That's why the Tarsnap key file format includes a checksum on each line, so tarsnap can tell you which line to look for the OCR (or typing) error on.
When I was writing this code I wasn't sure if it would ever matter but figured "hey, why not..." -- but I've probably had a dozen emails since then from users thanking me for including those.
It's base64, but with added checksums, yeah. There's very little that I use off-the-shelf in Tarsnap -- generally speaking I take existing standards and then make tweaks as needed to optimize for Tarsnap's use case. (Sometimes, as with scrypt, I decide that existing standards simply aren't any good and develop my own from the ground up.)
