This is a write down of the Mozila Personas protocol (1) as if the messages were exchanged over instant messaging.

In the first exchange, a website (in this example example.com) asks for the browser for an assertion:

navigator.id.get(callback);

The browser takes it from there and switches to a private group with the user to figure out which IDP they'd like to use:

Knowing which IDP to use, the Browser uses a convention to figure out how to load the provisioning URL:

https://email.com/.well-known/browserid

Which points the user to:

https://email.com/browserid/provision.html,

The Browser then proceeds to chat with the IDP:

This conversation happens via newly introduced APIs:

 // get parameters of provisioning
 navigator.id.beginProvisioning(function(email, cert_duration) {

    // ... check if the current user is authenticated as 'email' ...
    if (notAuthenticated()) {
        navigator.id.raiseProvisioningFailure("user isn't authenticated");
        return;
    }

    // request a keypair be generated by browserid and get the public key
    navigator.id.genKeyPair(function(pubkey) {

        // ... interact with the server to sign the public key and get
        // a certificate ...
        someServerInteraction(function(cert){
            // pass the certificate back to BrowserID and complete the
            // provisioining process
            navigator.id.registerCertificate(cert);
        });
    });
 });

With a newly minted certificate, the browser takes it back to the chat with example.com:

And the user is now logged in.

This is a large oversimplification, but should give you a sense of the kinda of privacy property it has.

Specifically, the IDP is unaware of most of the interaction here, namelly:

• which relying party the user is signing-in to
• at which moment the user is signing in

References

• post mortem
• ben's perspective