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Protocol Examples: Key Establishment Anonymity

18739A: Foundations of Security and Privacy. Protocol Examples: Key Establishment Anonymity. Dilsun Kaynar (Substituting for Anupam Datta) CMU, Fall 2009. Outline. Just Fast Keying (JFK) Shared secret creation Mutual authentication with identity protection

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Protocol Examples: Key Establishment Anonymity

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  1. 18739A: Foundations of Security and Privacy Protocol Examples: Key Establishment Anonymity Dilsun Kaynar (Substituting for Anupam Datta) CMU, Fall 2009

  2. Outline Just Fast Keying (JFK) Shared secret creation Mutual authentication with identity protection Protection against DoS Protocols for anonymous communication High-latency Chaum Mixes as a building block Low-latency Onion Routing and Tor Hidden location servers

  3. Part I: Jast Fast Keying (JFK) Protocol

  4. JFK in this course Just Fast Keying (JFK) protocol State-of-the-art key establishment protocol [Aiello, Bellovin, Blaze, Canetti, Ioannidis, Keromytis, Reingold CCS 2002] “Rational derivation” of the JFK protocol Combine known techniques for shared secret creation, authentication, identity and anti-DoS protection [Datta, Mitchell, Pavlovic Tech report 2002] Modeling JFK in applied pi calculus Specification of security properties as equivalences [Abadi,Fournet POPL 2001] [Abadi, Blanchet, Fournet ESOP 2004] Later lecture

  5. Design Objectives for Key Exchange Shared secret Create and agree on a secret which is known only to protocol participants Authentication Participants need to verify each other’s identity Identity protection Eavesdropper should not be able to infer participants’ identities by observing protocol execution Protection against denial of service Malicious participant should not be able to exploit the protocol to cause the other party to waste resources

  6. Ingredient 1: Diffie-Hellman A  B: ga B  A: gb Shared secret: gab Diffie-Hellman guarantees perfect forward secrecy Authentication Identity protection DoS protection

  7. Ingredient 2: Challenge-Response A  B: m, A B  A: n, sigB{m, n, A} A  B: sigA{m, n, B} Shared secret Authentication A receives his own number m signed by B’s private key and deduces that B is on the other end; similar for B Identity protection DoS protection

  8. DH + Challenge-Response ISO 9798-3 protocol: A  B: ga, A B  A: gb, sigB{ga, gb, A} A  B: sigA{ga, gb, B} Shared secret: gab Authentication Identity protection DoS protection m := ga n := gb

  9. Ingredient 3: Encryption Encrypt signatures to protect identities: A  B: ga, A B  A: gb, EK{sigB{ga, gb, A}} A  B: EK{sigA{ga, gb, B}} Shared secret: gab Authentication Identity protection (for responder only!) DoS protection

  10. Refresher: Anti-DoS Cookie Typical protocol: Client sends request (message #1) to server Server sets up connection, responds with message #2 Client may complete session or not (potential DoS) Cookie version: Client sends request to server Server sends hashed connection data back Send message #2 later, after client confirms Client confirms by returning hashed data Need extra step to send postponed message

  11. Ingredient 4: Anti-DoS Cookie “Almost-JFK” protocol: A  B: ga, A B  A: gb, hashKb{gb, ga} A  B: ga, gb, hashKb{gb, ga} EK{sigA{ga, gb, B}} B  A: gb, EK{sigB{ga, gb, A}} Shared secret: gab Authentication Identity protection DoS protection? Doesn’t quite work: B must remember his DH exponential b for every connection

  12. Additional Features of JFK Keep ga, gb values medium-term, use (ga,nonce) Use same Diffie-Hellman value for every connection (helps against DoS), update every 10 minutes or so Nonce guarantees freshness More efficient, because computing ga, gb, gab is costly Two variants: JFKr and JFKi JFKr protects identity of responder against active attacks and of initiator against passive attacks JFKi protects only initiator’s identity from active attack Responder may keep an authorization list May reject connection after learning initiator’s identity

  13. JFKr Protocol [Aiello et al.] If initiator knows group g in advance xi=gdi Ni, xi R I xr=gdr DH group tr=hashKr(xr,Nr,Ni,IPi) Same dr for every connection Ni, Nr, xr, gr, tr xidr=xrdi=x Ka,e,v=hashx(Ni,Nr,{a,e,v}) derive a set of keys from shared secret and nonces Ni, Nr, xi, xr, tr, ei, hi ei=encKe(IDi,ID’r,sai,sigKi(Nr,Ni,xr,xi,gr)) hi=hashKa(“i”,ei) er, hr check integrity before decrypting “hint” to responder which identity to use er=encKe(IDr,sar,sigKr(xr,Nr,xi,Ni)) hr=hashKa(“r”,er) real identity of the responder

