tryst: making local service discovery confidential
DESCRIPTION
Tryst: Making Local Service Discovery Confidential. Jeffrey Pang Ben Greenstein Srinivasan Seshan David Wetherall. 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. - PowerPoint PPT PresentationTRANSCRIPT
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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: [email protected]
“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