Yet Another Delegation Thread

[Recordon, David]

So been going through all of this up in Seattle with Drummond and think
I fully have my head around this.

Thinking we have the following cases, which Draft 10 basically already
addresses.  In any of the responses, the IdP MAY return a differing
value for "openid.identity" than the RP requested.  This obviously has
varying degrees of usefulness depending on the specific situation.  See
below for "rules" RP must follow to protect itself from bogus
assertions.

1) IdP Registered
	a) Entered http://user.myidp.com   (IdP implicitly knows it
owns)
            <URI> -> http://myidp.com/server.cgi

         Request
            openid.identity -> http://user.myidp.com
         Response
            openid.identity -> http://user.myidp.com

	b) Entered http://user.example.com (IdP has an out of band
"registration" process where it verifies via discovery)
            <URI> -> http://myidp.com/server.cgi

         Request
            openid.identity -> http://user.example.com
         Response
            openid.identity -> http://user.example.com

2) Delegated (IdP knows nothing about what the user entered)
	a) Entered http://user.example.com
            <URI>             -> http://myidp.com/server.cgi
            <OpenID:Delegate> -> http://user.myidp.com (IdP Controlled)

         Request
            openid.identity -> http://user.myidp.com
         Response
            openid.identity -> http://user.myidp.com

	b) Entered =user.example (or @2idi for Directed Identity case)
            <URI>             -> http://myidp.com/server.cgi
            <OpenID:Delegate> -> http://user.myidp.com (IdP Controlled)

         Request
            openid.identity -> iNumber (LocalID ? <LocalID /> :
<CanonicalID />)
         Response
            openid.identity -> iNumber

3) Directed Identity
      Entered http://myidp.com (IdP Registered)
            <URI>  -> http://myidp.com/server.cgi
            <Type> -> http://openid.net/identifier_select/2.0

      Request
            openid.identity -> http://openid.net/identifier_select/2.0
      Response
            openid.identity -> http://user.myidp.com (IdP Registered,
though not necessarily on same domain)


I would argue, that this actually accomplishes what is needed; providing
the following rules:
1) Before starting a transaction, the RP MUST validate that the IdP is
authoritative for the URI it is requesting an assertion about through
the discovery process.

2a) If the RP ever receives a response value from IdP differing in the
URI assertion it requested, it MUST validate that the IdP is
authoritative for that URI via the discovery process.  This is due to
that the IdP's response value for "openid.identity" does not have to be
the same as the RP's request value in any of these three cases.

2b) In the Directed Identity case, the RP MUST always validate that the
IdP is authoritative for the URI returned in its assertion via the
discovery process.

So the one time that an additional parameter is useful, is in the
Delegated Case where it would be useful to the IdP to know what the user
entered at the RP.  This has certain privacy implications, though most
of the data is public to begin with in terms of the HTML/XRDS semantics.
There also is the concern that since the IdP is signing the response, it
would have to also check that it is authoritative over this other
parameter.  If this is still a design goal, I'd propose we add
"openid.display" (or whatever we want to call it) which the RP sends in
the request solely to aid the IdP when interacting with the user.  The
IdP would then still make an assertion about the value of
"openid.identity" and thus not return "openid.display" in the response.
This thus continues placing the burden on the RP to verify all of the
assertions made in the protocol.

In the end this remains backwards compatible with 1.x, though also
broadens the delegation model as Josh and Mart have previously
described.  This also places no additional burden on the IdP, since in
every case the RP is responsible for verifying that the IdP is
authoritative for the assertion made.  While the IdP should not make
assertions that are not true, at the end of the day the RP should be
verifying the validity of every assertion in any case to protect itself.

--David