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Transcript
Anonymity - Background
R. Newman
Topics

Defining anonymity

Need for anonymity

Defining privacy

Threats to anonymity and privacy

Mechanisms to provide anonymity

Applications of anonymity technology
User Observability in Networks



Attacker: can observe messages

Message contents: Data disclosure

Message headers: Traffic analysis
Attacker: may be able to affect/inject messages

Destroy/delay

Replay

Modification

Fabrication
Attacker: may have compromised node(s)

Observe whatever the node can see

Perform actions as that node
Network Protocols
Simplified version of what goes
on when a message is sent
Application Message
Socket – map process/port via OS
Transport Header Application Message
Interface – provide address for routing through network
Network Header
Transport Header Application Message
Medium Access – provide MAC address and deliver to next node in path
Link Header Network Header
Transport Header Application Message
Link Trailer
Physical – modulate/sense medium, synchronize symbols, bits, boundaries
PHY Link Header Network Header
Transport Header Application Message
Link Trailer
Where to protect your wires?

Protect actual wires

Link Layer Encryption

Network Layer Encryption

Transport Layer Encryption


Allows policies at port and connection levels
Application Layer Encryption

Allows for specificity, but reveals a lot!
Physical Mechanisms

Prevent eavesdropping on wires

Prevent tapping

Fiber optics

Special cabling


Still need appropriate protocols in case nodes are
compromised
Still need EMI emission elimination (TEMPEST)
Link Encryption

Encrypt all traffic at link level

Network header is not observable

But....

Can still have linkability:



Frame lengths
Frame timing
Node compromise

Reveals everything!
Network Layer Encryption

Encrypt at network level

If network addresses encrypted, must broadcast

Not scalable

Implicit addressing

Encrypt contents



Compromise of router doesn’t lose content confidentiality
But allows for traffic analysis
So – encrypt true destination, encapsulate, and
send to intermediate nodes


These become Mixes
Mix unpacks and resends
Transport Layer Encryption

Encrypt at transport layer

If port numbers encrypted, host has no way to route
to processes/sockets

Can be transparent to applications

Encrypt contents

Allows for endpoint (IP address/Port number) traffic
analysis
Network Anonymity Forms

Recipient Anonymity


Message linkability


Know who sent a message, but not who received it
Know a message was sent, but don’t know which of
the incoming messages correspond to an outgoing
message
Sender Anonymity

Know who received a message, but not who sent it
Recipient Anonymity

Broadcast

All nodes receive all messages

Scaling problems!

Implicit addressing – recognize msgs for you

Invisible – only destination can determine attribute


Visible – if not invisible


Public key distribution (like covert channel)
Can use pseudonyms
Public vs. Private


Public if known to all principals
Public <=> Not invisible – else linkable
Sender-Receiver Unlinkability

Mixes

Sender sends to Mix

Mix resends to Recipient


Must prevent linking incoming messages with
outgoing messages
More on this when covering Chaum Mix papers
Sender Anonymity

Superposed Sending






DC-networks
Every station generates at least one key bit per
message bit
Key bit is sent over secure channel to exactly one
other station
To send a bit, each station XORs all key bits it sent
or received, plus the bit it wants to send (if any)
Makes multiple access collision channel
Need anonymity-preserving multiple access
protocol

Slotted ring w/sender remove, e.g.
Performance Issues


End-to-end delay

Store-and-forward vs. Cut-through

Introduced delays (Mixes)
Reliability


End-to-end retransmission problematic
Scalability

Network load

Station load
Next

Chaum Mixes

Generalized Mixes

Measuring information leakage