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.