Howto fix the problem:

Tarsnap seems like a well-engineered system, from reading the author's blog, but as far as I know
the server-side code is proprietary. The Tahoe-LAFS project (I'm a contributor) has excellent
encryption and erasure-coding features and if you like duplicity you can use Tahoe-LAFS as a
backend for duplicity. Also Tahoe-LAFS comes with its own integrated backup system which has
different trade-offs than the duplicity backend. (Duplicity does deltas for you, but you can't view or
download your files without going through duplicity. The Tahoe-LAFS integrated backup doesn't do
deltas, but it stores the files in a time-machine-style layout which you can browse and download
through the web.)