On the topic of three parties.... I think this covers the case of authoritative sources of claims... and the possibility of proxy services...
"An identity information exchange should involve just three principal parties: the user, their agent, and a third party. The user’s agent is where they authenticate themselves and a repository where they store their identity information and the third party is a relying party requesting identity information or an authoritative source providing identity information. Non-principal parties may participate in an information exchange by providing facilitating services, such as proxying or caching."
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