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## Network Security

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**Network Security**• introduction • cryptography • authentication • key exchange • required reading: text section 7.1**Network Security**• intruder may • eavesdrop • remove, modify, and/or insert messages • read and playback messages • important issues: • cryptography: secrecy of info being transmitted • authentication: proving who you are and having correspondent prove his/her/its • identity**Security in Computer Networks**User resources: • login passwords often transmitted unencrypted in TCP packets between applications (e.g., telnet, ftp) • passwords provide little protection**Network resources:**• often completely unprotected from intruder eavesdropping, injection of false messages • mail spoofs, router updates, ICMP messages, network management messages Bottom line: • intruder attaching his/her machine (access to OS code, root privileges) onto network can override many system-provided security measures • users must take a more active role**Encryption**plaintext: unencrypted message ciphertext: encrypted form of message intruder may • intercept ciphertext transmission • intercept plaintext/ciphertext pairs • obtain encryption decryption algorithms**A simple encryption algorithm**substitution cipher: abcdefghijklmnopqrstuvwxyz poiuytrewqasdfghjklmnbvczx • replace each plaintext character inmessage with matching ciphertext character: plaintext:Charlotte, my love ciphertext:iepksgmmy, dz sgby**key is pairing between plaintext characters and ciphertext**characters • symmetric key: sender and receiver use same key • 26! (approx 10**26) different possible keys: unlikely to be broken by random trys • substitution cipher subject to decryption using observed frequency of letters • 'e' most common letter, ;the' most common word**DES: Data Encryption Standard**• encrypts data in 64-bit chunks • encryption/decryption algorithm is a published standard • everyone knows how to do it • substitution cipher over 64-bit chunks: 56-bit key determines which of 56! Substitution ciphers used • substitution: 19 stages of transformations, 16 involving functions of key**decryption done by reversing encryption steps**• sender and receiver must use same key**Key Distribution Problem**• problem: how do communicant agree on symmetric key? • N communicants implies N keys • trusted agent distribution: • keys distributed by centralized trusted agent • any communicant need only know key to communicate with trusted agent • for communication between I and j, trusted agent will provide a key**Public Key Cryptography**• separate encryption/decryption keys • receiver makes known (!) its encryption key • receiver keeps its decryption key secret • to send to receiver B, encrypt message M using B's publicly available key, EB • send EB(M) • to decrypt, B applies its private decrypt key DB to receiver message: • compute DB( EB(M) ) gives M**knowing encryption key does not help with decryption:**decryption is a non-trivial inverseof encryption • only receiver can decrypt message • question: good encryption/decryption algorithms**RSA: public key encryption/decryption**RSA: a public key algorithm for encrypting/decrypting entity wanting to receive encrypted messages: • choose two prime numbers, p, q greater than 10**100 • compute n=pq and z = (p-1)(q-1) • choose number d which has no common factors with z • compute e such that ed = 1 mod z, i.e., integer-remainder( (ed) / ((p-1)(q-1)) ) = 1, i.e., ed = k(p-1)(q-1) +1 • three numbers: • e, n made public • d kept secret**RSA (continued)**to encrypt: • divide message into i blocks, bi of size k: 2**k < n • encrypt: encrypt(bi) = bi**e mod n to decrypt: • bi = encrypt(bi)**d to break RSA • need to know p, q, given pq=n, n known • factoring 200 digit n into primes takes 4 billion years using known methods**RSA example**• choose p=3, q=11, gives n=33, (p-1)(q-1)=z=20 • choose d = 7 since 7 and 20 have no common factors • compute e = 3, so that ed = k(p-1)(q-1)+1 (note: k=1 here)**Further notes on RSA**why does RSA work? • crucial number theory result: of p, q prime then bi**((p-1)(q-1)) mod pq = 1 • using mod pq arithmetic: (b**e)**d = b**(ed) = b**(k(p-1)(q-1)+1) for some k = b b**(p-1)(q-1) b**(p-1)(q-1) ... b**(p-1)(q-1) = b 1 1 ... 1 = b Note: we can also encrypt with d and encrypt with e. • this will be useful shortly**How to break RSA?**Brute force: get B's public key • for each possible bi in plaintext, compute bi**e • for each observed bi**e, we then know bi • more: choose size of bi "big enough"**Authentication**Question: how does a receiver know that remote communicating entity is who it is claimed to be?**Approach 1: peer-peer key-based authentication**• A, B (only) know secure key for encryption/decryption • A sends encrypted msf to B and B decrypts: A to B: msg = encrypt("I am A") B computes: if decrypt(msg)=="I am A" then A is verified else A is fradulent • failure scenarios?