Censorship Resistance: Parrots Amir Houmansadr CS660: Advanced Information Assurance Spring 2015 Content may be borrowed from other resources. See the last slide for acknowledgements!

Classes of Information Hiding Digital watermarking Steganography Covert channels Anonymous communications Protocol obfuscation CS660 - Advanced Information Assurance - UMassAmherst 2

Definition Protocol obfuscation: Concealing the type of the underlying network protocol from a traffic monitoring entity 3 CS660 - Advanced Information Assurance - UMassAmherst

Why Hide the Protocol? Bypass ISP restrictions: – BitTorrent blocked on campus – Skype blocked in some corporate networks Bypass nation-state censorship (censorship circumvention): – Tor is blocked by various countries – VPN is blocked by the Great Firewall of China 4 CS660 - Advanced Information Assurance - UMassAmherst

Types of Protocol Obfuscation De-identification: look like nothing Impersonation: look like some other protocol 5 CS660 - Advanced Information Assurance - UMassAmherst

Internet Censorship

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The Non-Democratic Republic of Repressistan Gateway 8 (IP=A.B.C.D) X A.B.C.D Censorship model CS660 - Advanced Information Assurance - UMassAmherst

Censorship circumvention 9 CS660 - Advanced Information Assurance - UMassAmherst

Using Tor for circumvention 10 The Non-Democratic Republic of Repressistan Blocked Destination Tor Network Tor Bridge Not effective anymore! Gateway Active probes Easily recognizable at the network level Deep Packet Inspection (DPI) Insider attacks CS660 - Advanced Information Assurance - UMassAmherst

Challenge! We need unobservable circumvention Censors should not be able to easily identify circumvention traffic or end-hosts through passive, active, or proactive techniques 11 CS660 - Advanced Information Assurance - UMassAmherst

Hide and seek! 12 The Non-Democratic Republic of Repressistan Blocked Destination Tor Bridge Tor Network Gateway CS660 - Advanced Information Assurance - UMassAmherst

Parrot systems Imitate a popular protocol – SkypeMorph (CCS’12) – StegoTorus (CCS’12) – CensorSpoofer (CCS’12) 13 CS660 - Advanced Information Assurance - UMassAmherst

SkypeMorph (CCS’12) 14 The Non-Democratic Republic of Repressistan Blocked Destination SkypeMorph Bridge Tor Network SkypeMorph Client Skype Client Traffic Shaping CS660 - Advanced Information Assurance - UMassAmherst

StegoTorus Client Censorship Region The Internet StegoTorus A Tor node StegoTorus Bridge HTTP Skype Ventrilo HTTP CS660 - Advanced Information Assurance - UMassAmherst 15

Dummy host Censorship Region The Internet CensorSpoofer Censored destination Spoofer RTP upstream RTP downstream SIP server CensorSpoofer Client CS660 - Advanced Information Assurance - UMassAmherst 16

The Parrot is Dead: Observing Unobservable Network Communications Amir Houmansadr, Chad Brubaker, Vitaly Shmatikov IEEE S&P (Oakland) 2013 Received the Best Practical Paper Award

Detecting SkypeMorph 18 The Non-Democratic Republic of Repressistan Blocked Destination Tor Bridge Tor Network SOM TCP control stream CS660 - Advanced Information Assurance - UMassAmherst

19 No, no..... no, 'e's stunned! CS660 - Advanced Information Assurance - UMassAmherst

SkypeMorph+ Let’s imitate the missing parts! Problem: hard to mimic dynamic behavior in response to active tests 20 CS660 - Advanced Information Assurance - UMassAmherst

Dropping UDP packets 21 CS660 - Advanced Information Assurance - UMassAmherst

Other tests TestSkypeSkypeMorph+ Flush Supernode cacheServes as a SNRejects all Skype messages Drop UDP packetsBurst of packets in TCP control No reaction Close TCP channelEnds the UDP streamNo reaction Delay TCP packetsReacts depending on the type of message No reaction Close TCP connection to a SN Initiates UDP probesNo reaction Block the default TCP port Connects to TCP ports 80 and 443 No reaction 22 CS660 - Advanced Information Assurance - UMassAmherst

23 Now that's what I call a dead parrot. CS660 - Advanced Information Assurance - UMassAmherst

Unobservability by imitation is fundamentally flawed! 24 CS660 - Advanced Information Assurance - UMassAmherst

Perfect imitation of a complex real system is extremely hard A complex protocol in it entirety Inter-dependent sub-protocols with complex, dynamic behavior Bugs in specific versions of the software User behavior 25 Not enough to mimic a "protocol," need to mimic a specific implementation with all its quirks CS660 - Advanced Information Assurance - UMassAmherst

So, what is the real problem?

