Defending against device theft with human notarization

People increasingly rely on mobile phones for storing sensitive information and credentials for access to services. Because these devices are vulnerable to theft, security of this data is put at higher risk-once the attacker is in physical possession of the device, recovering these credentials and impersonating the owner of the phone is hard to defend by purely local means. We introduce the concept of `notarization', a process by which a remote notary verifies the identity of the device user through video chat. We describe the design and implementation of a system that leverages notarization to protect cryptographic keys that the device uses to decrypt device data (e.g., website passwords) or perform signatures in support of client-side TLS, without trusting the notary with these keys. Through a lab-based study with 56 participants, we show that notarization even by strangers is effective for combating device theft.