Is it possible to sync files between devices which are rarely if ever turned on at the same time? I could run a caching relay but would prefer it to be “dumb.” That is, it shouldn’t have access to the files being synchronized.
I use two laptops alternately which are almost never both online or turned on at the same time. I would like to synchronize files between them. Some of these files are sensitive, including the password store. Technically, it’s encrypted but even the metadata (file structure for example) is rather revealing.
Is this use case covered or too exotic?
No, you need devices to be online to sync. Relay does not hold data, it’s just there to help connect, so no, this usecase is not covered and it’s not a usecase syncthing is made for. You are probably better of with a centralised offering, dropbox et al.
Personally I have “solved” this use case by using my NAS (powered 24/7) as a third device for backup purposes. Therefore, when A is on but B is off, A syncs with the NAS, and when A is off but B is on, it gets the data from the NAS.
I don’t know if this can be considered as the “caching relay” you referred to, but it’s true that the data sits on my NAS unencrypted. In any case, I don’t see how you could solve this without using a third device. So if your concern is privacy, your only chance would be to ensure that data on that third device is always encrypted. Maybe use something like Cryptomator. I’ve never used it along with Syncthing, but already did with Google Drive and it worked fine IIRC and I don’t see why it wouldn’t work with Syncthing.
I would be interested in knowing how it turned out for you if you try it with Syncthing
I’ve a pretty similar use case with devices that are often not online at the same time. I’m also running a central device, in my case it’s a private server which is reachable from pretty much everywhere. It’s simply storing the data which is synced between devices, to have it available at all times.
For your use case I would recommend a similar setup - if your bandwith allows it, you could host it at home (a small raspberry pi with an USB disk attached could do it), otherwise you could go for some (trusted) VPS or so. Maybe encrypt the disk too if that gives you a better feeling.