I'm thinking of using Tarsnap. Can I absolutely, positively, definitely trust that everything on Tarsnap's end is encrypted to best practice standards and that there is no reasonable way to get to my data (outside of the usual contract provided by encryption I mean)?
I don't have the option to know for sure by analyzing the source code myself so I'll have to trust the popular opionion of Very Smart People here on HN (well, I suppose I could if I spent a non-trivial chunk of the coming year reading up on crypto stuff).
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).
I don't have the option to know for sure by analyzing the source code myself so I'll have to trust the popular opionion of Very Smart People here on HN (well, I suppose I could if I spent a non-trivial chunk of the coming year reading up on crypto stuff).