Class 13 Introduction to Anonymity CIS 755: Advanced Computer Security Spring 2014 Eugene Vasserman eyv/CIS755_S14

Class 13Introduction to Anonymity

CIS 755: Advanced Computer SecuritySpring 2014

Eugene Vasserman

http://www.cis.ksu.edu/~eyv/CIS755_S14/

Administrative stuff

• Quizzes and Exam II– Scheduling– Format

Anonymity Concepts

• Privacy– Confidentiality

• Anonymity/Pseudonymity– Unobservability– Unlinkability

Dining Cryptographers

• Three people toss coins: heads=1, tails=0• Menus hide right-hand coin• XOR your coin flip result and left

neighbor’s result• Report value to everyone• Report opposite value to send a single bit• If the sum is odd, someone sent a message

Dining Cryptographers II

• Slow• Error-prone• Needs tamper detection• Does not scale• Provides unobservability

Unobservability

• k-anonymity (scalable dining cryptographers)– Must be implemented very carefully

• Link padding– Inefficient– Cover traffic knowledge

Unlinkability

• Sender can’t identify receiver• Receiver can’t identify sender• Neither knows who the other is

– How do we handle authentication?

• Unobservability implies unlinkability (?)

For Bob For Bob from Alicefrom AliceFor Carol For Carol from Alicefrom AliceFor David For David from Alicefrom Alice

Onion Encryption

Message for BobWrapping for CarolWrapping for Doug

Onion Encryption IIBob

Alice

Wrapping for Edward

Edward

Doug

Carol

Chaum MixesBob

Alice

Output in lexographic order

Global AdversaryBob

Alice

Chaum Mix CascadeBob

Alice

Anonymous Reply

• Address for replies:

• Reply:

• Mix0 decrypts N,A; sends:

• Mix decrypting reply does not know destination• Mix encrypting reply does not know source

Mixminion

A

B

C

D

E Bob

A,B,C,D,E

Alice

Bob

Problems with MixMinon

• Centralized entities required– Availability failure– Anonymity failure (how?)

• Malicious nodes:– Control entry and exit– Unlikely

Anonymous Email

• High-latency• Low-throughput• Provides unlinkability

– Have to be careful about authentication

• No default end-to-end confidentiality (PGP)– Actually, there is for replies

• Secure against global adversary

Anonymous Web Browsing

• Low-latency• Medium-throughput• Server does not know client• Provides sender unlinkability

– Have to be careful about authentication

• No default end-to-end confidentiality (SSL)• NOT secure against global adversary

Tor

A

B

C

TCP over TCP (UGH!)

Anonymous Web Services

• Web service does not know client• Client does not know web service• Provides sender and receiver unlinkability

• Rendezvous

Tor Hidden Services

A

B

C

D

E

F

Questions?

Reading discussion