CoverUp: Upload and Download via Passive Participation · 2019-03-05 · Participation alone raises suspicion Little deniability Bootstrapping Problem Motivation: Deniability and

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CoverUp: Upload and Download via Passive Participation

David Sommer, Aritra Dhar, Luka Malisa

Esfandiar Mohammadi, Srdjan Čapkun, Daniel Ronzani

NSDI'19 - 28.02.2019David Sommer, Aritra Dhar 1

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Were you Ever Afraid to …

… download something that is easily accessible?

Maybe someone is watching?

2NSDI'19 - 28.02.2019David Sommer, Aritra Dhar

mass surveillance↯

whistleblowers

free speech

accessing primary sources (e.g., WikiLeaks)

(essential for an informed democracy)

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Participation alone raises suspicion Little deniability

Bootstrapping Problem

Motivation: Deniability and Participation

ACN - Strong anonymity Hide which users are connected to whom

Limits surveillance and censorship


small anonymity set

Low number of connected users

unattractive degree of anonymity

Unattractive latency and/or bandwidth

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Our contribution: Passive Participation

4

passive

❶

❷❸cover traffic

NSDI'19 - 28.02.2019David Sommer, Aritra Dhar

participant

active

passivecover traffic

❶

❷❸

participantreal traffic

Web site visitors passively produce cover traffic

User visits reddit

Reddit respondsand includes a pieceof JavaScript code

This JS code producescover traffic

❶

❷

❸

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Our contribution: Passive Participation

5

passive

❶

❷❸cover traffic


participant

active

passivecover traffic

❶

❷❸

participantreal traffic

Web site visitors passively produce cover traffic

Indistinguishability Larger anonymity set

Anonymity set size = active + passive

Mitigates bootstrapping

Provides deniability

≈

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CoverUp: Contributions

Uses Passive Participation Uni-directional channel: Feed

Bi-directional channel: Transfer

Working Prototype

Analyzed Network Timing leakage


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JS code in sandboxed iframe due to Same-Origin-Policy

Attacker controls: Network (monitor/drop/fake)

Entry Server (reddit)

CoverUp server (delivers js code)

Feed Server (delivers feed)

Active user’s machine not compromised

.

.

.

passive participants

Feed Server

CoverUp server

activeparticipants

(3) connects clients via JS to

(2) triggers clients to connect to

(4) sendsmessages to

CoverUp Tool

(5) extract feed

browser

(1) connects to

feed feed

feed

CoverUp Tool

(5) extract feed

browser

feed


CoverUp: Feed

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.

.

.

passive participants

Feed Server

CoverUp server

activeparticipants




CoverUp Tool

(5) extract feed

browser

(1) connects to

feed feed

feed

CoverUp Tool

(5) extract feed

browser

feed


CoverUp: Feed

Indistinguishability Active and passive participants: same protocol

Difference: CoverUp Tool

Provides Deniability

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Protecting Passive Participants

9

Feed Server

CoverUp Tool

localstorage

passive participant

+ +

=

active participant

Fountain Codes + All-or-Nothing Scheme

Only one packet stored

→ protects passive participants


Browser

JavaScript

reddit

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CoverUp: Transfer

CoverUp Toolbrowser

JavaScript

Extension

response8

localstorage

request1

7

3

6

2

5

4

www

Bi-directional channel Adds upstream channel

Involves extension

Using TLS

Indistinguishability Trust Transfer Server

Trust CoverUp Server

Augments Feed


Active participant

Transfer Server

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Evaluating the Indistinguishability Assertion

Protocol transcripts are indistinguishable Everything else identical?

But active users have CoverUp tool and browser extension (in Transfer)

What can network attacker do? Measure execution time by network timestamps

Timing leakage Evaluation

Mitigation


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Periodic Periodic

Setup: LAN, entry, transfer, and feed server

Feed and Transfer scenarios:

Strong attacker model: No other processes running on the system

High-precision time resolution

3 Million measurements

Loading

CoverUp JavaScript

Transfer Server

time

Browser

CoverUp: Experimental Setup


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Request dispatch time: add truncated Gaussian noise

Continual observation for half a year

< 5 hours of visiting the entry server

(Periodic-observations) per day

< 50 connecting to the entry server

(Loading-observations) per day

t

p

0


*t0 w

p

t0 w

p

CoverUp: Privacy Budget

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CoverUp: Implementation

CoverUp Tool Implemented in Java

Features: feed, chat and interactive browsing

Uses crypto APIs from whisper systems and JCA

Browser extension Chrome extension based on WebExtension API

Feed/Transfer and CoverUp server Implemented using Java EE Servlet API

Hosted on Apache Tomcat webserver

Available for download and testing: http://coverup.ethz.ch


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CoverUp: Performance

Performance Packet size: 75KB every 60s avg.

Goodput: 10KBit/s

Per user overhead Around 660 MB/month or 22MB/day

Privacy guarantee Attacker’s advantage < 2∙ 10−3


cnn.com: 4.0MBamazon.com: 5.0MBalibaba.com: 5.4MB

google.com: 0.3MB

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CoverUp: Summary

Passive Participation Increases anonymity set (Bootstrapping)

Hides Intention (Deniability)

Adding Noise reduces Timing Leakage Maintains feasible usability

Measurements available


Available for testing: https://coverup.ethz.chAvailable for download: https://github.com/sommerda/CoverUp-source-code

Feed Server

CoverUp server




CoverUp Tool

(5) extract feed

browser

feed

https://coverup.ethz.ch/

Documents

CoverUp: Upload and Download via Passive Participation · 2019-03-05 · Participation alone raises suspicion Little deniability Bootstrapping Problem Motivation: Deniability and