This would be a locally hosted service that could be used to encrypt the documents under a designated folder on the fly. Whenever the documents are modified, the service would encrypt it again and copy over to a cloud folder if specified. In the initial version we would not do in-place encryption, and just coipy over the encrypted files the cloud folder.
In later revisions, we would do in-place encryption and also provide UI to launch windows/native applications to open the files.
Later we would release mobile application to do the same on various mobile platforms