Network Security

1. Network Security • introduction • cryptography • authentication • key exchange • Reading: Tannenbaum, section 7.1 Ross/Kurose, Ch 7 (which is incomplete)

2. Network Security Intruder may • eavesdrop • remove, modify, and/or insert messages • read and playback messages

3. Important issues: • cryptography: secrecy of info being transmitted • authentication: proving who you are and having correspondent prove his/her/its identity

4. Security in Computer Networks User resources: • login passwords often transmitted unencrypted in TCP packets between applications (e.g., telnet, ftp) • passwords provide little protection

5. 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

6. Encryption plaintext: unencrypted message ciphertext: encrypted form of message Intruder may • intercept ciphertext transmission • intercept plaintext/ciphertext pairs • obtain encryption decryption algorithms

7. A simple encryption algorithm Substitution cipher: abcdefghijklmnopqrstuvwxyz poiuytrewqasdfghjklmnbvczx • replace each plaintext character in message with matching ciphertext character: plaintext:Charlotte, my love ciphertext:iepksgmmy, dz sgby

8. 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 trials • substitution cipher subject to decryption using observed frequency of letters • 'e' most common letter, 'the' most common word

9. 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

10. decryption done by reversing encryption steps • sender and receiver must use same key

11. 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

12. We will cover in more detail shortly

13. 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: • computing DB( EB(M) ) gives M

14. knowing encryption key does not help with decryption; decryption is a non-trivial inverse of encryption • only receiver can decrypt message Question: good encryption/decryption algorithms

15. 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

16. RSA (continued) to encrypt: • divide message into blocks, {b_i} of size j: 2^j < n • encrypt: encrypt(b_i) = b_I^e mod n to decrypt: • b_i = encrypt(b_i)^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

17. 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)

19. Further notes on RSA why does RSA work? • crucial number theory result: if p, q prime then b_i^((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

20. How to break RSA? Brute force: get B's public key • for each possible b_i in plaintext, compute b_i^e • for each observed b_i^e, we then know b_i • moral: choose size of b_i "big enough"

21. man-in-the-middle: intercept keys, spoof identity:

22. Authentication Question: how does a receiver know that remote communicating entity is who it is claimed to be?

23. Authentication Protocol (ap) • Ap 1.0 • Alice to Bob: “I am Alice” • Problem: intruder “Trudy” can also send such a message • Ap 2.0 • Authenticate source IP address is from Alice’s machine • Problem: IP Spoofing (send IP packets with a false address) • Ap 3.0: use a secret password • Alice to Bob: “I am Alice, here is my password” (e.g., telnet) • Problem: Trudy can intercept Alice’s password by sniffing packets

24. Authentication Protocol Ap 3.1:use encryption use a symmetric key known to Alice and Bob • Alice & Bob (only) know secure key for encryption/decryption 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: playback attack • Trudy can intercept Alice’s message and masquerade as Alice at a later time

25. Authentication Using Nonces Problem with ap 3.1: same password is used for all sessions Solution: use a sequence of passwords pick a "once-in-a-lifetime-only" number (nonce) for each session Ap 4.0 A to B: msg = "I am A" /* note: unencrypted message! */ B to A: once-in-a-lifetime value, n A to B: msg2 = encrypt(n) /* use symmetric keys */ 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?

26. Authentication Using Public Keys Ap 4.0 uses symmetric keys for authentication Question: can we use public keys? symmetry: DA( EA(n) ) = EA ( DA(n) ) AP 5.0 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

27. Problems with Ap 5.0 • Bob needs Alice’s public key for authentication • Trudy can impersonate as Alice to Bob • Trudy to Bob: msg = “I am Alice” • Bob to Alice: nonce n (Trudy intercepts this message) • Trudy to Bob: msg2= DT(n) • Bob to Alice: send me your public key (Trudy intercepts) • Trudy to Bob: send ET (claiming it is EA) • Bob: verify ET(DT(n)) == n and authenticates Trudy as Alice!! • Moral: Ap 5.0 is only as “secure” as public key distribution