  14. Part II: Protocols for Anonymous Communication 18739A: Foundations of Security and Privacy

  15. Privacy on Public Networks Internet is designed as a public network Machines on your LAN may see your traffic, network routers see all traffic that passes through them Routing information is public IP packet headers identify source and destination Even a passive observer can easily figure out who is talking to whom Encryption does not hide identities Encryption hides payload, but not routing information Even IP-level encryption (tunnel-mode IPSec/ESP) reveals IP addresses of IPSec gateways

  16. Applications of Anonymity (I) Privacy Hide online transactions, Web browsing, etc. from intrusive governments, marketers and archivists Untraceable electronic mail Corporate whistle-blowers Political dissidents Socially sensitive communications (online AA meeting) Confidential business negotiations Law enforcement and intelligence Sting operations and honeypots Secret communications on a public network

  17. Applications of Anonymity (II) Digital cash Electronic currency with properties of paper money (online purchases unlinkable to buyer’s identity) Anonymous electronic voting Censorship-resistant publishing

  18. What is Anonymity? Anonymity is the state of being not identifiable within a set of subjects You cannot be anonymous by yourself! Hide your activities among others’ similar activities Unlinkability of action and identity For example, sender and his email are no more related after observing communication than they were before Unobservability (hard to achieve) Any item of interest (message, event, action) is indistinguishable from any other item of interest

  19. Attacks on Anonymity Passive traffic analysis Infer from network traffic who is talking to whom To hide your traffic, must carry other people’s traffic! Active traffic analysis Inject packets or put a timing signature on packet flow Compromise of network nodes Attacker may compromise some routers It is not obvious which nodes have been compromised Attacker may be passively logging traffic Better not to trust any individual router Assume that some fraction of routers is good, don’t know which

  20. Chaum’s Mix Early proposal for anonymous email David Chaum. “Untraceable electronic mail, return addresses, and digital pseudonyms”. Communications of the ACM, February 1981. Public key crypto + trusted re-mailer (Mix) Untrusted communication medium Public keys used as persistent pseudonyms Modern anonymity systems use Mix as the basic building block Before spam, people thought anonymous email was a good idea 

  21. Basic Mix Design {r1,{r0,M}pk(B),B}pk(mix) {r0,M}pk(B),B {r5,M’’}pk(B),B {r2,{r3,M’}pk(E),E}pk(mix) {r3,M’}pk(E),E {r4,{r5,M’’}pk(B),B}pk(mix) B A C E D Mix Adversary knows all senders and all receivers, but cannot link a sent message with a received message

  22. Anonymous Return Addresses A,{{r2,M’}K2}K1 {K1,A}pk(mix), {r2,M’}K2 Response MIX M includes {K1,A}pk(mix), K2 where K2 is a fresh public key {r1,{r0,M}pk(B),B}pk(mix) {r0,M}pk(B),B B MIX A Secrecy without authentication (good for an online confession service )

  23. Mix Cascade Messages are sent through a sequence of mixes Can also form an arbitrary network of mixes (“mixnet”) Some of the mixes may be controlled by attacker, but even a single good mix guarantees anonymity Pad and buffer traffic to foil correlation attacks

  24. Disadvantages of Basic Mixnets Public-key encryption and decryption at each mix are computationally expensive Basic mixnets have high latency Ok for email, not Ok for anonymous Web browsing Challenge: low-latency anonymity network Use public-key cryptography to establish a “circuit” with pairwise symmetric keys between hops on the circuit Then use symmetric decryption and re-encryption to move data messages along the established circuits Each node behaves like a mix; anonymity is preserved even if some nodes are compromised

  25. A simple idea: Basic Anonymizing Proxy Channels appear to come from proxy, not true originator Appropriate for Web connections etc.: SSL, TSL (Lower cost symmetric encryption) Example: The Anonymizer Simple, focuses lots of traffic for more anonymity Main disadvantage: Single point of failure, compromise, attack

  26. Another Idea: Randomized Routing Hide message source by routing it randomly Popular technique: Crowds, Freenet, Onion routing Routers don’t know for sure if the apparent source of a message is the true sender or another router

  27. Onion Routing [Reed, Syverson, Goldschlag ’97] R R R4 R R3 R R1 R R2 Alice R Bob • Sender chooses a random sequence of routers • Some routers are honest, some controlled by attacker • Sender controls the length of the path

