Tryst: Making Local Service Discovery Confidential

1. Tryst: Making Local Service Discovery Confidential Jeffrey Pang Ben Greenstein Srinivasan Seshan David Wetherall

2. Find my friend’s PSP Find my friend’s iTunes Authentication Setup encryption What is Local Service Discovery? Find an 802.11 network Find a local printer Proceeds automatically, often without user’s knowledge

3. Method 1: Announcement • Services broadcast their existence • Interested clients discover them • E.G., 802.11 APs announce network names (SSIDs)

4. iTunes here! iChat here! Privacy Threats: Inventory Phone Here! • “The devices I have” • Example: cell phone pirates break into cars to steal phones that announce their presence [Cambridge Evening News 2005] • “The applications I am running” • Example: Apple mDNS “announces” to hackers that they are vulnerable to a buffer overflow[CERT 2007]

5. Method 2: Probing • Clients broadcast queries for familiar services • Present services respond • E.G., 802.11 clients probe for SSIDs they have associated with before

6. Is “Anna, Jeff, and Mark’s Net” here? Privacy Threats: History • “Where I have been before” • Example: Probing for 802.11 SSIDs can expose where you live [WiGLE Wardriving Database]

7. Privacy Threats: History • “Where I have been before” • Example: Probing for 802.11 SSIDs can expose where you live [WiGLE Wardriving Database] 23% of devices at SIGCOMM 2004 probed for an SSID that WiGLE isolates to one city

8. 010294859 Is “010294859” here? Privacy Threats: History • “Where I have been before” • Example: Even opaque SSIDs can be correlated with other databases, such as Google’s business directory Is “Juvenile Detention Classroom” here?

9. Solution Requirement • Security during discovery • Confidentiality: unlinkable discovery attempts • Authenticity: prevent masquerading • Departure from common practice • Clients and services want privacy from third parties • Tryst • Access control for discovery messages

10. Proof of Identity Verify Source Identity Identity-Hiding Encryption How to Provide Access Control Sender Application Receiver Application Service Discovery Message

11. Protocol Design Details • Existing theoretical protocol [Abadi ’04] • Based on public key cryptography • Problem 1: Message size scales linearly with number of intended recipients • Typically OK: 90% of 802.11 clients probe for fewer than 12 unique SSIDs [OSDI 2006] • Problem 2: Messages can’t be addressed  must try to decrypt every message • Decryption is 168x slower than 802.11 line-rate • Opens up receivers to denial-of-service attacks

12. Protocol Design Details • Observation 1: Common case is to rediscover known services • Can negotiate a secret symmetric key the first time • Symmetric key cryptography is fast • Observation 2: Linkability at short timescales is usually OK • Compute temporary unlinkable addresses known only to a client and a service [similar to Cox ’07] • Messages not for me are discarded at 802.11 line-rate • Thus: • Prioritize symmetric key protocol • Use spare cycles for public key protocol

13. How Do I Obtain the Initial Keys? • Existing key establishment is not enough • Pairing: E.G., Bluetooth peripherals • Can not always physically identify service • User must discover service before device discovers service! • Discovery is also used to find new services • Goal: Automatically expand the trust horizon • E.G., new services in trusted domains • E.G., new services trusted transitively

14. ? New Services in Trusted Domains x x Trusted Alice Bob x “Discover Alice’s iPhone” Strawman Solution

15. “bob.laptop” “alice.laptop” “bob.psp” “alice.ds” “bob.zune” “alice.iphone” ? New Services in Trusted Domains Trusted Trusts: alice@att.com Alice Bob “Discover Alice’s iPhone” Anonymous Identity Based Encryption

16. Conclusion • Local service discovery exposes sensitive info • Tryst enables confidential service discovery • Progress: • Implementation of Tryst access control • Integration with a real 802.11 protocol stack • Future Work: • Implement automated key establishment • Evaluate how people use Tryst in the wild

17. Questions?

18. Service Discovery is Widely Used • Example 1: 85% devices send 802.11 probes(SIGCOMM 2004) • Example 2:ApplicationProtocols(OSDI 2006)

19. IR_Guest Pittsburgh Seattle Berkeley Cambridge Privacy Threats: Location x • “The fact that my service is present” • Example: Common practice to disable 802.11 beacons to (try to) hide access points[O’Reilly 802.11 Guide] • “Where my service is located” • Example: Knowledge of 802.11 SSID at one site can tell you where other sites are [WiGLE Wardriving Database]

20. Privacy Threats: Identity • “Fingerprints who I am” • Example: Both 802.11 and application level probes accurately identify a person[Our MobiCom 2007 Paper] “IR_Guest”, “djw”, “University of Washington” “IR_Guest”, “djw”, “University of Washington” = = ………..

21. Is the network“djw” here? Privacy Threats: History • “Where I have been before” • Example: Probing for 802.11 SSIDs can expose where you live [SSID Lookup in WiGLE]

22. More Threats in the Future • Emerging social devices also offer “services” • Microsoft Zune: music sharing service • PSP, Nintendo DS: multiplayer gaming service • Service discovery exposes social contacts

23. We tackle this problem Reasons for Privacy Threats • Plug-and-Play Automatic • Infrastructure Independent Broadcast • Before Security Setup  No Authentication, Encryption

24. Find networks that Alice trusts Alice’s secret Alice trusts “Alice’s Home” Transitive Trust Alice’s secret Alice trusts bob.laptop New Services Transitively Trusted “Alice’s Home” Trust Alice Bob Attestation