RFC 9237

Internet Engineering Task Force (IETF)                        C. Bormann
Request for Comments: 9237                        Universität Bremen TZI
Category: Standards Track                                    August 2022
ISSN: 2070-1721

    An Authorization Information Format (AIF) for Authentication and
            Authorization for Constrained Environments (ACE)


   Information about which entities are authorized to perform what
   operations on which constituents of other entities is a crucial
   component of producing an overall system that is secure.  Conveying
   precise authorization information is especially critical in highly
   automated systems with large numbers of entities, such as the
   Internet of Things.

   This specification provides a generic information model and format
   for representing such authorization information, as well as two
   variants of a specific instantiation of that format for use with
   Representational State Transfer (REST) resources identified by URI

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in 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

Copyright Notice

   Copyright (c) 2022 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.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Information Model
     2.1.  REST-Specific Model
     2.2.  Limitations
     2.3.  REST-Specific Model with Dynamic Resource Creation
   3.  Data Model
   4.  Media Types
   5.  IANA Considerations
     5.1.  Media Types
       5.1.1.  application/aif+cbor
       5.1.2.  application/aif+json
     5.2.  Registries
     5.3.  Content-Format
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References

   Author's Address

1.  Introduction

   Constrained devices, as they are used in the Internet of Things, need
   security in order to operate correctly and prevent misuse.  One
   important element of this security is that devices in the Internet of
   Things need to be able to decide which operations requested of them
   should be considered authorized, ascertain that the authorization to
   request the operation does apply to the actual requester as
   authenticated, and ascertain that other devices they make requests of
   are the ones they intended.

   To transfer detailed authorization information from an authorization
   manager (such as an ACE-OAuth authorization server [RFC9200]) to a
   device, a compact representation format is needed.  This document
   defines such a format -- the Authorization Information Format (AIF).
   AIF is defined both as a general structure that can be used for many
   different applications and as a specific instantiation tailored to
   REST resources and the permissions on them, including some provision
   for dynamically created resources.

1.1.  Terminology

   This memo uses terms from the Constrained Application Protocol (CoAP)
   [RFC7252] and the Internet Security Glossary [RFC4949]; CoAP is used
   for the explanatory examples as it is a good fit for constrained

   The shape of data is specified in Concise Data Definition Language
   (CDDL) [RFC8610] [RFC9165].  Terminology for constrained devices is
   defined in [RFC7228].

   The term "byte", abbreviated by "B", is used in its now customary
   sense as a synonym for "octet".

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

2.  Information Model

   Authorizations are generally expressed through some data structures
   that are cryptographically secured (or transmitted in a secure way).
   This section discusses the information model underlying the payload
   of that data (as opposed to the cryptographic armor around it).

   The semantics of the authorization information defined in this
   document are that of an _allow-list_: everything is denied until it
   is explicitly allowed.

   For the purposes of this specification, the underlying access control
   model will be that of an access matrix, which gives a set of
   permissions for each possible combination of a subject and an object.
   We are focusing the AIF data item on a single row in the access
   matrix (such a row has often been called a "capability list") without
   concern to the subject for which the data item is issued.  As a
   consequence, AIF MUST be used in a way that the subject of the
   authorizations is unambiguously identified (e.g., as part of the
   armor around it).

   The generic model of such a capability list is a list of pairs of
   object identifiers (of type Toid) and the permissions (of type Tperm)
   that the subject has on the object(s) identified.

   AIF-Generic<Toid, Tperm> = [* [Toid, Tperm]]

                    Figure 1: Definition of Generic AIF

   In a specific data model (such as the one specified in this
   document), the object identifier (Toid) will often be a text string,
   and the set of permissions (Tperm) will be represented by a bit set,
   which in turn is represented as a number (see Section 3).

