I took a look https://www.tarsnap.com/gettingstarted.html and I don't understand why "If you have multiple machines, you almost certainly want to create a separate key file for each machine."? Can you explain why, if I'd like to access the same data from the different machines? Or is the main assumption that every machine has its own, nor shareable, backup? Isn't the main advantage of an online service to have the same data accessible from more machines?
Tarsnap isn't Dropbox. It's a backup system. Its cost structure and security model is optimized for backup, which is why you can't e.g. read your Tarsnap files from a web interface at Tarsnap.
