The Entity Category Security Assertion Markup Language (SAML) Attribute Types
RFC 8409

Independent Submission                                     I. Young, Ed.
Request for Comments: 8409                                   Independent
Category: Informational                                     L. Johansson
ISSN: 2070-1721                                                    SUNET
                                                               S. Cantor
                                                   Shibboleth Consortium
                                                             August 2018

     The Entity Category Security Assertion Markup Language (SAML)
                            Attribute Types

Abstract

   This document describes two SAML entity attributes: one that can be
   used to assign category membership semantics to an entity and another
   for use in claiming interoperation with or support for entities in
   such categories.

   This document is a product of the working group process of the
   Research and Education FEDerations (REFEDS) group.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8409.

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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................3
      1.1. REFEDS Document Process ....................................3
   2. Notation and Conventions ........................................4
   3. Entity Category Attribute .......................................4
      3.1. Syntax .....................................................4
      3.2. Semantics ..................................................5
      3.3. Entity Category Example ....................................6
   4. Entity Category Support Attribute ...............................7
      4.1. Syntax .....................................................7
      4.2. Semantics ..................................................7
      4.3. Entity Category Support Example ............................9
   5. IANA Considerations .............................................9
   6. Security Considerations .........................................9
   7. References .....................................................11
      7.1. Normative References ......................................11
      7.2. Informative References ....................................11
   Acknowledgements ..................................................12
   Authors' Addresses ................................................12

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1.  Introduction

   This document describes a SAML attribute called the "entity category
   attribute".  Values of this attribute represent entity types or
   categories.  When used with the SAML V2.0 Metadata Extension for
   Entity Attributes [SAML2MetadataAttr], each such entity category
   attribute value represents a claim that the entity thus labeled 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.  Therefore, this document
   does not specify a controlled vocabulary for assigning such values;
   they may 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.

   This document also describes a SAML attribute called the "entity
   category support attribute".  This attribute contains URI values that
   represent claims that an entity supports and/or interoperates with
   entities in a given category or categories.  These values, defined in
   conjunction with specific entity category attribute values, provide
   entities in a category with the means to identify peer entities that
   wish to interact with them in a fashion described by the category
   specification.

   This document does not specify any values for either the entity
   category attribute or the entity category support attribute.

1.1.  REFEDS Document Process

   The Research and Education FEDerations [REFEDS] group is the voice
   that articulates the mutual needs of research and education identity
   federations worldwide.  It aims to represent the requirements of
   research and education in the ever-growing space of access and
   identity management.

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   From time to time, REFEDS will publish a document in the RFC Series.
   Such documents will be published as part of the Independent
   Submission stream [RFC4844]; however, the REFEDS Working Group sign-
   off process will have been followed for these documents, as described
   in the REFEDS Participant's Agreement [REFEDS.agreement].

   This document is a product of the REFEDS Working Group process.

2.  Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The notation "@example" is used as a shorthand for an XML attribute
   with attribute name "example".

3.  Entity Category Attribute

3.1.  Syntax

   Entity category attribute values MUST be URIs.  Such values are also
   referred to as "category URIs" in this document.

   It is RECOMMENDED that http:-scheme or https:-scheme URIs are used;
   it is further RECOMMENDED that a category URI resolves to a human-
   readable document defining the category.

   Authorities defining entity categories MUST produce a specification
   of the entity category and SHOULD make arrangement for the category
   URI to resolve to the specification in human-readable form.

   Authorities defining entity categories MAY use versioning of category
   URIs where appropriate; if versioning is used, each version of the
   specification of the entity category SHOULD clearly indicate the
   latest version of the category URI (and hence of the specification).
   The specification SHOULD include a description of how the authority
   defining the entity category implements governance for the
   specification if the specification is updated.

   When used in SAML metadata or protocol elements, 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.

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   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 metadata extension defined in [SAML2MetadataAttr].  In
   this extension, the Attribute element is contained within an
   mdattr:EntityAttributes element directly contained within an
   md:Extensions element directly contained within the entity's
   md:EntityDescriptor.

   The meaning of the entity category attribute is not defined by this
   specification if it appears anywhere else within a metadata instance
   or within any other XML document.

   If the entity category attribute appears more than once in the
   metadata for an entity, relying parties SHOULD interpret the combined
   set of associated attribute values as if they all appeared together
   within a single entity category attribute.

3.2.  Semantics

   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.

   An entity may be claimed to be a member of more than one category.
   In this case, the entity is claimed to meet the requirements of each
   category independently unless otherwise specified by the category
   definitions themselves.

   This document intentionally does not define "category", in order to
   leave the concept as general as possible.  However, to be useful,
   category definitions SHOULD include the following as appropriate:

   o  A definition of the authorities who may validly assert membership
      in the category.  While membership in some categories may be self-
      asserted informally by an entity's owner, others may need to be
      validated by third parties such as the entity's home federation or
      other registrar.

   o  A set of criteria by which an entity's membership in the category
      can be objectively assessed.

   o  A definition of the processes by which valid authorities may
      determine that an entity meets the category's membership criteria.

   o  A description of the anticipated uses for category membership by
      relying parties.

