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

Application Message

Application MessageTransport Header

Application MessageTransport HeaderNetwork Header

Application MessageTransport HeaderNetwork HeaderLink Header Link Trailer

Application MessageTransport HeaderNetwork HeaderLink Header Link TrailerPHY

Socket – map process/port via OS

Interface – provide address for routing through network

Medium Access – provide MAC address and deliver to next node in path

Physical – modulate/sense medium, synchronize symbols, bits, boundaries

Simplified version of what goes on when a message is sent

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 Know who sent a message, but not who received it

Message linkability 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

Public key distribution (like covert channel) Visible – if not invisible

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. Can reduce traffic by PRNG distribution

End-to-end delay Store-and-forward vs. Cut-through Introduced delays (Mixes)

Reliability End-to-end retransmission problematic

Scalability Network load Station load

Performance Issues

Chaum Mixes Generalized Mixes Measuring information leakage

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