Proposal: Chrome privacy CA

Hi folks,

Please see attached a proposal from Google regarding the "Privacy CA" model
that Chrome will be adopting. The idea is to open this up for discussion
(maybe on the call today, but definitely at TPAC next week).

Please note that this document is a WIP, but I wanted to make sure we give
everyone an early glimpse into our thinking so we could refine the proposal
as we go along while making sure we have the necessary plumbing in WebAuthN
to support this model.

I'll also be cross-posting this to the FIDO2 TWG.

Regards,
Christiaan