What is an identity federation (Identitätsföderierung)?

An identity federation links the identity systems of different organisations or domains so that subjects can be authenticated and authorised across system and organisational boundaries; it requires the domains to trust each other in defined respects, backed by explicit and implicit agreements (SLAs).

The model has three entities, subject, relying party and identity provider; the subject always communicates with both, authenticates at the IdP, and the event is passed to the RP as an authentication assertion. Federation protocols are SAML, OAuth 2.0 and OpenID Connect. In e-government a public authority can act as relying party and as IAM service provider at the same time.

Go deeper:

• Federated identity (Wikipedia) — linking identities and attributes across separately managed systems.