  28. Route Establishment R2 R4 Alice R3 Bob R1 {M}pk(B) {B,k4}pk(R4),{ }k4 {R4,k3}pk(R3),{ }k3 {R3,k2}pk(R2),{ }k2 {R2,k1}pk(R1),{ }k1 • Routing info for each link encrypted with router’s public key • Each router learns only the identity of the next router

  29. Tor Second-generation onion routing network http://tor.eff.org Developed by Roger Dingledine, Nick Mathewson and Paul Syverson Specifically designed for low-latency anonymous Internet communications Running since October 2003 100 nodes on four continents, thousands of users “Easy-to-use” client proxy Freely available, can use it for anonymous browsing

  30. Tor Circuit Setup (1) Client proxy establish a symmetric session key and circuit with Onion Router #1

  31. Tor Circuit Setup (2) Client proxy extends the circuit by establishing a symmetric session key with Onion Router #2 Tunnel through Onion Router #1

  32. Tor Circuit Setup (3) Client proxy extends the circuit by establishing a symmetric session key with Onion Router #3 Tunnel through Onion Routers #1 and #2

  33. Using a Tor Circuit Client applications connect and communicate over the established Tor circuit Datagrams are decrypted and re-encrypted at each link

  34. Tor Management Issues Many applications can share one circuit Multiple TCP streams over one anonymous connection Tor router doesn’t need root privileges Encourages people to set up their own routers More participants = better anonymity for everyone Directory servers Maintain lists of active onion routers, their locations, current public keys, etc. Control how new routers join the network “Sybil attack”: attacker creates a large number of routers Directory servers’ keys ship with Tor code

  35. Location Hidden Servers Goal: deploy a server on the Internet that anyone can connect to without knowing where it is or who runs it Accessible from anywhere Resistant to censorship Can survive full-blown DoS attack Resistant to physical attack Can’t find the physical server!

  36. Creating a Location Hidden Server Client obtains service descriptor and intro point address from directory Server gives intro points’ descriptors and addresses to service lookup directory Server creates onion routes to “introduction points”

  37. Using a Location Hidden Server If server chooses to talk to client, connect to rendezvous point Client sends address of the rendezvous point and any authorization, if needed, to server through intro point Client creates onion route to a “rendezvous point” Rendezvous point mates the circuits from client & server

  38. Deployed Anonymity Systems Free Haven project has an excellent bibliography on anonymity Linked from the reference section of course website Tor (http://tor.eff.org) Overlay circuit-based anonymity network Best for low-latency applications such as anonymous Web browsing Mixminion (http://www.mixminion.net) Network of mixes Best for high-latency applications such as anonymous email

  39. Dining Cryptographers Clever idea how to make a message public in a perfectly untraceable manner David Chaum. “The dining cryptographers problem: unconditional sender and recipient untraceability.” Journal of Cryptology, 1988. Guarantees information-theoretic anonymity for message senders This is an unusually strong form of security: defeats adversary who has unlimited computational power Impractical, requires huge amount of randomness In group of size N, need N random bits to send 1 bit

  40. Three-Person DC Protocol Three cryptographers are having dinner. Either NSA is paying for the dinner, or one of them is paying, but wishes to remain anonymous. Each diner flips a coin and shows it to his left neighbor. Every diner will see two coins: his own and his right neighbor’s Each diner announces whether the two coins are the same. If he is the payer, he lies (says the opposite). Odd number of “same”  NSA is paying; even number of “same”  one of them is paying But a non-payer cannot tell which of the other two is paying!

  41. Non-Payer’s View: Same Coins “same” “different” ? payer payer “same” “different” ? Without knowing the coin toss between the other two, non-payer cannot tell which of them is lying

  42. Non-Payer’s View: Different Coins “same” “same” ? payer payer “same” “same” ? Without knowing the coin toss between the other two, non-payer cannot tell which of them is lying

  43. Superposed Sending This idea generalizes to any group of size N For each bit of the message, every user generates 1 random bit and sends it to 1 neighbor Every user learns 2 bits (his own and his neighbor’s) Each user announces own bit XOR neighbor’s bit Sender announces own bit XOR neighbor’s bit XOR message bit XOR of all announcements = message bit Every randomly generated bit occurs in this sum twice (and is canceled by XOR), message bit occurs once

  44. DC-Based Anonymity is Impractical Requires secure pairwise channels between group members Otherwise, random bits cannot be shared Requires massive communication overhead and large amounts of randomness DC-net (a group of dining cryptographers) is robust even if some members collude Guarantees perfect anonymity for the other members

  45. Acknowledgement Part 1 of this lecture was based on slides by Anupam Datta Part 2 of this lecture was based on slides by Vitaly Shmatikov

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