7: Network Security1 Chapter 7: Network security Foundations: r what is security? r cryptography r authentication r message integrity r key distribution and certification Security in practice: r application layer: secure r transport layer: Internet commerce, SSL, SET r network layer: IP security r Firewalls

7: Network Security2 Friends and enemies: Alice, Bob, Trudy r well-known in network security world r Bob, Alice (lovers!) want to communicate “securely” r Trudy, the “intruder” may intercept, delete, add messages Figure 7.1 goes here

7: Network Security3 What is network security? Secrecy: only sender, intended receiver should “understand” msg contents m sender encrypts msg m receiver decrypts msg 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

7: Network Security4 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 A B C src:B dest:A payload

7: Network Security5 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 A B C src:B dest:A payload

7: Network Security6 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 A B C SYN

7: Network Security7 The language of cryptography symmetric key crypto: sender, receiver keys identical public-key crypto: encrypt key public, decrypt key secret Figure 7.3 goes here plaintext ciphertext K A K B

7: Network Security8 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?

7: Network Security9 Perfect cipher r Definition: m Let C = E[M] m Pr[C=c] = Pr[C=c | M] r Example: one time pad m Generate random bits b 1... b n m E[M 1... M n ] = (M 1  b 1... M n  b n ) r Cons: size r Pseudo Random Generator m G(R) = b 1... b n m Indistinguishable from random (efficiently)

7: Network Security10 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

7: Network Security11 Symmetric key crypto: DES initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation DES operation

7: Network Security12 Block Cipher chaining r How do we encode a large message m Would like to guarantee integrity r Encoding: m C i = E[M i  C i-1 ] r Decoding: m M i = E[C i ]  C i-1 r Malfunctions: m Loss m Reorder/ integrity

7: Network Security13 Key Exchange r Diffie & Helman m Based on DISCRETE LOG. r Alice chooses K A and a prime p r Alice selects g (a generator) mod p r Alice sends to Bob (g, p, g K A mod p) r Bob send to Alice (g, p, g K B mod p) r The common key is m K A+B = g (K A *K B ) mod p m How is the key computed?

7: Network Security14 Exponentiation r Compute g x mod n Exp g,n (x) r Assume x = 2y + b r Let z = Exp g,n (y) r R=z 2 r If (b=1) R = g R mod n r Return R r Complexity: logarithmic in x

7: Network Security15 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 encryption key public (known to all) r decryption key private (known only to receiver)

7: Network Security16 Public key cryptography Figure 7.7 goes here

7: Network Security17 Public key encryption algorithms need d ( ) and e ( ) such that d (e (m)) = m B B B B.. need public and private keys for d ( ) and e ( ).. B B Two inter-related requirements: 1 2 RSA: Rivest, Shamir, Adelson algorithm

7: Network Security18 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 and 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).

7: Network Security19 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!

7: Network Security20 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:

7: Network Security21 RSA: Why: m = (m mod n) e mod n d Number theory results: Euler Theorem: x p-1 mod p =1 Chinese Remainder Theorem: Primes q i Eq. X mod q i =a i A unique S, S  q i, such that S mod q i =a i Consider the eq. mod either p or q (primes!) R = (m e mod p) d mod p = m ed mod p ed = k(p-1) +1 R = m mod p Chinese Remainder Theorem: unique solution

7: Network Security22 Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice” Failure scenario??

7: Network Security23 Authentication: another try Protocol ap2.0: Alice says “I am Alice” and sends her IP address along to “prove” it. Failure scenario??

7: Network Security24 Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. Failure scenario?

7: Network Security25 Authentication: yet another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Failure scenario? I am Alice encrypt(password)

7: Network Security26 Authentication: yet another try Goal: avoid playback attack Failures, drawbacks? Figure 7.11 goes here Nonce: number (R) used onlyonce in a lifetime ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key

7: Network Security27 Figure 7.12 goes here Authentication: ap5.0 ap4.0 requires shared symmetric key m problem: how do Bob, Alice agree on key m can we authenticate using public key techniques? ap5.0: use nonce, public key cryptography

7: Network Security28 Figure 7.14 goes here ap5.0: security hole ? Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Need “certified” public keys (more later …)

7: Network Security29 Figure 7.14 goes here ap5.0: security hole ? Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Need “certified” public keys (more later …)

7: Network Security30 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 verify that Bob, and no one else, signed document. r Assumption: m e B (d B (m)) = d B (e B (m)) m RSA Simple digital signature for message m: r Bob decrypts m with his public key d B, creating signed message, d B (m). r Bob sends m and d B (m) to Alice.

7: Network Security31 Digital Signatures (more) r Suppose Alice receives msg m, and digital signature d B (m) r Alice verifies m signed by Bob by applying Bob’s public key e B to d B (m) then checks e B (d B (m) ) = m. r If e B (d B (m) ) = m, whoever signed m must have used Bob’s private key. Alice thus verifies that: m Bob signed m. m No one else signed m. m Bob signed m and not m’. Non-repudiation: m Alice can take m, and signature d B (m) to court and prove that Bob signed m.