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   o  A statement indicating the applicability or otherwise of
      membership of the entity category to different SAML role
      descriptors and any protocol support restrictions that may be
      relevant.

   Entity categories SHOULD NOT be used to indicate the certification
   status of an entity regarding its conformance to the requirements of
   an identity assurance framework.  The SAML extension defined in
   [SAML2IDAssuranceProfile] SHOULD be used for this purpose.

   If significant changes are made to a category definition, the new
   version of the category SHOULD be represented by a different category
   URI so that the old and new versions can be distinguished by a
   relying party.  It is for this reason that authorities defining
   entity categories MAY employ some form of versioning for category
   URIs.  When versioning is used, each version of the entity category
   MUST be treated as a separate URI.

   No ordering relation is defined for entity category attribute values.
   Entity category attribute values MUST be treated as opaque strings
   for the purpose of comparison.  In particular, if the specification
   defining the entity category relies on versioning of the category
   URI, a relying party MUST NOT assume any particular ordering between
   different versions of the category URI.  Any order between versions
   MUST be spelled out in the specification.

3.3.  Entity Category Example

   <md:EntityDescriptor xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata"
     entityID="https://service.example.com/entity">
     <md:Extensions>
       <mdattr:EntityAttributes
         xmlns:mdattr="urn:oasis:names:tc:SAML:metadata:attribute">
         <Attribute xmlns="urn:oasis:names:tc:SAML:2.0:assertion"
           NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
           Name="http://macedir.org/entity-category">
           <AttributeValue
             >http://example.org/category/dog</AttributeValue>
           <AttributeValue>urn:oid:1.3.6.1.4.1.21829</AttributeValue>
         </Attribute>
       </mdattr:EntityAttributes>
     </md:Extensions>
     ...
   </md:EntityDescriptor>

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4.  Entity Category Support Attribute

4.1.  Syntax

   Entity category support attribute values MUST be URIs.  Such values
   are also referred to as "category support URIs" in this document.

   It is RECOMMENDED that http:-scheme or https:-scheme URLs are used;
   it is further RECOMMENDED that each such value resolves to a human-
   readable document defining the value's semantics.

   A given category URI MAY be associated with multiple category support
   URIs in order to allow for multiple forms of support, participation,
   or interoperation with entities in the category.  The authority
   defining the category URI and category support URIs MUST clearly
   describe the relationship between (all versions of) the category URI
   and (all versions of) the category support URIs as applicable in the
   entity category specification.

   The entity category support 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-support.

   Claims that a SAML entity implements support for one or more
   categories are represented by including the Attribute element
   described here in the entity's metadata through use of the metadata
   extension defined in [SAML2MetadataAttr].  In this extension, the
   Attribute element is contained within an mdattr:EntityAttributes
   element directly contained within an md:Extensions element directly
   contained within the entity's md:EntityDescriptor.

   The meaning of the entity category support attribute is not defined
   by this specification if it appears anywhere else within a metadata
   instance or within any other XML document.

   If the entity category support attribute appears more than once in
   the metadata for an entity, relying parties SHOULD interpret the
   combined set of associated attribute values as if they all appeared
   together within a single entity category support attribute.

4.2.  Semantics

   The presence of the entity category support attribute within an
   entity's entity attributes represents a series of claims (one for
   each attribute value) that the entity supports peer entities in a

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   category in a particular fashion.  The precise semantics of such a
   claim depend on the definition of the category support URI itself.
   Category support claims will often be defined to be self-asserted.

   An entity may be claimed to support more than one category.  In this
   case, the entity is claimed to meet the support requirements of each
   category independently unless otherwise specified by the category
   definitions themselves.

   This document intentionally does not define "support" for a category,
   in order to leave the concept as general as possible.  It is assumed
   that entity category definitions MAY define one or more category
   support URIs signifying particular definitions for "support" by peers
   as motivated by use cases arising from the definition of the category
   itself.

   A common case is expected to be the definition of a single category
   support URI whose value is identical to the category URI.

   If significant changes are made to a category support definition, the
   new version SHOULD be represented by a different category support URI
   so that the old and new versions can be distinguished by a relying
   party.  It is for this reason that authorities defining entity
   categories support MAY employ some form of versioning.  When
   versioning is used, each version of the category support URI MUST be
   treated as a separate URI.

   No ordering relation is defined for entity category support attribute
   values.  Entity category support attribute values MUST be treated as
   opaque strings for the purpose of comparison.  In particular, if the
   specification defining the category support URIs relies on
   versioning, a relying party MUST NOT assume any particular ordering
   between different versions of the category support URI.  Any order
   between versions MUST be spelled out in the specification.

