Attacker can see evidence of a sent message, but the sender appears no more likely to be the originator than any other potential sender in the system

1. Anonymity on the Internet

2. Types of Anonymity
 Pseudonymity
– Susceptible to subpoenas
 Sender
– Receiver / observer can’t
identify sender
 Receiver
– Observer can’t identify receiver
 Sender-receiver
– Observer can’t identify that
communication has been sent

3. Uses of Anonymity
Positive
 Free speech for political claims as well as non-political
comments
 engage in whistle-blowing
 conduct commercial transactions
 freedom from detection, retribution, and embarrassment
 New York Times Co. vs. Sullivan, 1964
"an author's decision to remain anonymous...is an aspect
of the freedom of speech protected by the First
Amendment"

4. Uses of Anonymity
Negative
 Spam
 DoS -
 Illegal activity – anonymous bribery,
copyright infringement, harassment, financial
scams, disclosure of trade secrets

5. Assumptions
 Weak attacker
– Eavesdrops on first and last hop
– Can introduce messages here
 Strong attacker
– Eavesdrops on all links
– Can introduce messages anywhere
 Attacker has finite time, computing power
 Multiple users

6. Types of Attackers
 Local eavesdropper
– Observes inbound and outbound messages on
user’s computer
 Administrator
– Operator or group of operators of anonymizing
systems attempting to foil their own system
 Remote attack
– Observation at the remote end by eavesdropper or
attack by the remote host

7. Attacks
 Timing Attack, Volume Attack
– Watches shape of traffic instead of content
 Flooding Attack
– With batch size n, attacker sends n-1 messages
 Usage Pattern Attack
– Consistent usage patterns leads to predictability

8. Levels of Anonymity
Absolute
Privacy
Beyond
Suspicion
Probable
Innocence
Possible
Innocence
Exposed Provably
Exposed
•Beyond Suspicion
•Attacker can see evidence of a sent message, but the sender appears no
more likely to be the originator than any other potential sender in the system
•Probable Innocence
•The sender is more likely the originator than any other potential sender, but
there is equal likelihood the sender is not the originator
•Possible Innocence
•The sender appears more likely to be the originator than to not be the
originator, but there’s still a non-trivial probability that the originator is
someone else

9. Capabilities
 Latency, Bandwidth, Anonymity
– Pick 2
 Human element
– Repetitive usage patterns make attacks easier
– Pizza effect

10. Proxy Anonymizers
 Use trusted centralized servers
 Anonymous remailers - Helsingius
 Anonymizer.com
 Hides IP address - NAT
 Users not anonymous to proxy server
 Susceptible to traffic analysis

11. Mixes
 Source routing chosen by user
 Shuffles order of packets
 Mix cascade consists of several mixes under
separate operators
 Encrypted for each mix in the path
 Processes packets in batches
 Used to counter traffic analysis

12. Mixes
A1, C1(A3, C3(A2, C2(S, M, r2), r3), r1)
A3, C3(A2, C2(S, M, r2), r3)
A2, C2(S, M, r2)
S, M
Mix 1
Mix 4Mix 3
Mix 2
1.
2.
3.
4.
1.
2.
3.
4.
Ai = Next Hop Address
Ci = Message encrypted with public key of Mix i
S = Destination Host address
M = Original message

13. Mixes
 Fine for non real-time (email)
 Not sufficient for VoIP, video, web
 Mix waits to accumulate inputs to process as
a batch (especially slow for low traffic)

14. Enhancements
 Messages all the same length
 Buffers messages until several can be sent at
once
 Dummy messages inserted
– Between mixes
– Between mixes and user
 Balance end to end throughput with anonymity
– Duration to wait for mixes to accumulate traffic
– Percentage of dummy traffic

15. P5
 Decentralized
– Harder to attack
 Allows choice of tradeoff between anonymity /
throughput
 Encrypted with public key of each node in route
 Nodes change packet order
 Fixed message size
 Users have broadcast map and route map
 Noise packets counter statistical traffic analysis

16. User A
User B
User A can send an anonymous message to User B via group */0, 1/1, 111/3, etc
User A can route messages between 00/2 and 01/2
01/2 is a subset of */0 – more efficient but less anonymous
Hash of User’s public key provides choice of groups.

17. P5
 Within a channel, P5
functions as a mix cascade
 Between channels, P5
provides greater anonymity per
bandwidth
– For 8192 users, 1.5 Mbps provides 200Kbps with 40% loss
 Resistant to Timing/Volume and DoS attacks
 Susceptible to Flood Attack (Mob Attack)
– User’s channel is flooded, prompting him to reveal more of his
mask to gain efficiency, thereby reducing his anonymity

18. Conclusion
 Costly to be anonymous (Use
Anonymous VPN Service)
– Tradeoff with throughput
 Can not be completely anonymous anyway
– No protection from monitoring usage patterns
 Aside from this, practical anonymity can be
achieved