**Authentication Using Nonces**to prove that A is alive, B sends "once-in-a-lifetime-only" number (nonce) to A, which Aencodes and returns to B A to B: msg = encrypt("I am A") B compute: if decrypt(msg)=="I am A" then A is OK so far B to A: once-in-a-lifetime value, n A to B: msg2 = encrypt(n) B computes: if decrypt(msg2)==n then A is verified else A is fradulent • note similarities to three way handshake and initial sequence number choice • problems with nonces?**Authentication Using Public Keys**B wants to authenticate A A has made its encryption key EA known A alone knows DA symmetry: DA( EA(n) ) = EA ( DA(n) ) A to B: msg = "I am A" B to A: once-in-a-lifetime value, n A to B: msg2 = DA(n) B computes: if EA (DA(n))== n then A is verified else A is fradulent**Digital Signatures Using Public Keys**Goals of digital signature: • sender can not repudiate message never sent ("I never sent that") • receiver can not fake a received message Suppose A wants B wants to "sign" a message M B sends DA(M) to A A computes if EA ( DA(M)) == M then A has signed M Question: can A plausibly deny having sent M?**Symmetric key exchange: trusted server**problem: how do distributed entitues agree on a key? assume: each entity has its own single key, which only it and trusted server know server: • will generate a one-time session key that A and B use to encrypt communication • will use A and B's single keys to communicate session key to A, B**Symmetric Key exchange: trusted server**Preceding scenario: 1. A sends encrypted msg to S, containing A, B, nonce RA: EA(A,B,RA) 2. S decrypts using DA, generates one time session key, K, sends nonce, key, and B-encrypted encoding of key to A: EA(RA,B,K,EB(K,A)) 3. A decrypts msg from S using DA and verifies nonce. Extracts K, saves it and send EB(K,A) to B. 4. B decrypts msg using DB, extracts K, generates new nonce RB, sends EK(RB) to A 5. A decrypts using K, extracts RB, computes RB-1 and encrypts using K. Sends EK(RB-1) to B 6. B decrypts using K and verifies RB-1**Public key exchange: trusted server**• public key retrieval subject to man-in-middle attack • locate all public keys in trusted server • everyone has server's encryption key (ED public) • suppose A wants to send to B using B's "public" key**Clipper Chip: technical aspects**US gov't proposed federal information processing standard (voluntary) • obviously need to encrypt many things passed over phone line • encryption technique for Clipper (skipjack algorithm) highly classified • voluntarily installed in telecommunications equipment (existing products)**call setup: A and B want to communicate**• A, B use standard public key techniques to agree on a session key • session key encrypted using clipped chips unit key • encrypted session key and unencrypted unit ID put into LEAF (Law Enforcement Access Field) which is sent • note: LEAF redundant, A and B know session K • session key transmitted so it can be intercepted! • session communication encrypted using session key**Privacy issues**Clipper I: device manufacturers split unit chip key in half: • unit chip key hardwired into tamper proof, non reverse-engineerable chip • half in escrow at NIST, half at Treasury • gov't wants to wiretap machine with known unit ID • gov't (e.g., FBI) presents court orders to both agencies, gets unit chip key • uses chip key to determine session key from LEAF • unencrypts using session key US gov't outlawed export of greater-than-40-bit key technology • Oct 96: 56 bit key technology selectively exportable for two year trial basis**firewall: network components (host/router+software) sitting**between inside ("us") and outside ("them) • packet filtering firewalls: drop packets on basis of source or destination address (i.e., IP address, port) • application gateways: application specific code intercepts, processes and/or relays application specific packets • e.g., email of telnet gateways • application gateway code can be security hardened • can log all activity**Security: Internet activity**IP layer: • authentication of header: receiver can authenticate sender using messageauthentication code (MAC) • encryption of contents: DES, RFC 1829 API • SSL - secure socket layer: support for authentication and encryption • port numbers: 443 for http with SSL, 465 for smtp with SSL Application Layer • Privacy Enhanced Mail • secure http: supports many authentication, encryption schemes**Security: conclusion**key concerns: • encyption • authentication • key exchange also: • increasingly an important area as network connectivity increases • digital signatures, digital cash, authentication, biometrics increasingly important • an important social concern • further reading: • Crypto Policy Perspectives: S. Landau et al., Aug 1994 CACM • Internet Security, R. Oppliger, CACM May 1997 • www.eff.org