Tarsnap is an online encrypted backup service. It presents a tar-like
command-line interface, but stores data online rather than locally;
using ideas taken from the author's FreeBSD Update and Portsnap
utilities, it maximizes performance by recognizing duplicate data and
only storing it once, and cryptographically encrypts and signs archives
using locally-held keys in order to guarantee that nobody without access
to the key file (including the author) can read or modify archives.