Download Lecture 3b - Anonymity in networks

Anonymity - Background
R. Newman
Topics
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Defining anonymity
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Need for anonymity
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Defining privacy
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Threats to anonymity and privacy
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Mechanisms to provide anonymity
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Applications of anonymity technology
User Observability in Networks
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Attacker: can observe messages
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Message contents: Data disclosure
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Message headers: Traffic analysis
Attacker: may be able to affect/inject messages
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Destroy/delay
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Replay
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Modification
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Fabrication
Attacker: may have compromised node(s)
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Observe whatever the node can see
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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?
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Protect actual wires
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Link Layer Encryption
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Network Layer Encryption
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Transport Layer Encryption
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Allows policies at port and connection levels
Application Layer Encryption
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Allows for specificity, but reveals a lot!
Physical Mechanisms
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Prevent eavesdropping on wires
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Prevent tapping
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Fiber optics
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Special cabling
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Still need appropriate protocols in case nodes are
compromised
Still need EMI emission elimination (TEMPEST)
Link Encryption
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Encrypt all traffic at link level
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Network header is not observable
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But....
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Can still have linkability:
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Frame lengths
Frame timing
Node compromise
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Reveals everything!
Network Layer Encryption
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Encrypt at network level
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If network addresses encrypted, must broadcast
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Not scalable
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Implicit addressing
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Encrypt contents
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Compromise of router doesn’t lose content confidentiality
But allows for traffic analysis
So – encrypt true destination, encapsulate, and
send to intermediate nodes
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These become Mixes
Mix unpacks and resends
Transport Layer Encryption
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Encrypt at transport layer
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If port numbers encrypted, host has no way to route
to processes/sockets
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Can be transparent to applications
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Encrypt contents
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Allows for endpoint (IP address/Port number) traffic
analysis
Network Anonymity Forms
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Recipient Anonymity
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Message linkability
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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
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Know who received a message, but not who sent it
Recipient Anonymity
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Broadcast
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All nodes receive all messages
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Scaling problems!
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Implicit addressing – recognize msgs for you
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Invisible – only destination can determine attribute
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Visible – if not invisible
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Public key distribution (like covert channel)
Can use pseudonyms
Public vs. Private
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Public if known to all principals
Public <=> Not invisible – else linkable
Sender-Receiver Unlinkability
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Mixes
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Sender sends to Mix
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Mix resends to Recipient
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Must prevent linking incoming messages with
outgoing messages
More on this when covering Chaum Mix papers
Sender Anonymity
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Superposed Sending
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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
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Slotted ring w/sender remove, e.g.
Performance Issues
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End-to-end delay
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Store-and-forward vs. Cut-through
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Introduced delays (Mixes)
Reliability
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End-to-end retransmission problematic
Scalability
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Network load
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Station load
Next
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Chaum Mixes
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Generalized Mixes
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Measuring information leakage