   AIF-Specific = AIF-Generic<tstr, uint>

              Figure 2: Commonly Used Shape of a Specific AIF

2.1.  REST-Specific Model

   In the specific instantiation of the REST resources and the
   permissions on them, we use the URI of a resource on a CoAP server
   for the object identifier (Toid).  More specifically, since the parts
   of the URI that identify the server ("authority" in [RFC3986]) are
   authenticated during REST resource access (Section 4.2.2 of [RFC9110]
   and Section 6.2 of [RFC7252]), they naturally fall into the realm
   handled by the cryptographic armor; we therefore focus on the "path"
   ("path-abempty") and "query" parts of the URI (_URI-local-part_ in
   this specification, as expressed by the Uri-Path and Uri-Query
   options in CoAP).  As a consequence, AIF MUST be used in a way that
   it is clear who is the target (enforcement point) of these
   authorizations (note that there may be more than one target that the
   same authorization applies to, e.g., in a situation with homogeneous

   For the permissions (Tperm), we use a simple permissions model that
   lists the subset of the REST (CoAP or HTTP) methods permitted.  This
   model is summarized in Table 1.

                    | URI-local-part | Permission Set |
                    | /s/temp        | GET            |
                    | /a/led         | PUT, GET       |
                    | /dtls          | POST           |

                         Table 1: An Authorization
                       Instance in the REST-Specific
                           AIF Information Model

   In this example, a device offers a temperature sensor /s/temp for
   read-only access, a LED actuator /a/led for read/write, and a /dtls
   resource for POST access.

   As shown in the data model (Section 3), the representations of REST
   methods provided are limited to those that have a CoAP method number
   assigned; an extension to the model may be necessary to represent
   permissions for exotic HTTP methods.

2.2.  Limitations

   This simple information model only allows granting permissions for
   statically identifiable objects, e.g., URIs for the REST-specific
   instantiation.  One might be tempted to extend the model towards URI
   templates [RFC6570] (for instance, to open up an authorization for
   many parameter values as in /s/temp{?any*}).  However, that requires
   some considerations of the ease and unambiguity of matching a given
   URI against a set of templates in an AIF data item.

   This simple information model also does not allow expressing
   conditionalized access based on state outside the identification of
   objects (e.g., "opening a door is allowed if it is not locked").

   Finally, the model does not provide any special access for a set of
   resources that are specific to a subject, e.g., that the subject
   created itself by previous operations (PUT, POST, or PATCH/iPATCH
   [RFC8132]) or that were specifically created for the subject by

2.3.  REST-Specific Model with Dynamic Resource Creation

   The _REST-specific model with dynamic resource creation_ addresses
   the need to provide defined access to dynamic resources that were
   created by the subject itself, specifically, a resource that is made
   known to the subject by providing Location-* options in a CoAP
   response or using the Location header field in HTTP [RFC9110] (the
   Location-indicating mechanisms).  (The concept is somewhat comparable
   to "Access Control List (ACL) inheritance" in the Network File System
   version 4 (NFSv4) protocol [RFC8881], except that it does not use a
   containment relationship but rather the fact that the dynamic
   resource was created from a resource to which the subject had
   access.)  In other words, it addresses an important subset of the
   third limitation mentioned in Section 2.2.

          | URI-local-part | Permission Set                    |
          | /a/make-coffee | POST, Dynamic-GET, Dynamic-DELETE |

             Table 2: An Authorization Instance in the REST-
               Specific AIF Information Model with Dynamic
                            Resource Creation

   For a method X, the presence of a Dynamic-X permission means that the
   subject holds permission to exercise the method X on resources that
   have been returned in a 2.01 (201 Created) response by a Location-
   indicating mechanism to a request that the subject made to the
   resource listed.  In the example shown in Table 2, POST operations on
   /a/make-coffee might return the location of a resource dynamically
   created on the coffee machine that allows GET to find out about the
   status of, and DELETE to cancel, the coffee-making operation.

   Since the use of the extension defined in this section can be
   detected by the mentioning of the Dynamic-X permissions, there is no
   need for another explicit switch between the basic and the model
   extended by dynamic resource creation; the extended model is always
   presumed once a Dynamic-X permission is present.

