Keybase is the closest I've seen to solving the key exchange problem, with their scheme for verifiably tying keys to user identity and user identities to users'

Keybase is the closest I've seen to solving the key exchange problem, with their scheme for verifiably tying keys to user identity and user identities to users' online presences (websites, social media, etc.). Unfortunately, it seems that key exchange alone isn't enough to build a sustainable business on, and, after attempting to be both Slack and Dropbox, they ended up getting aquihired by Zoom to do something only tangentially related. So the product's mostly stagnated.