Universal Device Pairing using an Auxiliary Device

1. Universal Device Pairing using an Auxiliary Device Nitesh Saxena, Md. Borhan Uddin and Jonathan Voris Polytechnic Institute of New York University SOUPS July 24, 2008

2. The "Pairing" Problem • How to bootstrap secure communication between two wireless devices when they have • No prior association • No common trusted third party • Examples • Pairing a Bluetooth cell phone with a headset • Pairing a WLAN laptop with an access point

3. Main Solution Idea • Utilize an Out-Of-Band (OOB) channel between the • devices • Created with human perceptible (audio, visual, tactile) output • The OOB channel is physically authenticatable • Place a minimal burden on device users • Usability is of extreme importance

4. Security Model • Devices are connected by two channel types: • An insecure, high bandwidth wireless channel • An authenticable, (typically) low bandwidth OOB channel • Adversary has complete control over the wireless • channel • Can eavesdrop on, delay, drop, replay, reorder, and modify messages • Adversary has a limited control over the OOB • channel • Can not modify messages, but can eavesdrop on, delay, drop, replay, and reorder messages

5. Secure with: • A weakly CR H() • An 80 bit permanent key • A 48 bit ephemeral key pkA pkB H(pkA) H(pkB) Prior Work • Seeing-is-Believing by McCune et al. [Oakland’05] • Based on protocol by Balfanz et al. [NDSS’02] Insecure Channel A B Authenticated Channel

6. An adversary can not succeed with • a probability greater than 2-k • k=15 offers reasonable security in practice pkA,H(RA,pad) pkB,RB RA,pad SAS Protocol • Short Authenticated Strings (SAS) pairing protocol • by Pasini-Vaudenay [PKC’06] Wireless Channel Unidirectional OOB Channel A B Accept (pkB,B) if Accept (pkB,A) if

7. Drawbacks of Prior Research • Geared for specific pairing scenarios • None are universally applicable • Require hardware and interfaces not common across all devices • User doesn’t know what method to use with what pair of devices  confusion! • We believe: universality would immensely improve security as well as usability

8. A Universal Pairing Method (1) Prasad-Saxena [ACNS’08] • Use existing SAS protocols • The strings transmitted by both devices over OOB channel are • the same, if everything is fine • different, if there is an attack or fault • Both devices encode these strings using a pattern of • Synchronized beeping/blinking • The user acts as a reader and verifies if the two patterns are the same or not

9. A Universal Pairing Method (2) • Usability? • Previous work has shown that human users are capable of efficiently performing • Blink-Blink • Beep-Blink • However, in practice users will commit mistakes • Due to a slight distraction, for example • Motivation for this paper: can we do better?

10. Success/Failure The Proposed Scheme • Automate the prior scheme based on manual • comparison • Utilize an Auxiliary Third Device (ATD) to perform the • comparison or Device1 Device2 ATD

11. Manual vs Automated or Device1 Device2 Manual Pairing using Blink-Blink or Audio-Blink or Device1 Device2 Success/Failure ATD Automated Pairing using Blink-Blink or Audio-Blink

12. Role of the User • When performing automated pairing, a user is responsible for • Pressing a button on one device to start pairing • Adjusting the ATD’s inputs to focus on the devices being paired • Pressing a button to activate the ATD’s receivers • Pressing a button on one device to start SAS transmission • Accepting or rejecting the pairing session based on the ATD’s output • Users do not have to remember what steps to take • The ATD will provide instructions

13. ATD Requirements • In the Blink-Blink setup, the ATD requires a camera as a receiver • For the Audio-Blink setup, the ATD requires a camera and a microphone as receivers • Both require a screen or speaker to output the pairing outcome • Today’s camera phones are suitable ATDs • The ATD does not connect over the wireless channel with the devices being paired • The ATD does not need to trusted with any cryptographic secret

14. Implementation • For testing, a laptop computer was used as an ATD • 2.0 megapixel, 30 FPS webcam • Devices being paired were simulated using a desktop computer • Visual output interface: LEDs connected via a parallel port • Audio output interface: Desktop speakers

15. Experimental Setup ATD’s receivers: Camera and microphone Overall setup Speaker used to simulate an audio output interface Laptop used as an ATD LEDs used to simulate visual output interfaces

16. Encoding Method • A ‘1’ SAS bit is expressed by activating the output • interface for a given signal interval • A ‘0’ SAS bit is represented by disabling the output • interface for the duration of the signal interval • Optimal intervals determined experimentally • Dependant on the ATD’s processing speed • Which output interfaces are used depends on which • pairing scheme is in use • In our experiments, we used a 15-bit SAS

17. Visual Data Processing/Decoding • Visual data was encoded using blinking LEDs • Signal interval: 250 ms • The ATD used saturation and luminance measurements to detect LEDs and capture their encoded SAS data • Overall transmission time: 4.5 seconds to transmit and capture 18 frames • 15 data frames • 3 control frames: All-OFF, All-ON, SYNC

18. Audio Data Processing/Decoding • Audio data was encoded as spoken English words using the Microsoft Speech API (SAPI) 5.0 Text-To-Speech engine • Signal interval: 400 ms • The ATD captured the audio data via a microphone and decoded it using the SAPI Speech Recognition engine • Overall transmission time: 7.2 seconds

19. Usability Testing • Schemes tested with 20 subjects • The same tests were performed with the manual and • automated setup • Each subject was presented 24 test cases • 20 reliability tests for the Blink-Blink and Audio-Blink schemes • 4 tests for the robustness of the ATD • Test goals: • Determine if the ATD could be used to reliably pair devices • Determine which scheme: • Demonstrated the least amount of errors • Safe errors or false positives, and • Fatal errors or false negatives • Users qualitatively preferred

20. Usability Testing Results Results of Automated Comparison Tests Results of Manual Comparison Tests • 80% of the subjects (16 out of 20) preferred the automated scheme • 20% of the subjects (4 out of 20) preferred the manual scheme.

21. Discussion (1) • Results indicate that the use of an ATD makes the • pairing process safer and less burdensome • No fatal errors • Reduced safe error rate • The higher safe error rate of Audio-Blink is attributable • to the ATD picking up background noise • The ATD’s audio robustness is expected to improve when implemented on a smartphone as opposed to the current proof-of-concept • Users of this scheme must be sure of the origin of the SAS • audio to guard against attacks

22. Discussion (2) • Whether the ATD is a help or hindrance in terms of • speed is dependant on its decoding rate for a • particular setup • Blink-Blink: Automated is faster than manual due to the fast visual decoding process • Audio-Blink: Automated is slower than manual due to the relatively slower audio decoding process

23. Conclusion • Both the manual and automated schemes are universally applicable to any pairing scenario • Use of an ATD is not mandatory, but test results show it increases usability when available • An ATD can handle SAS data that is longer than what a human user can

24. Thank you!