Protocol Examples: Key Establishment Anonymity

Dilsun Kaynar(Substituting for Anupam Datta)

CMU, Fall 2009

18739A: Foundations of Security and Privacy

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

Part I: Jast Fast Keying (JFK) Protocol

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

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

Ingredient 1: Diffie-Hellman

A B: ga

B A: gb

Shared secret: gab Diffie-Hellman guarantees perfect forward secrecy

Authentication Identity protection DoS protection

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

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

Ingredient 3: EncryptionEncrypt 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

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

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 mustremember his DH exponential bfor every connection

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

JFKr Protocol [Aiello et al.]

I R

Ni, xi

xi=gdi

Ni, Nr, xr, gr, tr

xr=gdr tr=hashKr(xr,Nr,Ni,IPi)

Ni, Nr, xi, xr, tr, ei, hi

ei=encKe(IDi,ID’r,sai,sigKi(Nr,Ni,xr,xi,gr))

xidr=xr

di=x Ka,e,v=hashx(Ni,Nr,{a,e,v})

hi=hashKa(“i”,ei)

er, hr

er=encKe(IDr,sar,sigKr(xr,Nr,xi,Ni)) hr=hashKa(“r”,er)

“hint” to responder which identity to use

derive a set of keys from shared secret and nonces

real identity of the responder

check integrity before decrypting

Same dr forevery connection

DH group

If initiator knowsgroup g in advance

Part II: Protocols for Anonymous Communication

18739A: Foundations of Security and Privacy

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

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

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

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

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

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

Basic Mix Design

A

C

D

E

B

Mix

{r1,{r0,M}pk(B),B}pk(mix)

{r0,M}pk(B),B

{r2,{r3,M’}pk(E),E}pk(mix)

{r4,{r5,M’’}pk(B),B}pk(mix)

{r5,M’’}pk(B),B

{r3,M’}pk(E),E

Adversary knows all senders and

all receivers, but cannot link a sent

message with a received message

Anonymous Return Addresses

A

BMIX

{r1,{r0,M}pk(B),B}pk(mix) {r0,M}pk(B),B

M includes {K1,A}pk(mix), K2 where K2 is a fresh public key

Response MIX

{K1,A}pk(mix), {r2,M’}K2A,{{r2,M’}K2

}K1

Secrecy without authentication(good for an online confession

service )

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

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

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

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

Onion Routing

R R4

R1

R2

R

RR3

Bob

R

R

R

Sender chooses a random sequence of routers Some routers are honest, some controlled by

attacker Sender controls the length of the path

[Reed, Syverson, Goldschlag ’97]

Alice

Route Establishment

R4

R1

R2R3

BobAlice

{R2,k1}pk(R1),{ }k1

{R3,k2}pk(R2),{ }k2

{R4,k3}pk(R3),{ }k3

{B,k4}pk(R4),{ }k4

{M}pk(B)

• Routing info for each link encrypted with router’s public key• Each router learns only the identity of the next router

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

Tor Circuit Setup (1) Client proxy establish a symmetric session key

and circuit with Onion Router #1

Tor Circuit Setup (2) Client proxy extends the circuit by

establishing a symmetric session key with Onion Router #2 Tunnel through Onion Router #1

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

Using a Tor Circuit

Client applications connect and communicate over the established Tor circuit Datagrams are decrypted and re-encrypted at each link

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

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!

Creating a Location Hidden Server

Server creates onion routesto “introduction points”

Server gives intro points’descriptors and addresses to service lookup directory

Client obtains servicedescriptor and intro pointaddress from directory

Using a Location Hidden Server

Client creates onion routeto a “rendezvous point”

Client sends address of therendezvous point and anyauthorization, if needed, toserver through intro point

If server chooses to talk to client,connect to rendezvous point

Rendezvous pointmates the circuitsfrom client & server

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

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

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.

1. 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

2. Each diner announces whether the two coins are the same. If he is the payer, he lies (says the opposite).

3. 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!

?

Non-Payer’s View: Same Coins

“same”

“different”

payer payer

?

“same”

“different”

Without knowing the coin tossbetween the other two, non-

payercannot tell which of them is

lying

?

Non-Payer’s View: Different Coins

“same”

“same”

payer payer

?

“same”

“same”

Without knowing the coin tossbetween the other two, non-

payercannot tell which of them is

lying

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

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

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