Towards Energy-Efficient Privacy-Preserving Active Authentication of Smartphone Users

Common smartphone authentication mechanisms such as PINs, graphical passwords, and fingerprint scans offer limited security. They are relatively easy to guess or spoof, and are ineffective when the smartphone is captured after the user has logged in. Multi-modal active authentication addresses these challenges by frequently and unobtrusively authenticating the user via behavioral biometric signals, such as touchscreen interaction, hand movements, gait, voice, and phone location. However, these techniques raise significant privacy and security concerns because the behavioral signals used for authentication represents personal identifiable data, and often expose private information such as user activity, health, and location. Because smartphones can be easily lost or stolen, it is paramount to protect all sensitive behavioral information collected and processed on these devices. One approach for securing behavioral data is to perform off-device authentication via privacy-preserving protocols. However, our experiments show that the energy required to execute these protocols, implemented using state-of-the-art techniques, is unsustainably high, and leads to very quick depletion of the smartphone's battery.
This research advances the state of the art of privacy-preserving active authentication by devising new techniques that significantly reduce the energy cost of cryptographic authentication protocols on smartphones. Further, this research takes into account signals that indicate that the user has lost possession of the smartphone, in order to trigger user authentication only when necessary. The focus of this project is in sharp contrast with existing techniques and protocols, which have been largely agnostic to energy consumption patterns and to the user1s possession of the smartphone post-authentication. The outcome of this project is a suite of new cryptographic techniques and possession-aware protocols that enable secure energy-efficient active authentication of smartphone users. These cryptographic techniques advance the state of the art of privacy-preserving active authentication by re-shaping individual protocol components to take into account complex energy tradeoffs and network heterogeneity, integral to modern smartphones. Finally, this project will focus on novel techniques to securely offload computation related to active authentication from the smartphone to a (possibly untrusted) cloud, further reducing the energy footprint of authentication. The proposed research will thus make privacy-preserving active authentication practical on smartphones, from both an energy and performance perspective.
This project is funded by the National Science Foundation.

HMOG: Hand Movement, Orientation, and Grasp

Hand Movement, Orientation, and Grasp (HMOG) is a set of behavioral features to continuously authenticate smartphone users. HMOG features unobtrusively capture subtle micro-movement and orientation dynamics resulting from how a user grasps, holds, and taps on the smartphone. In this project, we evaluated authentication and biometric key generation (BKG) performance of HMOG features on data collected from 100 subjects typing on a virtual keyboard.
Data was collected under two conditions: sitting and walking. We achieved authentication EERs as low as 7.16% (walking) and 10.05% (sitting) when we combined HMOG, tap, and keystroke features. We performed experiments to investigate why HMOG features perform well during walking. Our results suggest that this is due to the ability of HMOG features to capture distinctive body movements caused by walking, in addition to the hand-movement dynamics from taps.
With BKG, we achieved EERs of 15.1% using HMOG combined with taps. In comparison, BKG using tap, key hold, and swipe features had EERs between 25.7% and 34.2%. We also analyzed the energy consumption of HMOG feature extraction and computation. Our analysis shows that HMOG features extracted at 16Hz sensor sampling rate incurred a minor overhead of 7.9% without sacrificing authentication accuracy.
This project is funded by DARPA.
The paper reporting on HMOG was published in the IEEE Transactions on Information Forensics and Security, and is available here. The team's work on HMOG was also presented as a poster at SenSys 2014. The poster is available here.
The HMOG dataset can be downloaded from here.