27 The Non-Democratic Republic of Repressistan Tor (and its flavors) Psiphon Ultrasurf Tor relays Ultrasurf proxies Psiphon proxies X X X Custom tunnels are easy to recognize! CS660 - Advanced Information Assurance - UMassAmherst

Wait! We already have lots of encrypted tunnels! 28 CS660 - Advanced Information Assurance - UMassAmherst

29 The Non-Democratic Republic of Repressistan VoIP VoIP servers (e.g., Skype) servers (e.g., Gmail) File sharing File hosts (e.g., BitTorent) Online games Gaming servers (e.g., Warcraft) Cloud storage Cloud servers (e.g., Amazon EC2) Tor CS660 - Advanced Information Assurance - UMassAmherst

Hide-within circumvention (or, parasites!)

Definition Tunneling circumvention traffic through a popular service provider via an allowed, already deployed network protocol 31 CS660 - Advanced Information Assurance - UMassAmherst

I Want My Voice to Be Heard: IP over Voice-over-IP for Unobservable Censorship Circumvention Amir Houmansadr, Thomas Riedl, Nikita Borisov, Andrew Singer NDSS 2013

FreeWave: IP over Voice-over-IP Target protocol: Voice-over IP (VoIP) Why VoIP – Widely used – Encrypted – Many VoIP provider options How to hide? – The dial-up modems are back! 33 CS660 - Advanced Information Assurance - UMassAmherst

34 The Non-Democratic Republic of Repressistan Gateway 34 Blocked Destination Tor Bridge Tor Network X FreeWave architecture CS660 - Advanced Information Assurance - UMassAmherst

System components 35 CS660 - Advanced Information Assurance - UMassAmherst

MoDem component A typical acoustic modem – QAM modulation Reliable transmission – Turbo codes – Use Preambles 36 CS660 - Advanced Information Assurance - UMassAmherst

Unobservability

Unobservability in hide-within 7. Application 6. PresentationData 5. Session 4. TransportSegments 3. NetworkPacket/Datagram 2. Data linkBit/Frame 1. PhysicalBit 38 The OSI model Parrot systems Hide-within systems CS660 - Advanced Information Assurance - UMassAmherst

Costs of censorship 39 More resource-intensive Slower More false positives Cheap and fast Doable at line speed Very accurate Machine learning Statistical analysis Proactive probing Active probing Inspecting protocol signatures Inspecting keywords IP filtering Hide-within Traditional systems CS660 - Advanced Information Assurance - UMassAmherst

Some of the tools used to analyze the attacks Probability theory to model types of traffic – E.g., Chernoff bound, stochastic processes Detection and estimation theory to distinguish instances of a traffic type – E.g., hypothesis testing, LRT tests Statistics to find deviations from a type – E.g., K-S test, Q-Q plot Information theory to derive the bounds of deviation 40 CS660 - Advanced Information Assurance - UMassAmherst

Unique properties of hide- within systems

1. Resistant to partial compromise

43 The Non-Democratic Republic of Repressistan Gateway 43 Blocked Destination Tor Bridge Tor Network X Tor Detecting one user makes it easier to detect other users Gateway X X CS660 - Advanced Information Assurance - UMassAmherst

44 The Non-Democratic Republic of Repressistan Circumvention user Benign user Oblivious server Tor X Hide-within Detecting one user does not help detect others CS660 - Advanced Information Assurance - UMassAmherst

45 The Non-Democratic Republic of Repressistan Gateway 45 Blocked Destination FreeWave Tor Bridge Tor Network X FreeWave Gateway CS660 - Advanced Information Assurance - UMassAmherst

This is a big step forward! 46 CS660 - Advanced Information Assurance - UMassAmherst

2. Censorship causes collateral damage

Censors are rational! 48 CS660 - Advanced Information Assurance - UMassAmherst

49 The Non-Democratic Republic of Repressistan Gateway 49 Blocked Destination Tor Bridge Tor Network X Tor Censoring Tor bridges has zero impact on benign users Gateway X X CS660 - Advanced Information Assurance - UMassAmherst

50 The Non-Democratic Republic of Repressistan Circumvention user Benign user Oblivious server Tor X X X X Hide-within Censoring disrupts benign users as well X CS660 - Advanced Information Assurance - UMassAmherst

51 The Non-Democratic Republic of Repressistan Gateway 51 Blocked Destination FreeWave Tor Bridge Tor Network FreeWave X Censoring FreeWave bridges disrupts benign users as well (collateral damage) Gateway X X X X CS660 - Advanced Information Assurance - UMassAmherst

This is another big step forward! 52 CS660 - Advanced Information Assurance - UMassAmherst

Challenge: designing efficient hide-within systems 53 CS660 - Advanced Information Assurance - UMassAmherst

Hide-within designs FreeWave v2 – Encode directly in video stream – Bandwidth and latency sufficient for interactive web browsing Additional hide-within designs: SWEET ( ) CloudTransport (Cloud services) Under development: – BitTorrent 54 CS660 - Advanced Information Assurance - UMassAmherst

Acknowledgement Some pictures are obtained through Google search without being referenced 55 CS660 - Advanced Information Assurance - UMassAmherst