What you mean is not "passphrase keys" but "public/private keypairs", and it sounds like you are using an agent as proxy, so you only have to authenticate local

What you mean is not "passphrase keys" but "public/private keypairs", and it sounds like you are using an agent as proxy, so you only have to authenticate locally once. This isn't an answer, but: password managers like LastPass can do this for the web. I let it generate secure passwords like "e^9u21$Fb%mJv"--in fact I don't know most of my passwords, it logs me in automatically, I just authenticate once with a passphrase. Highly recommended (integration with most browsers, other security features, see the website); definitely worth a look for the interface considerations alone. The answer: it's probably too hard to get there from here now. Too many changes to UI's, re-education of users, and really they don't care if you have to type in a password every time or click an extra page. Look at the popularity of decentralized private/public key trading with (e.g.) PGP. NOT. Sure, the paranoid elite have learned how to use the keyservers and mixmaster remailers, but there needs to be a killer UI that's simple enough (or maybe just smarter than the user) before people.. will... post to facebooks with it? Client SSL certificates were supposed to be a thing once back in the 90's--everyone would have one, it would identify you to your bank uniquely, and everyone would be secure. I'm pretty sure the protocol still allows for it, but I think I only saw one website use/require it, and not sure it was this century.