Anonymous Communication -- a brief survey Pan Wang North Carolina State University

Outline Why anonymous communication Definitions of anonymities Traffic analysis attacks Some anonymous communication protocols for Internet Some anonymous communication schemes for MANET and sensor networks Potential research problems

Why Anonymous Communication Privacy issue Some covert missions may require anonymous communication In hostile environments, end-hosts may need hidden their communications to against being captured

Anonymity in terms of unlinkability* Sender anonymity A particular message is not linkable to any sender and that to a particular sender, no message is linkable Recipient anonymity A particular message cannot be linked to any recipient and that to a particular recipient, no message is linkable Relationship anonymity The sender and the recipient cannot be identified as communicating with each other, even though each of them can be identified as participating in some communication. A. Pfizmann and M. Waidner, Networks without User Observability. Computers & Security 6/2 (1987) 158-166

Traffic Analysis Attacks against an Anonymous Communication System Contextual attacks Communication pattern attacks Packet counting attacks Intersection attack Brute force attack Node flushing attack Timing attacks Massage tagging attack On flow marking attack

Some Anonymous Communication Protocols for Internet Mix-NET Feb 1981, D. Chaum Crowd June 1997, Michael K. Reiter and Aviel D. Rubin Tarzan Nov 2002, Michael J. Freedman and Robert Morris K-Anonymous Message Transmission Oct, 2003, Luis von Ahn, Andrew Bortz and Nicholas J. Hopper

Mix-NET* Basic idea: Traffic sent from sender to destination should pass one or more Mixes Mix relays data from different end-to-end connections, reorder and re-encrypt the data So, incoming and outgoing traffic cannot be related *D. Chaum, Untraceable Electric Mail, Return Address and Digital Pseudonyms, Communication of A.C.M 24.2 (Feb 1981), 84-88

Mix-NET (cont-1)

Mix-NET (cont-2) Trust one mix server: the entire Mix-NET provides anonymity

Crowds* P2P anonymizer network for Web Transactions Uses a trusted third party (TTP) as centralized crowd membership server (“blender”) Provides sender anonymity and relationship anonymity *M. Reiter and A. Rubin, Crowd: Anonymity for Web Transactions. ACM Transactions on Information and System Security, 1(1) June 1998

Crowd (cont) A nodes decide randomly whether to forward the request to another node or to send it to the server Webserver

Tarzan* All nodes act as relays, Mix-net encoding Each node selects a set of mimics Tunneling data traffic through mimics Exchanging cover traffic with mimics Constant packet sending rate and uniformed packet size Network address translator Anonymity against corrupt relays and global eavesdropping M. Freedman and R. Morris, Tarzan: A Peer-to-Peer Anonymizing Network Layer, CCS 2002, Washington DC

Tarzan (cont-1) User PNAT

Tunnel Private Address Tarzan (Cont-2) User Tunnel Private Address Public Alias Address Real IP PNAT

k-Anonymous Message Transmission* Based on secure multiparty sum protocol Local group broadcast The adversaries, trying to determine the sender/receiver of a particular message, cannot narrow down its search to a set of k suspects Robust against selective non-participations L.Ahn, A.Bortz and N.Hopper, k-Anonymous Message Transmission, CCS 2003, Washington DC

k-Anonymous Message Transmission (cont) Group-D Group-S

Some anonymous communication schemes for MANET and sensor networks Anonymous on demand routing (ANODR) Jun 2003, Jiejun Kong and Xiaoyan Hong Phantom flooding protocol Jun 2005, Pandurang Kamat, Yanyong Zhang, Wade Trappe and Celal Ozturk

ANODR* Assuming salient adversaries Broadcast with trapdoor Route pseudonym J.Kong and X.Hong, ANODR: Anonymous On Demand Routing with Untraceable for Mobile Ad-hoc Networks, MobiHoc, 2003, Annapolis, MD

ANODR (cont)

Source-Location Privacy in Sensor network Network model: A sensor reports its measurement to a centralized base station (sink) Attack model: Adversaries may use RF localization to hop-by-hop traceback to the source’s location Why location privacy

Phantom Flooding Protocol* Random work plus local broadcast P. Kamat, et. al., Enhancing Source-Location Privacy in Sensor Network Routing, ICDCS 2005, Columbus, OH

Potential Research Problems Anonymity vs accountability Detect malicious users Efficiency vs anonymity More?

Questions?