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Message Splitting Against the Partial Adversary. Andrei Serjantov The Free Haven Project (UK) Steven J Murdoch University of Cambridge Computer Laboratory. Outline. Mix Systems. Criticisms. too strong threat model(!) intersection attack when >1 msg (too much data) sent Weaker threat model

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Message splitting against the partial adversary

Message Splitting Against the Partial Adversary

Andrei Serjantov

The Free Haven Project (UK)

Steven J Murdoch

University of Cambridge Computer Laboratory


Outline
Outline

  • Mix Systems. Criticisms.

    • too strong threat model(!)

    • intersection attack when >1 msg (too much data) sent

  • Weaker threat model

  • Sending each message via random route

    • “non connection-based system”

  • Empirical observations about Mixmaster Mixminion

  • Characteristic delay function [Dan04] is difficult to esitmate


Mix systems
Mix Systems

  • Well known to this audience

  • Implemented

    • Mixmaster

    • Mixminion

  • Threat Model

    • Global Passive Adversary (GPA)

    • GPA with some (all but one?) compromised mixes


Criticisms
Criticisms

  • GPA does not exist

    • (a matter of some debate)

  • The mix system (Chaum 81) allows one fixed-sized message to be sent anonymously

    • Great for votes

    • Ok for email

    • Bad for Web Browsing

    • Awful for Bit Torrent

  • If >1 message (more than 32K data), anonymity is degraded


Intersection attack

1

1

1

D

A

Mix 3

Mix 1

1

1

E

B

1

2

Mix 2

2

2

Mix 4

F

C

Intersection Attack

Receivers

Senders

Attacker



Intersection attack1
Intersection Attack

  • [BPS00] On the Disadvantages of Free Mix Routes (PET2001)

  • [WALS02] An Analysis of the Degradation of Anonymous Protocols (NDSS’02)

  • [KAP02] Limits of Anonymity in Open Environments (IH2002)

  • [Dan03] Statistical Disclosure (I-NetSec03)

    • [DS04] (IH2004)

  • [Dan04] The traffic analysis of continuous-time mixes (PET2004)

    etc


  • The common wisdom
    The Common Wisdom

    • Intersection attacks are:

      • Realistic

      • Powerful (reduce anonymity quickly)

      • Hard to protect against

        • Require lots of dummy traffic


    A weaker model

    Attacker observes:

    not all inputs

    not all outputs

    Not

    interesting

    A Weaker Model

    1

    1

    1

    A

    D

    Mix 1

    2

    Mix 2

    2

    2

    E

    B

    Mix 3

    Mix 4

    F

    C


    A better threat model
    A Better Threat Model

    • A Partial Adversary

      • Does not observe all Sender to Mix links

      • (alternatively not all mixes which senders can send to)

      • Ignore compromised mixes


    Observed mix
    Observed Mix

    Attacker sends all his messages via one single route theough

    the mix system

    1

    1

    1

    A

    D

    Mix 1

    Mix 2

    2

    2

    2

    B

    Mix 3

    E

    Mix 4


    Splitting data
    Splitting Data

    Sender B splits his stream of data and sends each message via a

    randomly chosen route

    1

    1

    1

    A

    E

    Mix 1

    Mix 2

    2

    1

    1

    2

    Mix 3

    1

    Mix 4

    B

    F

    1

    The problem: how do you choose

    the first mix?

    C


    The details
    The Details

    • Problem:

      • mixes to send to

        • compromised, the rest not (but no idea which ones)

      • P packets

      • What are the s.t. a random subset (attacker)

        of size gives least information about

      • Note that (dummy traffic)

      • No proof or optimal solution in this paper!

        • See one possible solution next


    One possible scheme
    One possible scheme

    • Pick (uniformly) at random a sequence of mixes

    • Pick from a geometric distribution with mean . Set

    • Pick from a geometric distribution with mean . Set

    • etc

    • Another in the paper (with some analysis)


    Part ii
    Part II

    • (Looking at a particular intersection attack and finding it not as easy as it looks at first glance)


    Another intersection attack
    Another Intersection Attack

    • Danezis 2004 (thanks for the diagrams)

    • The Idea:



    The characteristic delay function
    The Characteristic Delay Function

    • What is this for

      • Mixes

      • Mixmaster

      • Mixminion

      • Tor

    • This maybe unfair – Danezis intended his attack for lwo latency systems (Tor)

    • Nevertheless interesting


    The characteristic delay function1
    The Characteristic Delay Function

    • Theory:

      • What is the delay of a mix (cascade/network)

      • Can say not very much about it (as usual)

        • Details in the paper

    • Practice:

      • Steven wrote a disciplined pinger

        • Does not ping too often, hope not to affect the results by sampling




    Comparing
    Comparing

    • Nothing surprising

      • Mixmaster has longer delay

      • Heavy tails


    Conclusions i
    Conclusions I

    • It is well known that the intersection attack is powerful

      • No reason to abandon investigation!

    • New interesting, mathematically well defined threat model

    • Splitting traffic amongst first nodes

      • Does not have the efficiency of Tor or other connection-based systems

      • Does gain anonymity advantage (but only by means of a weaker threat model)


    Conclusions ii
    Conclusions II

    • Characteristic function of Mixmaster, Mixminion difficult to work out in theory or estimate empirically

    • Data at:

    • All references at “Anonymity Bibliography”

      Thank you


    The anonymity advantage
    The Anonymity Advantage

    100

    The Network

    (Mixmaster)

    17

    Alice

    10

    87

    5

    Total observed packets

    100

    The Network

    (Mixmaster)

    170

    10

    87

    Alice

    5


    Intersection attack2

    Attacker

    Intersection Attack

    Receivers

    Senders

    Mixes


    A weaker model1

    Attacker observes:

    not all inputs

    not all outputs

    Not

    interesting

    A Weaker Model


    Observed mix1
    Observed Mix

    Attacker sends all his messages via one single route theough

    the mix system


    Splitting data1
    Splitting Data

    Attacker splits his stream of data and sends each message via a

    randomly chosen route

    The problem: how do you choose

    The first mix?




    Comparing1
    Comparing

    • Nothing surprising

      • Mixmaster has longer delay

      • Heavy tails


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