To some extent, this is similar to SSL&#x2F;TLS certificates that can be used to authenticate the user (not just the server). If I recall correctly, <a href="http:&#x2F;&#x2F;startssl.com&#x2F;" rel="nofollow">http:&#x2F;&#x2F;startssl.com&#x2F;</a> does that.<p>It&#x27;s not a very intuitive mode of authentication, but if the UI was improved, and combined with a sync service (&amp; encrypted with a passphrase), I guess it could be usable? (also, presumably it requires the site to use https, but that&#x27;s also much more expected today than it was 10 years ago)