7: Network Security32 Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length,easy to compute digital signature, “fingerprint” r apply hash function H to m, get fixed size message digest, 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) r computationally infeasible to find any two messages m and m’ such that H(m) = H(m’).

7: Network Security33 Digital signature = Signed message digest Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message:

7: Network Security34 Hash Function Algorithms r Internet checksum would make a poor message digest. m Too easy to find two messages with same checksum. r MD5 hash function widely used. m Computes 128-bit message digest in 4-step process. m arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x. r SHA-1 is also used. m US standard m 160-bit message digest

7: Network Security35 Trusted Intermediaries Problem: m How do two entities establish shared secret key over network? Solution: m trusted key distribution center (KDC) acting as intermediary between entities Problem: m 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: m trusted certification authority (CA)

7: Network Security36 Key Distribution Center (KDC) r Alice,Bob need shared symmetric key. r KDC: server shares different secret key with each registered user. r Alice, Bob know own symmetric keys, K A-KDC K B-KDC, for communicating with KDC. r Alice communicates with KDC, gets session key R1, and K B-KDC (A,R1) r Alice sends Bob K B-KDC (A,R1), Bob extracts R1 r Alice, Bob now share the symmetric key R1.

7: Network Security37 Certification Authorities r Certification authority (CA) binds public key to particular entity. r Entity (person, router, etc.) can register its public key with CA. m Entity provides “proof of identity” to CA. m CA creates certificate binding entity to public key. m Certificate digitally signed by CA. r When Alice wants Bob’s public key: r gets Bob’s certificate (Bob or elsewhere). r Apply CA’s public key to Bob’s certificate, get Bob’s public key

7: Network Security38 Secure generates random symmetric private key, K S. encrypts message with K S also encrypts K S with Bob’s public key. sends both K S (m) and e B (K S ) to Bob. Alice wants to send secret message, m, to Bob.

7: Network Security39 Secure (continued) Alice wants to provide sender authentication message integrity. Alice digitally signs message. sends both message (in the clear) and digital signature.

7: Network Security40 Secure (continued) Alice wants to provide secrecy, sender authentication, message integrity. Note: Alice uses both her private key, Bob’s public key.

7: Network Security41 Pretty good privacy (PGP) r Internet encryption scheme, a 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:

7: Network Security42 Secure sockets layer (SSL) r PGP provides security for a specific network app. r SSL works at transport layer. Provides security to any TCP-based app using SSL services. r SSL: used between WWW browsers, servers for I- commerce (https). r SSL security services: m server authentication m data encryption m 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 Visit your browser’s security menu to see its trusted CAs.

7: Network Security43 SSL (continued) Encrypted SSL session: r Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server. r Using its private key, server decrypts session key. r Browser, server agree that future msgs will be encrypted. r All data sent into TCP socket (by client or server) is encrypted with session key. r SSL: basis of IETF Transport Layer Security (TLS). r SSL can be used for non- Web applications, e.g., IMAP. r Client authentication can be done with client certificates.

7: Network Security44 Secure electronic transactions (SET) r designed for payment-card transactions over Internet. r provides security services among 3 players: m customer m merchant m merchant’s bank All must have certificates. r SET specifies legal meanings of certificates. m apportionment of liabilities for transactions r Customer’s card number passed to merchant’s bank without merchant ever seeing number in plain text. m Prevents merchants from stealing, leaking payment card numbers. r Three software components: m Browser wallet m Merchant server m Acquirer gateway r See book for description of SET transaction.

7: Network Security45 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 principle 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 service agreement (SA) r Each SA unidirectional. r Uniquely determined by: m security protocol (AH or ESP) m source IP address m 32-bit connection ID

7: Network Security46 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.

7: Network Security47 Authentication Header (AH) Protocol r Provides source host authentication, data integrity, but not secrecy. r AH header inserted between IP header and IP data field. r Protocol field = 51. r Intermediate routers process datagrams as usual. AH header includes: r connection identifier r authentication data: signed message digest, calculated over original IP datagram, providing source authentication, data integrity. r Next header field: specifies type of data (TCP, UDP, ICMP, etc.)

7: Network Security48 Firewalls Two firewall types: m packet filter m application gateways To prevent denial of service attacks: m SYN flooding: attacker establishes many bogus TCP connections. Attacked host alloc’s TCP buffers for bogus connections, none left for “real” connections. To prevent illegal modification of internal data. m e.g., attacker replaces CIA’s homepage with something else To prevent intruders from obtaining secret info. isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. firewall

7: Network Security49 Packet Filtering r Internal network is connected to Internet through a router. r Router manufacturer provides options for filtering packets, based on: m source IP address m destination IP address m TCP/UDP source and destination port numbers m ICMP message type m TCP SYN and ACK bits 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.

7: Network Security50 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.

7: Network Security51 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.

7: Network Security52 Network Security (summary) Basic techniques…... r cryptography (symmetric and public) r authentication r message integrity …. used in many different security scenarios r secure r secure transport (SSL) r IP sec r Firewalls