In an implicit authentication system, a user profile is used as an additional factor to strengthen the authentication of mobile users. The profile consists of features that are constructed using the history of user actions on her mobile device over time. The profile is stored on a server and is used to authenticate an access request originated from the device at a later time. An access request will include a vector of recent features measurements on the device that will be matched against the stored features to accept or reject the request. The features however include private information such as user location or web sites they have visited. In this paper we propose privacy-preserving implicit authentication which achieves implicit authentication without revealing unnecessary information about the users’ usage profiles to the server. We propose an architecture, give formal security models, and propose constructions with provable security. We consider two security models, namely for cases where the device behaves semi-honestly or maliciously.