Simply printing stuff out onto alkaline/archival paper and sticking it in a vault still remains in much of the world an important part of retention/preservation schemes and contracts (and in some jurisdictions/cases may even be a regulatory requirement). Properly stored quality paper can have a lifespan of 500-1000 years, can have some favorable tradeoffs in terms of reader future-proofing and upkeep vs purely digital storage, etc. No reason not to want to take advantage of signing/encryption just because it's paper, or be able to easily retain important messages in their native format.
You could argue that this doesn't need to be part of a message format itself because it'd be perfectly possible to write an intermediation layer for print that simply translates arbitrary input to an OCR favorable output and back, and maybe that'd make sense anyway if there are other desirable choices for properties like parity unique to print/archival/OCR. Still, to the extent that a format base choice is arbitrary and makes no particular difference to the humans or computers involved since they'll be intermediating through software anyway, better OCR properties doesn't seem like an entirely unreasonable metric to consider as part of the design considerations if there isn't a compelling reason otherwise.
Isn't that situation a good place to use QR codes? It might be a large code (im not sure the limitations of QR codes) but it would eliminate the need for OCR. Nobody is going to be decrypting it by hand so you don't really need letters that are human readable. The base62 form works well for posting online but for printing QR seems like a good format to me
So QR codes are great as part of a designed workflow where the reading app is well understood. The challenge comes in more ad hoc scenarios.
Also QR codes hit a certain practicality limit with size (2953 bytes to stay in spec)
Perhaps a meta-point is that when you are trying to design a general purpose interchange format, there will always be scenarios that you didn't imagine. In this case I have raised OCR and (legitimately) many people's responses have been a rather polite WTF (although I did garner one downvote). Experience teaches us that formats will be used in unexpected ways.
I freely admit that it's a stretch, but I'd rather scan/photograph a public key than try and copy it by hand (which base32 is also better at). And I won't put a USB device of unknown provenance into a device I use. So sometimes pictures and or printouts are a good way of sharing data.
It also occurs to me that OCR implies a computer, but it could just as legitimately mean a human reading text
If you're sharing this stuff optically, you'd almost certainly be better off with something like a big QR code. QR can be scanned at angles, has built-in error correction, and (depending on the font you're comparing it against) can probably pack in more bytes. OCR feels more like a hack than anything; you're using a human-readable format with an error-prone medium to communicate non-human-friendly data between two machines!
I don't see it either. I could see someone printing their backup key, and then scanning it back in to their system, but the data is authenticated so OCR flipping characters would be detected -- also if you're doing that you're better off using a QRcode with native ECC.
