Seamless, secure, private, collaborative file synchronization across trust boundaries, typically as a companion to a store and sync file service. Information needed to recover a file is stored within the file itself, without giving away secret data. User specific personal keys are preferably only stored on the users' device(s). A unique ID is also created for each protected file; a password is generated that depends on (a) a key value that can either be (i) the user's personal key in the case of a file that is to be private or (ii) a shared key in the case of a file that is to be shared with other users, and (b) the unique file ID. The password is then encrypted using a recovery key and also stored in the file itself. The file is secured using a format that supports password-based content encryption.