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4.3.  Entity Category Support Example

   <md:EntityDescriptor xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata"
     entityID="https://idp.example.edu/entity">
     <md:Extensions>
       <mdattr:EntityAttributes
         xmlns:mdattr="urn:oasis:names:tc:SAML:metadata:attribute">
         <Attribute xmlns="urn:oasis:names:tc:SAML:2.0:assertion"
           NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
           Name="http://macedir.org/entity-category-support">
           <AttributeValue
             >http://example.org/category/dog/basic</AttributeValue>
           <AttributeValue
             >http://example.org/category/dog/advanced</AttributeValue>
           <AttributeValue>urn:oid:1.3.6.1.4.1.21829</AttributeValue>
         </Attribute>
       </mdattr:EntityAttributes>
     </md:Extensions>
     ...
   </md:EntityDescriptor>

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   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:

   o  The integrity of the metadata delivered to the relying party, for
      example, as assured by a digital signature.

   o  If the entity category attribute is carried within a signed
      assertion, the assertion itself must be evaluated.

   o  The policies and procedures of the immediate source of the
      metadata, in particular, any procedures the immediate source has
      with regard to aggregation of metadata from other sources.

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   o  The policies and procedures implemented by agents along the
      publication path from the original metadata registrar.  This may
      be determined by examination of the published procedures of each
      agent in turn or may be simplified if the entity metadata includes
      publication path metadata in mdrpi:PublicationPath elements as
      described in Section 2.3.1 of [SAML2MetadataRPI].

   o  The policies and procedures implemented by the original metadata
      registrar.  The registrar's identity may be known implicitly or
      may be determined from the entity metadata if it includes an
      mdrpi:RegistrationInfo element and corresponding
      @registrationAuthority as described in Section 2.1.1 of
      [SAML2MetadataRPI].

   o  The definition of the category itself, in particular, any
      statements it makes about whether membership of the category may
      be self-asserted or may only be asserted by particular
      authorities.

   Although entity category support attribute values will often be
   defined as self-asserted claims by the containing entity, the
   provenance of the metadata remains relevant to a relying party's
   decision to accept a claim of support as legitimate, and the specific
   definition of a support claim will influence the assurance required
   to act on it.

   The conclusion that a claim of category membership or support is
   justified and should be acted upon may require a determination of the
   origin of the claim.  This may not be necessary if the immediate
   source of the metadata is trusted to such an extent that the trust
   calculation is essentially delegated to it.

   In many cases, a claim will be included in an entity's metadata by
   the original metadata registrar on behalf of the entity's owner, and
   the mdrpi:RegistrationInfo element's @registrationAuthority is
   available to carry the registrar's identity.  However, any agent that
   is part of the chain of custody between the original registrar and
   the final relying party may have added, removed, or transformed
   claims according to local policy.  For example, an agent charged with
   redistributing metadata may remove claims it regards as untrustworthy
   or add others that were not already present if they have value to its
   intended audience.

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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [SAML2MetadataAttr]
              Cantor, S., Ed., "SAML V2.0 Metadata Extension for Entity
              Attributes Version 1.0", August 2009,
              <http://docs.oasis-open.org/security/saml/Post2.0/
              sstc-metadata-attr-cs-01.pdf>.

   [SAML2MetadataRPI]
              La Joie, C., Ed., "SAML V2.0 Metadata Extensions for
              Registration and Publication Information Version 1.0",
              April 2012, <http://docs.oasis-open.org/
              security/saml/Post2.0/saml-metadata-rpi/v1.0/cs01/
              saml-metadata-rpi-v1.0-cs01.pdf>.

7.2.  Informative References

   [REFEDS]   "Research and Education FEDerations (REFEDS) Group",
              <http://www.refeds.org/>.

   [REFEDS.agreement]
              Research and Education Federations, "REFEDS Participant's
              Agreement",
              <https://refeds.org/about/refeds-participants-agreement>.

   [RFC4844]  Daigle, L., Ed. and Internet Architecture Board, "The RFC
              Series and RFC Editor", RFC 4844, DOI 10.17487/RFC4844,
              July 2007, <https://www.rfc-editor.org/info/rfc4844>.

   [SAML2IDAssuranceProfile]
              Morgan, RL., Ed., Madsen, P., Ed., and S. Cantor, Ed.,
              "SAML V2.0 Identity Assurance Profiles Version 1.0",
              November 2010, <http://docs.oasis-open.org/security/saml/
              Post2.0/sstc-saml-assurance-profile-cs-01.pdf>.

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Acknowledgements

   This work has been a collaborative effort within the REFEDS and
   MACE-Dir communities.  Special thanks to the following individuals
   (in no particular order):

   o  RL 'Bob' Morgan

   o  Ken Klingenstein

   o  Keith Hazelton

   o  Steven Olshansky

   o  Mikael Linden

   o  Nicole Harris

   o  Tom Scavo

Authors' Addresses

   Ian A. Young (editor)
   Independent

   Email: ian@iay.org.uk

   Leif Johansson
   SUNET

   Email: leifj@sunet.se

   Scott Cantor
   Shibboleth Consortium

   Email: cantor.2@osu.edu

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