Proximity-Based Authentication of Mobile Devices Eyal de Lara Department of Computer Science University of Toronto Alex Varshavsky, Adin Scannel, Anthony LaMarca

Secure Spontaneous Interaction Phone + hotel room TV and keyboard Exchange of private info Phone and hands free Paying for groceries, tickets, cola

Naïve Solution Diffie-Hellman a Alice b Bob

Naïve Solution Diffie-Hellman a Alice b Bob g, g a

Naïve Solution Diffie-Hellman a Alice b K  g ab Bob g, g a

Naïve Solution Diffie-Hellman a Alice b K=g ab Bob g, g a gbgb

Naïve Solution Diffie-Hellman a K=g ba Alice b K=g ab Bob g, g a gbgb

Who is my device really communicating with? The Problem

Who is my device really communicating with? Spoofing The Problem a Alice b Bob

Who is my device really communicating with? Spoofing The Problem a Alice b Bob x X

Who is my device really communicating with? Spoofing The Problem a Alice x X

Who is my device really communicating with? Spoofing The Problem a Alice x Bob

Who is my device really communicating with? Spoofing The Problem aK=gxaaK=gxa Alice x K=g ax Bob g, g a gxgx

Who is my device really communicating with? Spoofing Man in the middle The Problem a Alice b Bob x X

Who is my device really communicating with? Spoofing Man in the middle The Problem a K 1 =g xa Alice b K 2 =g xb Bob g, g a gxgx x K 1 =g ax K 2 =g bx X g, g x gbgb

Who is my device really communicating with? Spoofing Man in the middle Solution: Ensure communication with device that is close  Assumption: attacker is not between legitimate devices The Problem a K 1 =g xa Alice b K 2 =g xb Bob g, g a gxgx x K 1 =g ax K 2 =g bx X g, g x gbgb

Existing Solutions Use a cable Use short range communication  Bluetooth  Infrared  Laser  Ultrasound  Near field communication (NFC) Ask user to verify pairing  Displaying keys  Playing music, images

Existing Solutions Use a cable Use short range communication  Bluetooth  Infrared  Laser  Ultrasound  Near field communication (NFC) Ask user to verify pairing  Displaying keys  Playing music, images BlueSniper Rifle by Flexis

Key Idea Secure pairing requires a shared secret Devices in close proximity perceive a similar radio environment Derive shared secret from common radio environment  Listen to traffic of ambient radio sources Use knowledge of common radio environment as proof of proximity

Advantages No extra hardware  Leverage radio already available on device No user involvement to verify pairing Not subject to eavesdropping  Secret derived by listening to ambient sources

Requirements on Radio Environment 1.Temporal variability Signal fluctuates randomly at a single location over time

Requirements on Radio Environment 2.Spatial variability Values at different locations have low correlation

Requirements on Radio Environment 3.Devices in proximity should perceive similar environment 5 cm 10 m 85% common pkts40% common pkts

Potential Authentication Methods Proximity-based authentication token  Diffie-Hellman  Authenticate using the token Proximity-based encryption keys  Directly from the common environment  Less CPU intensive?

Amigo: Diffie-Hellman + Proximity Token Devises monitor radio environment following Diffie-Hellman key exchange Send to each other a signature Each device verifies that signature similar to own observation  Signature does not have to remain secret after exchange is over

Signature Verification Signature: sequence of hash of packet + RSSI Segment size 1 second

Classifier 2 stage boosted binary stump classifier Stage 1: Filters noisy data  Marks as invalid instances with % of common pkts bellow threshold (75% works well) Stage 2: Assigns a score to valid instances  Function of differences in signal strength  Converts scores into votes based on threshold  Tally votes for all instances

Commitment Protocol Reveal man-in-middle attack while exchanging signatures Forces attacker to forge data Break signature S into n blocks Generate nonce Each period exchange K nonce ( Hash (K session_key ),Hash(id),s i ) Send nonce a K 1 =g xa Alice b K 2 =g xb Bob K nA (H(K 1 )H(A)S i ) x K 1 =g ax K 2 =g bx X K nB (H(K 2 )H(B)S i )

Scenario 1 : Simple Attacker 6 laptops  Friendly 5cm away  Attackers 1,3,5,10 meters WiFi – Orinoco Gold All at same height Line of sight 1m 3m 10m 5m Best case for attacker

Traces 2 traces: training and testing  2 months apart  2 different location in the lab 10 minute trace 30 – 50 thousand pkts per laptop 11 access points 45 – 58 WiFi radio sources

Simple Attacker Can pair within 5 seconds Can detect attacker 3 meters away or more 1 meter is a problem

Local Entropy: Obstacles False Positives Line-of-sight (1m)81% Drywall (10cm)100% Human (1m)12% Concrete wall (30cm)0% Human blocking attacker’s line of sight goes a long way to improve performance

Local Entropy: Movement Hand waving helps!

5 laptops  Friendly 1 m away  Attackers 3,5,10 meters All at same height Line of sight Stretching Co-Location 1m 3m 10m 5m

Stretching Co-Location

Scenario 2 : Attacker with Site Knowledge Before pairing  Attacker samples exact pairing spot  Creates RSSI distribution for every wireless source it hears While pairing  Pkts from know source  assign RSSI from distribution  Pkts from unknown source Option 1 Discard Option 2 Leave unchanged(best)

Scenario 2 : Attacker with Site Knowledge With hand waving false rate positives reaches 0 within 5 seconds

Scenario 3: “Omnipotent” Attacker Controls all radio sources  Knows which pkts were received by victim Oracle: RSSI from current distribution

Conclusions Possible to use knowledge of radio environment to prove physical proximity Advantages  No extra hardware  No user involvement to verify pairing  Not subject to eavesdropping Two potential methods  Location-based authentication token  Location-based encryption keys

Future Work System robustness  Different cards and antennas  Different environments Improve accuracy  Software radios  Multiple radios Proximity-based encryption keys

Questions? Eyal de Lara Varshavsky, Scannell, LaMarca, de Lara “Amigo: Proximity-based Authentication of Mobile Devices” 9th Int. Conference on Ubiquitous Computing (UbiComp) Innsbruck, Austria, Sep. 2007