Transcript I Want My Voice to Be Heard: IP over Voice-over-IP for Unobservable Censorship Circumvention Amir Houmansadr (The University of Texas at Austin) Thomas Riedl (University.

I Want My Voice to Be Heard:
IP over Voice-over-IP
for Unobservable Censorship
Circumvention
Amir Houmansadr (The University of Texas at Austin)
Thomas Riedl (University of Illinois at Urbana-Champaign)
Nikita Borisov (University of Illinois at Urbana-Champaign)
Andrew Singer (University of Illinois at Urbana-Champaign)
Internet Censorship
• The Internet is a big threat to repressive regimes!
• IP filtering, DNS hijacking, Deep packet-inspection, etc.
• Circumvention systems
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• Repressive regimes censor the Internet:
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New stage in the arms race
Past: detect circumvention end-points
Now: detect circumvention traffic also
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• The threat model has changed
We need traffic unobservability
against passive, active, or proactive analysis
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A recent approach
• A promising approach: hide circumvention traffic within
popular Internet protocols
• A new trend: mimic the target protocol
FLAWED
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• Censors are unlikely to completely block that protocol
• SkypeMorph, StegoTorus, and CensorSpoofer (CCS’12)
• It’s hard to imitate network protocols
The Parrot is Dead: Observing Unobservable Network
Communications [Oakland’13]
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Our approach
• We seek the same objective, but take a different approach:
• By running the target protocol no need to worry about
implementation quirks, bugs, protocol details
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Run the target protocol
• Challenge: how to efficiently encapsulate traffic into the target
protocol
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FreeWave: IP over Voice-over-IP
• Why VoIP
• Widely used protocol (only 663 Million Skype users)
• Collateral damage to block
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• Target protocol: Voice-over IP (VoIP)
• Encrypted
• How to hide?
• The dial-up modems are back!
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Client
FreeWave
Server
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FreeWave architecture
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The
Internet
FreeWave Client
Web
Browser
HTTP
traffic
Audio
MoDem
The
Internet
VoIP
peers
VoIP
traffic
VoIP
client
Audio
VSC
FreeWave Server
Audio
VoIP
client
VoIP
peer
The
Internet
HTTP
traffic
Audio
VSC
VoIP
traffic
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System components
MoDem
Proxy
HTTP
traffic
Censored
Destinations
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MoDem component
• A typical acoustic modem
• QAM modulation
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• Reliable transmission
• Turbo codes
• Use Preambles
Raw data
(IP traffic)
Channel
Encoder
Preamble
Interleaver
Signal
Training 1
QAM
mapper
Data 1
Data 1
Training 2
A data frame
Data 2
Data 2
……..
……..
Data N
Training N
Data N
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Evaluations
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FreeWave’s unobservability
• Traffic analysis (packet rates and sizes)
Fixed rate codecs (e.g., G.7 series)
• Not an issue 
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• Comprehensive unobservability at the protocol level
Variable bit-rates (e.g., Skype’s SILK)
• Simple analysis
• Superimpose with recoded conversation
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Client
FreeWave
Server
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Server obfuscation
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Future directions
• Embed into Video of VoIP
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… IP over Voice over IP over Voice-over-IP
• Find other protocol to tunnel
• Look for better efficiency
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Questions!
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