Syncthing is a decentralized continuous file synchronization software with many security features, like end-to-end encryption, device authentication, permission management and more. Both the protocol specification and the Syncthing software are free and open source.
With Syncthing files can be easily kept in sync across multiple devices, e.g. one desktop PC, one notebook and several mobile devices, *without* relying on third-party infrastructure (optional). File syncing can be limited to a local Wifi network, avoiding sending files to the outside world altogether. Here’s how.