R)) = R A 849 ap5.0: security hole Man (woman) in the middle attack: Eve poses as Alice (to Bob) and as Bob (to Alice) m = K (K (m)) A A I am Alice R K (R) A Send me your public key K A K (m) A Eve gets m = K (K (m)) sends T m to T Alice encrypted with Alice s public key I am Alice R K (R) T Send me your public key K T K (m) T

26 ap5.0: security hole Man (woman) in the middle attack: Eve 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 Eve receives all messages as well! 851 Apps:

27 Secure q Alice wants to send confidential , m, to Bob. m K S. K S ( ) K S (m ) K S (m ) K S (.) m K S K B (. ) K B K B (K S ) Internet K B (K S ) K B K S K B (. ) Alice: q generates random symmetric private key, K S. q encrypts message with K S (for efficiency) q also encrypts K S with Bob s public key. q sends both K S (m) and K B (K S ) to Bob. 853 Secure q Alice wants to send confidential , m, to Bob. m K S. K S ( ) K S (m ) K S (m ) K S (.) m K S K B (. ) K B K B (K S ) Internet K B (K S ) K B K S K B (. ) Bob: q uses his private key to decrypt and recover K S q uses K S to decrypt K S (m) to recover m

28 Secure (continued) Alice wants to provide sender authentication message integrity. m K A H(. ) K A (. ) K A (H(m)) K A (H(m)) K A K A (. ) H(m ) m Internet m compare H(.) H(m ) Alice digitally signs message. sends both message (in the clear) and digital signature. 855 Secure (continued) Alice wants to provide secrecy, sender authentication, message integrity. K A. K A (H(m)) m m H(.) K A ( ) K S K S (.) K B (. ) K B Alice uses three keys: her private key, Bob s public key, newly created symmetric key K S K B (K S ) Internet

29 Pretty good privacy (PGP) Internet encryption scheme, defacto standard. uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described. provides secrecy, sender authentication, integrity. inventor, Phil Zimmerman, was target of 3year federal investigation. A PGP signed message: 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 lo8ge4vb3mqjhfevzp9t6n7g6m5gw 2 END PGP SIGNATURE 857 Apps: SSL

30 Secure sockets layer (SSL) provides transport layer security to any TCPbased application using SSL services. e.g., between Web browsers, servers for ecommerce (shttp) security services: server authentication, data encryption, client authentication (optional) TCP socket Application TCP IP Application SSL sublayer TCP IP SSL socket TCP API TCP enhanced with SSL 859 SSL: three phases 1. Handshake: Bob establishes TCP connection to Alice authenticates Alice via CA signed certificate creates, encrypts (using Alice s public key), sends master secret key to Alice nonce exchange not shown create Master Secret (MS) TCP SYN TCP SYNACK SSL hello certificate K A (MS) TCP ACK decrypt using K A to get MS

31 SSL: three phases 2. Key Derivation: Alice, Bob use shared secret (MS) to generate 4 keys: E B : Bob>Alice data encryption key E A : Alice>Bob data encryption key M B : Bob>Alice MAC key M A : Alice>Bob MAC key encryption and MAC algorithms negotiable between Bob, Alice why 4 keys? 861 SSL: three phases 3. Data transfer TCP byte stream b 1 b 2 b 3 b n block n bytes together d H(.) M B compute MAC d d H(d) H(d) E(). E B SSL seq. # encrypted, MAC, SSL seq. # SSL record format Type Ver Len d H(d) unencrypted encrypted using E B

32 Apps: IPSec 863 IPsec: Network Layer Security networklayer secrecy: sending host encrypts the data in IP datagram TCP and UDP segments; ICMP and SNMP messages. networklayer authentication destination host can authenticate source IP address two principal protocols: authentication header (AH) protocol encapsulation security payload (ESP) protocol for both AH and ESP, source, destination handshake: create networklayer logical channel called a security association (SA) each SA unidirectional. uniquely determined by: security protocol (AH or ESP) source IP address 32bit connection ID

33 Authentication Header (AH) Protocol provides source authentication, data integrity, no confidentiality AH header inserted between IP header, data field. protocol field: 51 intermediate routers process datagrams as usual AH header includes: connection identifier authentication data: source signed message digest calculated over original IP datagram. next header field: specifies type of data (e.g., TCP, UDP, ICMP) IP header AH header data (e.g., TCP, UDP segment) 865 ESP Protocol provides secrecy, host authentication, data integrity. data, ESP trailer encrypted. next header field is in ESP trailer. authenticated encrypted ESP authentication field is similar to AH authentication field. Protocol = 50. IP header ESP header TCP/UDP segment ESP trailer ESP authent

34 Wireless Security 867 IEEE security wardriving: drive around your favorite business or residential neighborhood see what networks available? 1,000s accessible from public roadways 2025% use no encryption/authentication packetsniffing and various attacks easy! Especially, traffic analysis securing encryption, authentication first attempt at security: Wired Equivalent Privacy (WEP): a failure more recent attempt: i

35 Wired Equivalent Privacy (WEP): authentication as in protocol ap4.0 host requests authentication from access point access point sends 128 bit nonce host encrypts nonce using shared symmetric key access point decrypts nonce, authenticates host no key distribution mechanism authentication: knowing the shared key is enough 869 WEP data encryption host/ap share 40 bit symmetric key (semipermanent) host appends 24bit initialization vector (IV) to create 64bit key 64 bit key used to generate stream of keys, k i IV k i IV used to encrypt ith byte, d i, in frame: c i = d i XOR k i IV IV and encrypted bytes, c i sent in frame

