The present disclosure relates to managing user accounts in one or more computer systems. In general, one aspect of the subject matter described in this specification can be embodied in methods that include the actions of storing on a client computing device a first cookie containing plaintext representations of one or more user identifications, and a second cookie containing encrypted versions of the one or more user identifications; updating the cookies in coordination with each other based on communications received from a central server system in response to actions by a user of the client computing device to log on or off of accounts managed by the computer server system; and mediating user access to data stored locally on the client computing device by using the first and second cookies and determining whether the data is associated with one of the user identifiers.