I would vie for a much radical solution. What do we really want ? - That a website could correlate our successful visits if we wish to, in order to present us the settings / data we saved - That two different web sites may not correlate our accounts (well, there is IP... but there are proxies) - That this identity could be shared freely between different devices, but be tied to a single individual I don't really think that we need a passport or passphrase for this. And as much as people tout it, I don't think that PGP is what we want either: PGP gives only a handful identities. I think the one person to solve this problem will be hailed for generations... and those who fail will be forgotten (at best).