1 making local service discovery confidential with tryst jeffrey pang cmu ben greenstein intel...

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Making Local Service Discovery Confidential with Tryst

Jeffrey PangCMU

Ben GreensteinIntel Research

Srinivasan SeshanCMU

David WetherallUniversity of Washington

Damon McCoyUniversity of Colorado

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What is Local Service Discovery?Find a WiFi networkFind a local printerFind my friend’s PSPFind my friend’s iTunes

• Proceeds automatically, often without user’s knowledge• Occurs before security associations are setup

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Service Discovery is Widely Used

• Example 1:ApplicationProtocols(OSDI 2006)

• Example 2: 85% devices send WiFi discovery probes(SIGCOMM 2004)

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Outline

• Existing mechanisms and their privacy threats

• Solution requirements

• Tryst

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Outline

• Existing mechanisms and their privacy threats– Announcement– Probing


• Tryst

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Method 1: Announcement

• Services broadcast their existence• Interested clients discover them

• E.G., WiFi access points (APs) announce network names

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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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Privacy Threats: Location

• “The fact that my service is present”– Example: Common practice to

disable WiFi annoucements to (try to) hide access points[O’Reilly 802.11 Guide]

• “Where my service is located”– Example: Knowledge of network

name at one site can tell you where other sites are [WiGLE Wardriving Database]

IR_Guest

Pittsburgh

Seattle

Berkeley

Cambridge

x

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Outline

• Existing mechanisms pose privacy threats– Announcement reveals inventory and location– Probing


• Tryst

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Outline

• Existing mechanisms pose privacy threats– Announcement reveals inventory and location– Probing


• Tryst

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Method 2: Probing

• Clients broadcast queries for familiar services• Present services respond

• E.G., WiFi clients probe for network names they have associated with before

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

– Example: Probing for network names can expose where you live [WiGLE Wardriving Database]

Is “Anna, Jeff, and Mark’s Net” here?

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23% of devices at SIGCOMM 2004 probed for an name that WiGLE isolates to one city

All 4 known home networks located towithin ~500 ft

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– Example: Even opaque names 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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Privacy Threats: Identity

• “Fingerprints who I am”– Example: Both WiFi and application level

probes accurately identify a device[Pang, J. et al. MobiCom 2007]

“IR_Guest”, “djw”, “University of Washington”

“IR_Guest”, “djw”,“University of Washington”= =

………..

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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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Outline

• Existing mechanisms pose privacy threats– Announcement reveals inventory and location– Probing reveals history and identity


• Tryst

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Outline



• Tryst

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So … Why Use Service Discovery?

• Plug-and-play networking – Setup networks without configuration

Automatic (no user intervention)

• Infrastructure independence– Always works; no special servers required

Broadcast (only need communication medium)

• Key Problem: Before Security Setup No Confidentiality

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Solution Requirements

• Provide security during discovery– Anonymity: unlinkable discovery attempts– Authenticity: prevent masquerading

• Challenges– Clients and services want confidentiality– We need mutual authentication before either can

learn of the other’s existence– We can’t rely on manual user action or trusted

infrastructure

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Outline


• Solution requirements1. Plug-and-play networking2. Infrastructure independence3. Anonymity4. Authenticity

• Tryst

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Outline



• Tryst– Access control for discovery messages provides 3 and 4

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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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KAlice

Identity-hiding encryption with Alice’s public key (e.g., ElGamal)

Public Key Protocol

• Existing theoretical public key protocol [Abadi ’04]

K-1Bob

“Bob to Alice at time T”

Digital signature with Bob’s private key (e.g., RSA, DSA)

Service Discovery Message

“Is Alice’s Laptop here?”

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???

Public Key Protocol

KBob

K-1Bob

“Bob to Alice at time T” Service Discovery Message

K-1Alice Decrypt with Alice’s private key

Verify with Bob’s public key

• Existing theoretical public key protocol [Abadi ’04]

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Efficiency Problems

• Problem 1: Message size scales linearly with number of intended recipients– Typically OK: 90% of WiFi clients probe for fewer than

12 unique network names [OSDI 2006]

• Problem 2: Messages can’t be addressed must try to decrypt every message– Public key decryption is slow– 168x slower than WiFi line-rate – Receivers susceptible to denial-of-service attacks

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Symmetric Key Protocol• Observation 1:

Common case is to rediscover known services– Can negotiate a shared symmetric key the first time– Symmetric key cryptography is fast

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KShared

Identity-hiding encryption Alice and Bob’s shared key (e.g., AES)

Symmetric Key Protocol

KShared

“Bob to Alice at time T”

Message authentication code with Alice and Bob’s shared key(e.g., HMAC-SHA1)

Service Discovery Message

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Symmetric Key Protocol• 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– Messages not for me are discarded at WiFi line-rate

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KShared

Symmetric Key Protocol

KShared

“Bob to Alice at time T” Service Discovery Message

AT = address at time T

AT-1A0 ATHash()

KShared

AT+1Hash()

KShared

… …

Random hash function (e.g., HMAC-SHA1)

secret

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Protocol Design Summary• 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– Messages not for me are discarded at WiFi line-rate

• Thus:– Prioritize symmetric key protocol– Use spare cycles for public key protocol

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Outline



• Tryst– Access control for discovery messages provides 3 and 4

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Outline



• Tryst– Access control for discovery messages provides 3 and 4– Automated key establishment maintains 1 and 2

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How Do I Obtain the Initial Keys?

• Existing key establishment is not enough– Certificates: E.G., Secure websites

• Neither client nor service can offer proof of identity first!

– Pairing: E.G., Bluetooth peripherals• Can not always physically identify service • User must perform discovery before device does!

• 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

Trusted

?

x

xStrawman Solution

x

“Discover Alice’s iPod”

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?

New Services in Trusted Domains

“Discover Alice’s iPod”

Trusted

Trusts: [email protected]

“alice.ds”

“alice.laptop”

“bob.zune”

“bob.psp”“bob.laptop”

Anonymous Identity Based Encryption

“alice.ipod”

mailto:[email protected]

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New Services Transitively Trusted“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

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Outline




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Ongoing Work

• Status:– Created usable implementation of Tryst– Integrated with WiFi protocol stack on Linux

• Future work:– Evaluate how well key establishment mechanisms

reflect real trust relationships– Design privacy policies that users can understand

• More information:– Tryst: The Case for Confidential Service Discovery.

HotNets VI, 2007.

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Summary




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Backup Slides

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Related Work• SmokeScreen [Cox ’07] – access control for discovering friends

– Similar to symmetric key protocol– Uses online social network for key exchange

• SSDS [Czerwinski ’00] – secure service discovery architecture– Relies on trusted infrastructure– Not meant for use in wireless environments

• Broadcast Encryption [e.g., Fiat ‘93] –encrypt message to many users– Making this private is an open problem

• JFK [Aiello ’93] – efficient Internet key exchange– No service privacy …– … or not resilient to man-in-the-middle attacks

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Is the network“djw” here?

1 making local service discovery confidential with tryst jeffrey pang cmu ben greenstein intel...

Documents

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use service discovery

wifi discovery probes

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ftprivacy threats

beforeprivacy threats

wifi networkfind

wifi annoucements