The encryption happens on your end, not Tarsnap's.
The bar you're setting, though, is impossibly high. Can you absolutely, positively, definitely trust that your machine is not rooted and some nefarious entity isn't quietly collecting your every keystroke and snickering in the dark while stroking a white cat?
At that level of paranoia, you're probably best off using a device personally soldered together with hand-selected transistors that XORs all your backups with the white noise collected from your tv (while disconnected from cable, of course).
The bar you're setting, though, is impossibly high. Can you absolutely, positively, definitely trust that your machine is not rooted and some nefarious entity isn't quietly collecting your every keystroke and snickering in the dark while stroking a white cat?
At that level of paranoia, you're probably best off using a device personally soldered together with hand-selected transistors that XORs all your backups with the white noise collected from your tv (while disconnected from cable, of course).