This document describes a SAML entity attribute which can be used to assign category membership semantics to an entity.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].
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This document describes a SAML attribute, referred to here as the "entity category attribute", values of which represent entity types or categories. When used with the SAML V2.0 Metadata Extension for Entity Attributes (Cantor, S., Ed., “SAML V2.0 Metadata Extension for Entity Attributes,” August 2009.) [SAML2MetadataAttr] each such entity category attribute value represents a claim that the entity thus labelled meets the requirements of, and is asserted to be a member of, the indicated category.
These category membership claims MAY be used by a relying party to provision policy for release of attributes from an identity provider, to influence user interface decisions such as those related to identity provider discovery, or for any other purpose. In general, the intended uses of any claim of membership in a given category will depend on the details of the category's definition, and will often be included as part of that definition.
Entity category attribute values are URIs, and this document does not specify a controlled vocabulary. Category URIs may therefore be defined by any appropriate authority without any requirement for central registration. It is anticipated that other specifications may provide management and discovery mechanisms for entity category attribute values.
Entity category attribute values MUST be URIs. It is RECOMMENDED that http:-scheme or https:-scheme URLs are used, and further RECOMMENDED that each such value resolves to a human-readable document defining the category.
The entity category attribute MUST be encoded as a SAML 2.0 Attribute element with @NameFormat urn:oasis:names:tc:SAML:2.0:attrname-format:uri and @Name http://macedir.org/entity-category.
A SAML entity is associated with one or more categories by including the Attribute element described here in the entity's metadata through use of the [SAML2MetadataAttr] (Cantor, S., Ed., “SAML V2.0 Metadata Extension for Entity Attributes,” August 2009.) metadata extension, in which the Attribute element is contained within an mdattr:EntityAttributes element directly contained within an md:Extensions element directly contained within the entity's md:EntitiesDescriptor. The meaning of the entity category attribute is undefined by this specification if it appears anywhere else within a metadata instance, or within any other XML document.
[[Issue: I removed the next paragraph because it seemed to me to go much further in restricting the application of the underlying entity attributes specification than is required for interoperability. I see no evidence, for example, that we aren't expecting anyone implementing [SAML2MetadataAttr] (Cantor, S., Ed., “SAML V2.0 Metadata Extension for Entity Attributes,” August 2009.) to handle duplicate values, split across multiple Attribute elements. I think I'd like to talk to Scott about this before resolving this one.]]
[[Removed: The entity category attribute MAY contain multiple AttributeValue elements. A single EntityAttributes element MUST NOT contain multiple separate Attribute elements for the same attribute type and instead such cases MUST be represented as multiple AttributeValue elements.]]
[[Issue: the [SAML2MetadataAttr] (Cantor, S., Ed., “SAML V2.0 Metadata Extension for Entity Attributes,” August 2009.) extension permits signed SAML assertions within EntityAttributes elements associated with entities (as opposed to with entity groups, where only Attribute values are permitted). I'm not sure how we should incorporate this aspect of the underlying specification. The above wording very carefully allows this, but perhaps not very obviously. If we did intent to permit this, it has implications for the security section. If we intended to disallow this, we'd need to say so. My personal guess is that we might want to say that this is something you SHOULD NOT do, as it's bound to cause interoperability problems vs. "light" entity attribute implementations and of course anything that's being passed from hand to hand a few times might find out just how brittle XML signatures are... Again, I think we need to get Scott's input on this.]]
The presence of the entity category attribute within an entity's entity attributes represents a series of claims (one for each attribute value) that the entity is a member of each named category. The precise semantics of such a claim depend on the definition of the category itself.
The definition of the concept of a category is intentionally not addressed in this document, in order to leave it as general as possible. However, to be useful, category definitions SHOULD include the following as appropriate:
If significant changes are made to a category definition, the new version of the category SHOULD be represented by a different category URI.
Entity category attribute value URIs MUST be treated as opaque strings.
<EntityDescriptor entityID="https://foo.example.com"> <Extensions> <EntityAttributes> <Attribute NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri" Name="http://macedir.org/entity-category"> <AttributeValue>urn:oid:188.8.131.52</AttributeValue> <AttributeValue>https://f.example.org/foo</AttributeValue> </Attribute> </EntityAttributes> </Extensions> ... </EntityDescriptor>
This work has been a collaborative effort within the REFEDS and MACE-Dir communities. Special thanks to (in no particular order):
This memo includes no request to IANA.
The presence of the entity category attribute within an entity's entity attributes represents a series of claims (one for each attribute value) that the entity is a member of the named categories. Before accepting and acting on such claims, any relying party needs to establish, at a level of assurance sufficient for the intended use, a chain of trust concluding that the claim is justified.
Some of the elements in such a chain of trust might include:
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|[SAML2MetadataAttr]||Cantor, S., Ed., “SAML V2.0 Metadata Extension for Entity Attributes,” August 2009 (PDF).|
|[SAML2MetadataDRI]||La Joie, C., Ed., “SAML V2.0 Metadata Extensions for Registration and Publication Information Version 1.0.”|
|Ian A. Young|
|UK Access Management Federation for Education and Research|
|Leif Johansson (editor)|