I did not know about Public Key Bot. That’s really useful, actually. A TOFU bot would presumably also be possible; I think it’s basically the same as a public invitation bot. Posting a QR code publicly is also TOFU.
A shared-secret key exchange protocol has also been suggested, and is used in some other messengers.
So to update my old summary of key-exchange methods:
- scanning keys in person - EASY
- through a common contact - EASY
- looking up keys on keyservers - VIA BOT
- trust-on-first-use - VIA BOT
- Shared secret - VIA BOT?
- manually exchanging key files (.asc or VCard) or QR codes or invite links
6a. including downloading them from keyservers and sending them in plaintext, even over horribly insecure networks
Six is moderately easy but not obvious in the UI.
It makes sense to favour the method that is easiest to do securely, and steer users to it. There are those who need another method, usually because they can’t physically meet friends, but they seem to be a minority. We could think about what the UI should steer them to (since I think “screenshot my QR and send it, often insecurely” is the current default).
Other methods are not hard to do, but not obvious in the UI. They require that the user understand them.