To some extent, this is similar to SSL/TLS certificates that can be used to authenticate the user (not just the server). If I recall correctly, http://startssl.

To some extent, this is similar to SSL/TLS certificates that can be used to authenticate the user (not just the server). If I recall correctly, http://startssl.com/ http://startssl.com/ does that. It's not a very intuitive mode of authentication, but if the UI was improved, and combined with a sync service (& encrypted with a passphrase), I guess it could be usable? (also, presumably it requires the site to use https, but that's also much more expected today than it was 10 years ago)