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IETF RFC 8409
The Entity Category Security Assertion Markup Language (SAML) Attribute Types
Last modified on Friday, August 24th, 2018
 Permanent link to RFC 8409
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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/RFC 8409.
Young, et al. Informational PAGE 1 
RFC 8409 Entity Category August 2018
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
Young, et al. Informational PAGE 2
RFC 8409 Entity Category August 2018
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.
Young, et al. Informational PAGE 3
RFC 8409 Entity Category August 2018
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 [RFC 4844]; 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 [RFC 2119] [RFC 8174] 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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RFC 8409 Entity Category August 2018
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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RFC 8409 Entity Category August 2018
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>
Young, et al. Informational PAGE 6
RFC 8409 Entity Category August 2018
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
Young, et al. Informational PAGE 7
RFC 8409 Entity Category August 2018
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.
Young, et al. Informational PAGE 8
RFC 8409 Entity Category August 2018
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.
Young, et al. Informational PAGE 9
RFC 8409 Entity Category August 2018
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.
Young, et al. Informational PAGE 10
RFC 8409 Entity Category August 2018
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[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>.
[RFC 4844] Daigle, L., Ed. and Internet Architecture Board, "The RFC
Series and RFC Editor", RFC 4844, DOI 10.17487/RFC 4844,
July 2007, <https://www.rfc-editor.org/info/RFC 4844>.
[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>.
Young, et al. Informational PAGE 11
RFC 8409 Entity Category August 2018
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
Young, et al. Informational PAGE 12
The Entity Category Security Assertion Markup Language (SAML) Attribute Types
RFC TOTAL SIZE: 24924 bytes
PUBLICATION DATE: Friday, August 24th, 2018
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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