We have an existing SP called S1 that authenticates to multiple Idps. We have another service called S2, I want S2 to delegate authentication to S1 then to Idp, then back and notify S2 when authentication succeeds. Its important to note that S2 has no connection to any Idps, it relies solely on S1.
In a sense this is a service-to-service authentication, is there an existing mechanism on SAML2 to achieve this? I can also make S2 an SP so it becomes an SP-to-SP if it makes things easier.