I wasn't sure on the implementation of that bit (the code's open but I haven't read it). So I asked in the irc channel:
17:02:04 : my understanding (I didn't write sideshow) is that you login to google, we verify the openid token is valid, then the persona google identity bridge issues assertions for persona logins.
17:02:22 : so, google can see that you logged into the persona bridge, but not individual RPs [e.g. Microcosm] that you logged into after that with persona
So Google would know you're using Persona but not the site you're logging in to.
I wasn't sure on the implementation of that bit (the code's open but I haven't read it). So I asked in the irc channel:
So Google would know you're using Persona but not the site you're logging in to.
Also: http://identity.mozilla.com/post/57712756801/persona-makes-signing-in-easy-for-gmail-users