3.  Data Model

   Different data model specializations can be defined for the generic
   information model given above.

   In this section, we will give the data model for simple REST
   authorization as per Sections 2.1 and 2.3.  As discussed, in this
   case the object identifier is specialized as a text string giving a
   relative URI (URI-local-part as the absolute path on the server
   serving as the enforcement point).  The permission set is specialized
   to a single number _REST-method-set_ by the following steps:

   *  The entries in the table that specify the same URI-local-part are
      merged into a single entry that specifies the union of the
      permission sets.

   *  The (non-dynamic) methods in the permission sets are converted
      into their CoAP method numbers, minus 1.

   *  Dynamic-X permissions are converted into what the number would
      have been for X, plus a Dynamic-Offset that has been chosen as 32
      (e.g., 35 is the number for Dynamic-DELETE as the number for
      DELETE is 3).

   *  The set of numbers is converted into a single number REST-method-
      set by taking two to the power of each (decremented) method number
      and computing the inclusive OR of the binary representations of
      all the power values.

   This data model could be interchanged in the JSON [RFC8259]
   representation given in Figure 3.


       Figure 3: An Authorization Instance Encoded in JSON (40 Bytes)

   In Figure 4, a straightforward specification of the data model
   (including both the methods from [RFC7252] and the new ones from
   [RFC8132], identified by the method code minus 1) is shown in CDDL
   [RFC8610] [RFC9165]:

   AIF-REST = AIF-Generic<local-path, REST-method-set>
   local-path = tstr   ; URI relative to enforcement point
   REST-method-set = uint .bits methods
   methods = &(
     GET: 0
     POST: 1
     PUT: 2
     DELETE: 3
     FETCH: 4
     PATCH: 5
     iPATCH: 6
     Dynamic-GET: 32; 0 .plus Dynamic-Offset
     Dynamic-POST: 33; 1 .plus Dynamic-Offset
     Dynamic-PUT: 34; 2 .plus Dynamic-Offset
     Dynamic-DELETE: 35; 3 .plus Dynamic-Offset
     Dynamic-FETCH: 36; 4 .plus Dynamic-Offset
     Dynamic-PATCH: 37; 5 .plus Dynamic-Offset
     Dynamic-iPATCH: 38; 6 .plus Dynamic-Offset
   Dynamic-Offset = 32

                           Figure 4: AIF in CDDL

   For the information shown in Table 1 and Figure 3, a representation
   in Concise Binary Object Representation (CBOR) [RFC8949] is given in
   Figure 5; again, several optimizations and improvements are possible.

   83                        # array(3)
      82                     # array(2)
         67                  # text(7)
            2f732f74656d70   # "/s/temp"
         01                  # unsigned(1)
      82                     # array(2)
         66                  # text(6)
            2f612f6c6564     # "/a/led"
         05                  # unsigned(5)
      82                     # array(2)
         65                  # text(5)
            2f64746c73       # "/dtls"
         02                  # unsigned(2)

       Figure 5: An Authorization Instance Encoded in CBOR (28 Bytes)

   Note that having chosen 32 as Dynamic-Offset means that all future
   CoAP methods that are registered can be represented both as
   themselves and in the Dynamic-X variant, but that only the dynamic
   forms of methods 1 to 21 are typically usable in a JSON form

4.  Media Types

   This specification defines media types for the generic information
   model, expressed in JSON (application/aif+json) or in CBOR
   (application/aif+cbor).  These media types have parameters for
   specifying Toid and Tperm; default values are the values "URI-local-
   part" for Toid and "REST-method-set" for Tperm, as per Section 3 of
   the present specification.

   A specification that wants to use generic AIF with different Toid
   and/or Tperm is expected to request these as media type parameters
   (Section 5.2) and register a corresponding Content-Format
   (Section 5.3).

