Android Remote Unlocking Service using Synthetic Password: A Hardware Security-preserving Approach

Remote unlocking for Android devices may benefit both users and manufacturers. Users can continue using the device without factory-resetting when they unexpectedly forget their passphrases. Manufacturers can improve non-face-to-face customer services in the COVID-19 era. Nevertheless, not many manufacturers support remote unlocking services for Android devices. If the remote unlocking service is triggered by requests over-the-air, it may increase the attack surface of Android security. Android security is hardware-based (e.g., hardware-backed Keystore), so we seek to preserve this security level by designing a new remote unlocking service without modifying trusted execution environments. Our design supports two-factor authentication, distributed authority, trust-boundary minimization, and key management. Since a synthetic password used for remote unlocking is not exposed to the outside of an Android device, the manufacturer still cannot unlock the device without user consent. We identify 208 security threats in the proposed remote unlocking service using the STRIDE model and ensure that our design has countermeasures for all high-level security threats. After passing quality verification and penetration tests, the proposed remote unlocking service has been officially installed on commercial devices