Wouldn't you have to upload at least a read-only subkey of your tarsnap key not your ssh key. Github works because sshd only requires a copy of the public key to be stored to log in with the private key.

This would destroy the secure model of tarsnap. If you truly want this then don't use tarsnap use dropbox.

I was not trying to propose a "better" way - my understanding of tarsnap's security is deeply limited. Yet, I love how it is exposed as a scriptable command.

In my (very) limited knowledge - I am able to do retrieve backups on my machine using my key, which I supply to the command line. I was thinking of how it would work in a web based environment. Perhaps I could copy paste my key into a textbox, and I could retrieve my backups from a webpage. This would only last for the session (tarsnap would not store the key).

I know that you can think of a million ways that this can be exploited, but that is MY cross to bear. If I am prepared to do this, because I need immediate access to my backups from another machine - then please let me do this (basically, give me a way to access and create my backups in less than two steps).

I personally do believe that this would be a killer feature.