36 WEP encryption IV (per frame) K S: 40bit secret symmetric key plaintext frame data plus CRC k 1 IV k 2 IV key sequence generator ( for given K S, IV) k 3 IV k N IV k N1 IV k N1 IV d 1 d 2 d 3 d N CRC 1 CRC header IV WEPencrypted data plus CRC c 1 c 2 c 3 c N c N1 c N4 Figure 7.8new1: WEP protocol Senderside WEP encryption 871 Breaking WEP encryption security hole: 24bit IV, one IV per frame, > IV s eventually reused IV transmitted in plaintext > IV reuse detected attack: Eve causes Alice to encrypt known plaintext d 1 d 2 d 3 d 4 Eve sees: c i = d i XOR k i IV Eve knows c i d i, so can compute k i IV Eve knows encrypting key sequence k 1 IV k 2 IV k 3 IV Next time IV is used, Eve can decrypt!

37 802.11i: improved security numerous (stronger) forms of encryption possible provides key distribution uses authentication server separate from access point i: four phases of operation STA: client station AP: access point wired network AS: Authentication server 1 Discovery of security capabilities 2 STA and AS mutually authenticate, together generate Master Key (MK). AP servers as pass through 3 STA derives Pairwise Master Key (PMK) 3 AS derives same PMK, sends to AP 4 STA, AP use PMK to derive Temporal Key (TK) used for message encryption, integrity

38 EAP: extensible authentication protocol EAP: endend client (mobile) to authentication server protocol EAP sent over separate links mobiletoap (EAP over LAN) AP to authentication server (RADIUS over UDP) wired network EAP over LAN (EAPoL) IEEE EAP TLS EAP RADIUS UDP/IP 875 Firewalls, IDSs

39 Firewalls firewall isolates organization s internal net from larger Internet, allowing some packets to pass, blocking others. administered network public Internet firewall 877 Firewalls: Why prevent denial of service attacks: SYN flooding: attacker establishes many bogus TCP connections, no resources left for real connections prevent illegal modification/access of internal data. e.g., attacker replaces CIA s homepage with something else allow only authorized access to inside network (set of authenticated users/hosts) three types of firewalls: stateless packet filters stateful packet filters application gateways

40 Stateless packet filtering Should arriving packet be allowed in? Departing packet let out? internal network connected to Internet via router firewall router filters packetbypacket, decision to forward/drop packet based on: source IP address, destination IP address TCP/UDP source and destination port numbers ICMP message type TCP SYN and ACK bits 879 Stateless packet filtering: example example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. all incoming, outgoing UDP flows and telnet connections are blocked. example 2: Block inbound TCP segments with ACK=0. prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside

41 Stateless packet filtering: more examples Policy No outside Web access. No incoming TCP connections, except those for institution s public Web server only. Prevent Webradios from eating up the available bandwidth. Prevent your network from being used for a smurf DoS attack. Prevent your network from being tracerouted Firewall Setting Drop all outgoing packets to any IP address, port 80 Drop all incoming TCP SYN packets to any IP except , port 80 Drop all incoming UDP packets except DNS and router broadcasts. Drop all ICMP packets going to a broadcast address (eg ). Drop all outgoing ICMP TTL expired traffic 881 Access Control Lists ACL: table of rules, applied top to bottom to incoming packets: (action, condition) pairs action source address dest address protocol source port dest port flag bit allow /16 outside of /16 TCP > any allow outside of / /16 TCP 80 > 1023 ACK allow /16 allow outside of /16 outside of / /16 UDP > UDP 53 > 1023 deny all all all all all all

42 Stateful packet filtering stateless packet filter: heavy handed tool admits packets that make no sense, e.g., dest port = 80, ACK bit set, even though no TCP connection established: action source address dest address protocol source port dest port flag bit allow outside of / /16 TCP 80 > 1023 ACK stateful packet filter: track status of every TCP connection track connection setup (SYN), teardown (FIN): can determine whether incoming, outgoing packets makes sense timeout inactive connections at firewall: no longer admit packets 883 Stateful packet filtering ACL augmented to indicate need to check connection state table before admitting packet action source address dest address proto source port dest port flag bit check conxion allow /16 allow outside of /16 outside of / /16 TCP > any TCP 80 > 1023 ACK x allow /16 outside of /16 UDP > allow outside of / /16 UDP 53 > 1023 x deny all all all all all all

43 Application gateways filters packets on application data as well as on IP/TCP/UDP fields. example: allow select internal users to telnet outside. hosttogateway telnet session application gateway gatewaytoremote 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. 885 Limitations of firewalls and gateways IP spoofing: router can t know if data really comes from claimed source if multiple app s. need special treatment, each has own app. gateway. client software must know how to contact gateway. e.g., must set IP address of proxy in Web browser filters often use all or nothing policy for UDP. tradeoff: degree of communication with outside world, level of security many highly protected sites still suffer from attacks

44 Intrusion detection systems packet filtering: operates on TCP/IP headers only no correlation check among sessions IDS: intrusion detection system deep packet inspection: look at packet contents (e.g., check character strings in packet against database of known virus, attack strings) examine correlation among multiple packets port scanning network mapping DoS attack 887 Intrusion detection systems 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

45 Network Security (summary) Basic techniques... cryptography (symmetric and public) message integrity endpoint authentication. used in many different security scenarios secure secure transport (SSL) IP sec Operational Security: firewalls and IDS

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