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 Transport Header Application MessageTransport HeaderNetwork Header Application MessageTransport HeaderNetwork HeaderLink HeaderLink Trailer Application MessageTransport HeaderNetwork HeaderLink HeaderLink 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 Next