Less than two months later, despite all the points I listed in favour of them in Paper SSH & GPG key backups, I’ve destroyed mine and am now just using flash drives. The reason is simple. To ensure availability (which is what the whole exercise is about) I had placed paper key backups in the glove box of my car. But I recently realised that for such locations paper has a big drawback.
To most people who might find them (so anyone who accessed my car, with or without my permission & knowledge), it would have been obvious the paper backups were valuable information of some sort. What might someone who discovered them do? They might take pictures and (for example) Tweet them/insert-alternative-social-network-verb-here-them. So there’s then a (potentially large) audience trying to work out what my backups are. Granted, I password protect both of my keys, but I only see that as a weak final-defence that buys me a small amount of time to revoke and replace a given key when I suspect it may be compromised.
I believe the discoverer of a flash drive in my car would be less likely to upload its contents for the world to puzzle over. Also, using a block device allows me to conveniently add an extra layer of encryption (LUKS) – so the content is totally opaque. Obviously I’m not trying to protect against an attack targeted at me here – only against accidental key compromise by an inquisitive discoverer.