5.  IANA Considerations

5.1.  Media Types

   IANA has added the following media types to the "Media Types"
   registry.  The registration entries are in the following subsections.

   | Name     | Template             | Reference           |
   | aif+cbor | application/aif+cbor | RFC 9237, Section 4 |
   | aif+json | application/aif+json | RFC 9237, Section 4 |

                    Table 3: New Media Types

5.1.1.  application/aif+cbor

   Type name:  application

   Subtype name:  aif+cbor

   Required parameters:  N/A

   Optional parameters:

         the identifier for the object for which permissions are
         supplied.  A value from the "Sub-Parameter Registry for
         application/aif+cbor and application/aif+json" subregistry for
         Toid.  Default value: "URI-local-part" (RFC 9237).

         the data type of a permission set for the object identified via
         a Toid.  A value from the "Sub-Parameter Registry for
         application/aif+cbor and application/aif+json" subregistry for
         Tperm.  Default value: "REST-method-set" (RFC 9237).

   Encoding considerations:  binary (CBOR)

   Security considerations:  Section 6 of RFC 9237

   Interoperability considerations:  N/A

   Published specification:  Section 4 of RFC 9237

   Applications that use this media type:  Applications that need to
      convey structured authorization data for identified resources,
      conveying sets of permissions.

   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers is as specified for "application/cbor".  (At
      publication of RFC 9237, there is no fragment identification
      syntax defined for "application/cbor".)

   Person & email address to contact for further information:  ACE WG
      mailing list (ace@ietf.org) or IETF Applications and Real-Time
      Area (art@ietf.org)

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author/Change controller:  IETF

   Provisional registration:  no

5.1.2.  application/aif+json

   Type name:  application

   Subtype name:  aif+json

   Required parameters:  N/A

   Optional parameters:

         the identifier for the object for which permissions are
         supplied.  A value from the media-type parameter subregistry
         for Toid.  Default value: "URI-local-part" (RFC 9237).

         the data type of a permission set for the object identified via
         a Toid.  A value from the media-type parameter subregistry for
         Tperm.  Default value: "REST-method-set" (RFC 9237).

   Encoding considerations:  binary (JSON is UTF-8-encoded text)

   Security considerations:  Section 6 of RFC 9237

   Interoperability considerations:  N/A

   Published specification:  Section 4 of RFC 9237

   Applications that use this media type:  Applications that need to
      convey structured authorization data for identified resources,
      conveying sets of permissions.

   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers is as specified for "application/json".  (At
      publication of RFC 9237, there is no fragment identification
      syntax defined for "application/json".)

   Person & email address to contact for further information:  ACE WG
      mailing list (ace@ietf.org) or IETF Applications and Real-Time
      Area (art@ietf.org)

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author/Change controller:  IETF

   Provisional registration:  no

5.2.  Registries

   For the media types application/aif+cbor and application/aif+json,
   IANA has created a subregistry within
   [IANA.media-type-sub-parameters] for the media-type parameters Toid
   and Tperm, populated with the following:

   | Parameter | name            | Description/        | Reference |
   |           |                 | Specification       |           |
   | Toid      | URI-local-part  | local-part of URI   | RFC 9237  |
   | Tperm     | REST-method-set | set of REST methods | RFC 9237  |
   |           |                 | represented         |           |

                   Table 4: New Media Type Parameters

   The registration policy is Specification Required [RFC8126].  The
   designated expert will engage with the submitter to ascertain whether
   the requirements of this document are addressed:

   *  The specifications for Toid and Tperm need to realize the general
      ideas of unambiguous object identifiers and permission lists in
      the context where the AIF data item is intended to be used, and
      their structure needs to be usable with the intended media types
      (application/aif+cbor and application/aif+json) as identified in
      the specification.

   *  The parameter names need to conform to Section 4.3 of [RFC6838],
      but preferably they are in [KebabCase] so they can be easily
      translated into names used in APIs with popular programming

   The designated experts will develop further criteria and guidelines
   as needed.

