How to change the issuer of an assertion

[Daniel Lutz]

For our upcoming SWITCH edu-ID system, we need a method to change the issuer
of an assertion after authentication, presumably in an intercept flow.
The issuer to use depends an a choice by the user, i.e. is not known
before this intercept flow is run.

For example, while the main entityID of the IdP is "https://idp.example.org/idp/shibboleth"
(as configured via the property "idp.entityID"), the issuer of the assertion should be
"https://idp.uni-a.org/idp/shibboleth" according to the choice by the user.

I haven't yet been able to find how to do this. May you give me a hint, please?

First, I tried to set the "responderId" of the current RelyingPartyConfiguration
(got from the RelyingPartyContext), but that's not allowed. I'm not yet familiar
enough with the whole RelyingParty configuration to find out myself.

In the default configuration, all RelyingPartyConfigurations have a fixed
responderId.

Can I use some available "Lookup" strategy? How would I configure multiple
available responders with their entityID and corresponding credentials?

Thanks for some hints.

-- Daniel