Anonymous Communication

Question 1

Alternative to cookies for user tracking

Answer 1

Browser fingerprinting based on features and configuration settings of the browser (language etc) that are accessible without any permission.

Question 2

Sender anonymity

Answer 2

Adversary knows receiver
adversary may learn message
sender is unknown

Question 3

receiver anonymity

Answer 3

adversary knows sender
adversary may choose message
receiver is unknown

Question 4

Sender-receiver unklinkability

Answer 4

adversary knows senders and receivers
link between senders and receivers is unknown
anonymity -> unlinkability

Question 5

unobservability

Answer 5

adversary cannot tell whether any com. is taking place
Wireless:: DSSS
Wired: always send
unobservability -> anonymity

Question 6

Batching and mixing

Answer 6

• proxy collects number of messages before forwarding and mix order of messages.
• messages need to be indistinguishable.
• use multiple proxies (mixes) to avoid single point of failure
• layered encryption

ISSUE:

• Intersection attack possible (without cover traffic)
• low performance (mainly due to batching)

Question 7

Intersection attack

Answer 7

• everytime a message is seen by the target, register sets of destinations
• narrow it down over time
  Solution: Covertraffic fopr unobservability

Question 8

Direct circuit setup

Answer 8

• establish state on relays by using normal packets
• sender knows the established keys in advance (based on long-term public keys of relays)
• -> no forward secrecy

Question 9

Telescopic circuit setup

Answer 9

• keys are negotiated one relay at a time
• ID of R2 is encrypted with ephemeral session key of R1
• slower but offers immediate forward secrecy.

Question 10

Circuit tear-down

Answer 10

• can be initiated by sender and by intermediate relays
• ## sender communicates tear-down to one relay at a time, starting from the furthest away

Question 11

Attacks on circuit based systems

Answer 11

• traffic analysis: flow fingerprinting, webseite fingerprinting
• TCP fingerprinting: analize TCP protocol implementation (sol: per hop tcp)
• trick users into downloading malware
• any gap will break anonymity

Question 12

Tor basics

Answer 12

circuits over 3 relays

• per hop tcp
• per hop tls (except last hop)
• features: cencorship resistance (bridges), exit policies, hidden services

Question 13

Tor relay_early

Answer 13

relay_early enforces maximal path length of 9 to prevent cheap DoS.

• extend cells can only be contained in relay_early cells.
• each relay allows only 8 relay_early cells per circuit

Question 14

hidden services

Answer 14

• anonymize server (Bob)
• hash of Bobs public key is identifier of hidden service
• Bob has connection so a set of introduction points
• Alice connects to IP and suggests rendezvous
• traffic never leaves tor network

Question 15

directory authorities

Answer 15

• 10 directory authorities (DA) run consensus to keep state of relays
• weakness: adversary needs to compromise 5 authority servers

Question 16

cencorshop resistance tor

Answer 16

• contains bridge relays which are not publicly listed
• Issue: deep packet inspection allows detection of tor traffic
• Sol: obfuscate traffic