5.3.  Content-Format

   IANA has registered Content-Format numbers in the "CoAP Content-
   Formats" subregistry, within the "Constrained RESTful Environments
   (CoRE) Parameters" registry [IANA.core-parameters], as follows:

   | Media Type           | Encoding | ID  | Reference |
   | application/aif+cbor | -        | 290 | RFC 9237  |
   | application/aif+json | -        | 291 | RFC 9237  |

                Table 5: New Content-Formats

   Note that applications that register Toid and Tperm values are
   encouraged to also register Content-Formats for the relevant

6.  Security Considerations

   The security considerations of [RFC7252] apply when AIF is used with
   CoAP; Section 11.1 of [RFC7252] specifically applies if complex
   formats such as URIs are used for Toid or Tperm.  Some wider issues
   are discussed in [RFC8576].

   When applying these formats, the referencing specification needs to
   be careful to ensure:

   *  that the cryptographic armor employed around this format fulfills
      the referencing specification's security objectives and that the
      armor or some additional information included in it with the AIF
      data item (1) unambiguously identifies the subject to which the
      authorizations shall apply and (2) provides any context
      information needed to derive the identity of the object to which
      authorization is being granted from the object identifiers (such
      as, for the data models defined in the present specification, the
      scheme and authority information that is used to construct the
      full URI), and

   *  that the types used for Toid and Tperm provide the appropriate
      granularity and precision so that application requirements on the
      precision of the authorization information are fulfilled and that
      all parties have the same understanding of each Toid/Tperm pair in
      terms of specified objects (resources) and operations on those.

   For the data formats, the security considerations of [RFC8259] and
   [RFC8949] apply.

   A plain implementation of AIF might implement just the basic REST
   model as per Section 2.1.  If it receives authorizations that include
   permissions that use the REST-specific model with dynamic resource
   creation (Section 2.3), it needs to either reject the AIF data item
   entirely or act only on the permissions that it does understand.  In
   other words, the semantics underlying an allow-list as discussed
   above need to hold here as well.

   An implementation of the REST-specific model with dynamic resource
   creation (Section 2.3) needs to carefully keep track of the
   dynamically created objects and the subjects to which the Dynamic-X
   permissions apply -- both on the server side to enforce the
   permissions and on the client side to know which permissions are

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,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [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>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

   [RFC9165]  Bormann, C., "Additional Control Operators for the Concise
              Data Definition Language (CDDL)", RFC 9165,
              DOI 10.17487/RFC9165, December 2021,

7.2.  Informative References

              IANA, "Constrained RESTful Environments (CoRE)

              IANA, "MIME Media Type Sub-Parameter Registries",

              "Kebab Case", 29 August 2014,

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,

   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

   [RFC8576]  Garcia-Morchon, O., Kumar, S., and M. Sethi, "Internet of
              Things (IoT) Security: State of the Art and Challenges",
              RFC 8576, DOI 10.17487/RFC8576, April 2019,

   [RFC8881]  Noveck, D., Ed. and C. Lever, "Network File System (NFS)
              Version 4 Minor Version 1 Protocol", RFC 8881,
              DOI 10.17487/RFC8881, August 2020,

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,

   [RFC9200]  Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments Using the OAuth 2.0 Framework
              (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,


   Jim Schaad, Francesca Palombini, Olaf Bergmann, Marco Tiloca, and
   Christian Amsüss provided comments that shaped the direction of this
   document.  Alexey Melnikov pointed out that there were gaps in the
   media type specifications, and Loganaden Velvindron provided a
   shepherd review with further comments.  Many thanks also to the IESG
   reviewers, who provided several small but significant observations.
   Benjamin Kaduk provided an extensive review as Responsible Area
   Director and indeed is responsible for much improvement in the

Author's Address

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Phone: +49-421-218-63921
   Email: cabo@tzi.org