28. Man-in-the-middle Attack • Trudy impersonates as Alice to Bob and as Bob to Alice • Alice Trudy Bob • “I am A” “I am A” • nonce n • DT(n) • send me ET • ET • nonce n • DA(n) • send me EA • EA • Bob sends data using ET, Trudy decrypts and forwards it using EA!! (Trudy transparently intercepts every message)

29. Digital Signatures Using Public Keys Goals of digital signatures: • sender cannot repudiate message never sent ("I never sent that") • receiver cannot fake a received message Suppose A wants B to "sign" a message M B sends DB(M) to A A computes if EB ( DB(M)) == M then B has signed M Question: can B plausibly deny having sent M?

30. Message Digests • Encrypting and decrypting entire messages using digital signatures is computationally expensive • Routers routinely exchange data • Does not need encryption • Needs authentication and verify that data hasn’t changed • Message digests: like a checksum • Hash function H: converts variable length string to fixed length hash • Digitally sign H(M) • Send M, EA(H(m)) • Can verify who sent the message and that it has been changed! • Property of H • Given a digest x, it is infeasible to find a message y such that H(y) = x • It is infeasible to find any two messages x and y such that H(x) = H(y)

31. Symmetric key exchange: trusted server Problem: how do distributed entities 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

33. 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 sends 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

34. 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

35. Protection against Intruders: Firewalls

36. 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

37. Secure Email • Requirements: • Secrecy • Sender authentication • Message integrity • Receiver authentication • Secrecy • Can use public keys to encrypt messages • Inefficient for long messages • Use symmetric keys • Alice generates a symmetric key K • Encrypt message M with K • Encrypt K with EB • Send K(M), EB(K) • Bob decrypts using his private key, gets K, decrypts K(M)

38. Secure Email • Authentication and Integrity (with no secrecy) • Alice applies hash function H to M (H can be MD5) • Creates a digital signature DA(H(M)) • Send M, DA(H(M)) to Bob • Putting it all together • Compute H(M), DA(H(M)) • M’= { H(M), DA(H(M)) } • Generate symmetric key K, compute K(M’) • Encrypt K as EB(K) • Send K(M’), EB(K) • Used in PGP (pretty good privacy)

39. Secure Sockets Layer (SSL) • SSL: Developed by Netscape • Provides data encryption and authentication between web server and client • SSL lies above the transport layer • Useful for Internet Commerce, secure mail access (IMAP) • Features: • SSL server authentication • Encrypted SSL session • SSL client authentication

40. Secure Socket Layer • Protocol: https instead of http • Browser -> Server: B’s SSL version and preferences • S->B: S’s SSL version, preferences, and certificate • Certificate: server’s RSA public key encrypted by CA’s private key • B: uses its list of CAs and public keys to decrypt S’s public key • B->S: generate K, encrypt K with with ES • B->S: “future messages will be encrypted”, and K(m) • S->B: “future messages will be encrypted”, and K(m) • SSL session begins…

41. SSL • SET: secure electronic transactions [Visa, Mastercard] • Designed for secure credit card payment • Includes client, merchant and merchant’s bank • Homework: read up on SET from KR 7.7.2 • Homework: get your own digital certificate • Click on “security” icon (next to “print” icon) in Netscape 4.7 • Click on “Certificates” and then on “obtain your certificate” • Send an email to yourself signed with your certificate • Also examine listed of trusted CAs built into the browser

42. 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 (PEM) • secure http: supports many authentication, encryption schemes

43. Secure Email PEM : • operates on top of SMTP • ASCII • msg authentication - MD2, MD5 • msg encryption - RSA, DES • authenticated encrypted msgs and encrypted authenticated msgs PGP (Pretty Good Privacy): secure file transfer (incl. email) • binary files

44. Security: conclusion key concerns: • encryption • authentication • key exchange also: • increasingly an important area as network connectivity increases • digital signatures, digital cash, authentication, 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