Then, the system determines whether the user successfully responded to the challenge (step 610). If so (YES at step 610), the client authenticates the user (step 612). Otherwise, if the user does not successfully respond to the challenge (NO at step 610), the client does not authenticate the user (step 614). Also, if the security score of a user drops below or goes above the threshold, the query component can actively push notifications about the change to registered clients.

Triggering Collection of Sensor Data

The disclosed embodiments provide a system that continually determines the identity of a user through sensor data gathered from one or more devices associated with the user. During operation, the system opportunistically collects sensor data from the devices associated with the user while maintaining a low resource footprint, wherein the sensor data is processed to facilitate verification and authentication of an identity of the user. In this way, the system can determine whether the registered user of a device is the one currently using the device, without significantly impacting battery life on battery-powered devices, such as smartphones, wearable devices, laptops, and Internet of Things (IoT) devices.

To achieve this result, the system carefully controls when and what kind of data is collected. Some of the associated techniques for triggering data collection on portable devices include the following.

• • Periodic silent push notifications (at constant or variable intervals) to “wake up” data collection on mobile devices.