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Tryst: Making Local Service Discovery Confidential

Jeffrey PangBen Greenstein

Srinivasan SeshanDavid Wetherall

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What is Local Service Discovery?Find an 802.11 networkFind a local printer

AuthenticationSetup encryption

Find my friend’s PSPFind my friend’s iTunes

Proceeds automatically, often without user’s knowledge

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Method 1: Announcement

• Services broadcast their existence• Interested clients discover them

• E.G., 802.11 APs announce network names (SSIDs)

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Privacy Threats: Inventory

• “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]

PhoneHere!

iTunes here!iChat here!

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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

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Privacy Threats: History• “Where I have been before”

– Example: Probing for 802.11 SSIDs can expose where you live [WiGLE Wardriving Database]

Is “Anna, Jeff, and Mark’s Net” here?

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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

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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?Is “010294859” here?

010294859

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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

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How to Provide Access Control

Service Discovery Message Verify Source Identity

Sender Application Receiver Application

Proof of Identity

Identity-Hiding Encryption

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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

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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

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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

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New Services in Trusted Domains

Bob Alice

Trusted

?

x

xStrawman Solution

x

“Discover Alice’s iPhone”

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?

New Services in Trusted Domains

Bob

“Discover Alice’s iPhone”

Alice

Trusted

Trusts: alice@att.com

“alice.ds”

“alice.laptop”

“bob.zune”

“bob.psp”“bob.laptop”

Anonymous Identity Based Encryption

“alice.iphone”

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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

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Questions?

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Service Discovery is Widely Used

• Example 1: 85% devices send 802.11 probes(SIGCOMM 2004)

• Example 2:ApplicationProtocols(OSDI 2006)

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Privacy Threats: Location

• “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]

IR_Guest

Pittsburgh

Seattle

Berkeley

Cambridge

x

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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”= =

………..

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Privacy Threats: History• “Where I have been before”

– Example: Probing for 802.11 SSIDs can expose where you live [SSID Lookup in WiGLE]

Is the network“djw” here?

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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

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Reasons for Privacy Threats

• Plug-and-Play Automatic

• Infrastructure Independent Broadcast

• Before Security Setup No Authentication, Encryption

We tackle this problem

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New Services Transitively Trusted

AliceBob

“Alice’s Home”

Trust

TransitiveTrust

Alice trustsbob.laptop

Alice’s secret

Alice trusts “Alice’s Home”

Alice’s secret

Find networks that Alice trusts

Attestation