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The Identity Federation operated by the German Research and Education Network (DFN) introduced Entity Categories (both for SPs and IdPs) in 2012 in order to support so-called "Virtual Sub-Federations". The setup is based on a whitelist maintained by a specific project or community and which is hooked up with the metadata registry. The project-specific EC is only available for entities listed on such a whitelist - a nightly check removes the EC automatically if an entity disappears from the respective whitelist. Using such an EC, (Shibboleth) SPs are able to select all project-related IdPs from the federation metadata and ignore the rest, while IdPs only have to set up one Attribute Filter Policy in order to release Attributes to a dynamic number of project-related SPs. This concept turned out to be quite popular, meanwhile (2015) three of these ECs are in use, a fourth one has been requested recently.
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