It can be useful to know who the Relying Party (RP) is during the discovery 
phase. That is, the RP should state their identify when they are looking up a 
user’s OpenID URL (Claimed Identifier).


Use case: Alice wants to use different OPs for different RPs, while keeping the 
same URL (eg For instance, when logging into a 
service hosting her backups she wants to use an OP that requires a one-time 
password from a hardware token for each access. However, when leaving comments 
on blogs Alice wants to authenticate using an OP that only requires a password 
and uses a persistent cookie so she only has to log in once a day.


Problem: Only one OP can be specified with a <link rel="openid2.provider"…/> 
element or in a Yadis document.

[A Yadis document may be able to list many OPs, but I don’t think there is any 
mechanism for the RP to pick the right one.]


Solution: The RP could include a From HTTP header when performing discovery.

Instead of serving a static HTML page (or Yadis document) at, the page could be dynamically generated based on the 
value of the From header.


Suggested text for the authentication spec (draft 12):

Add the following paragraph at the end of section 7.3 Discovery:

“The Relying Party MUST include a From HTTP header field in each HTTP request 
made during discovery. The From field holds an email address for the RP (eg 
From: [EMAIL PROTECTED]) [RFC2616]. This enables the discovered information to 
vary based on the RP. The From field is not authenticated so it is not 
appropriate to use for access control.”


Other solutions:

The source IP address of the discovery request will often identify the RP, but 
this would be an unreliable mechanism due to proxies, clusters, load balancing, 
and changes at the RP.

Separate user-supplied identifiers could be used, but that unnecessarily 
complicates the system for users.

OPs can offer different authentication mechanisms based on the openid.return_to 
or openid.realm parameter in an authentication request. However, the user has 
less flexibility when they have to relying on OPs.


James Manger

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