An Authorization Information Format (AIF) for ACE
draft-ietf-ace-aif-04

ACE Working Group                                             C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Standards Track                         28 January 2022
Expires: 1 August 2022

           An Authorization Information Format (AIF) for ACE
                         draft-ietf-ace-aif-04

Abstract

   Constrained Devices as they are used in the "Internet of Things" need
   security.  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, need to ascertain
   that the authorization to request the operation does apply to the
   actual requester, and need to ascertain that other devices they place
   requests on are the ones they intended.

   To transfer detailed authorization information from an authorization
   manager (such as an ACE-OAuth Authorization Server) to a device, a
   compact representation format is needed.  This document provides a
   suggestion for 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 refinement that
   describes REST resources (potentially dynamically created) and the
   permissions on them.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-ace-aif/.

   Discussion of this document takes place on the Authentication and
   Authorization for Constrained Environments (ace) Working Group
   mailing list (mailto:ace@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/ace/.

   Source for this draft and an issue tracker can be found at
   https://github.com/cabo/ace-aif.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Information Model . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  REST-specific Model . . . . . . . . . . . . . . . . . . .   4
     2.2.  Limitations . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Extended REST-specific Model  . . . . . . . . . . . . . .   6
   3.  Data Model  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Media Types . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Media Types . . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Registries  . . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Content-Format  . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

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1.  Introduction

   Constrained Devices as they are used in the "Internet of Things" need
   security.  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, need to ascertain
   that the authorization to request the operation does apply to the
   actual requester, and need to ascertain that other devices they place
   requests on are the ones they intended.

   To transfer detailed authorization information from an authorization
   manager (such as an ACE-OAuth Authorization Server
   [I-D.ietf-ace-oauth-authz]) to a device, a compact representation
   format is needed.  This document provides a suggestion for 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 refinement that describes REST
   resources (potentially dynamically created) and the permissions on
   them.

1.1.  Terminology

   This memo uses terms from [RFC7252] and [RFC4949]; CoAP is used for
   the explanatory examples as it is a good fit for Constrained Devices.

   The shape of data is specified in CDDL [RFC8610].  Terminology for
   Constrained Devices is defined in [RFC7228].

   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.  These words may also appear in this
   document in lower case as plain English words, absent their normative
   meanings.

   (Note that this document is itself informational, but it is
   discussing normative statements that MUST be put into concrete terms
   in each specification that makes use of this document.)

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

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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).

   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 do not concern the AIF format with the subject for which the AIF
   data item is issued, so we are focusing the AIF data item on a single
   row in the access matrix (such a row traditionally is also called a
   capability list).  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 and the permissions 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, the object identifier (Toid) will often be
   a text string, and the set of permissions (Tperm) will be represented
   by a bitset in turn represented as a number (see Section 3).

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

                  Figure 2: Likely shape of a specific AIF

2.1.  REST-specific Model

   In the specific instantiation of the REST resources and the
   permissions on them, for the object identifiers (Toid), we use the
   URI of a resource on a CoAP server.  More specifically, the parts of
   the URI that identify the server ("authority" in [RFC3986]) are
   considered the realm of the authentication mechanism (which are
   handled in the cryptographic armor); we therefore focus on the "path-
   absolute" 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 devices).

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   For the permissions (Tperm), we simplify the model permissions to
   giving the subset of the CoAP methods permitted.  This model is
   summarized in Table 1.

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

                         Table 1: An authorization
                            instance in the AIF
                             Information Model

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

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 object.

   This simple information model also does not allow further
   conditionalizing access based on state outside the identification of
   objects (e.g., "opening a door is allowed if that 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
   others.

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2.3.  Extended REST-specific Model

   The extended REST-specific model 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 [RFC7231] (the Location-indicating mechanisms).
   (The concept is somewhat comparable to "ACL inheritance" in NFSv4
   [RFC8881], except that it does not use a containment relationship but
   the fact that the dynamic resource was created from a resource to
   which the subject had access.)  In other words, it addresses the
   third limitation mentioned in Section 2.2.

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

              Table 2: An authorization instance in the AIF
                            Information Model

   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 by a Location-indicating mechanism to a request
   that the subject made to the resource listed (/a/make-coffee in the
   example shown in Table 2, which might return the location of a
   resource that allows GET to find out about the status 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 extended
   model; 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 basic REST
   authorization as per Section 2.1 and Section 2.3.  As discussed, in
   this case the object identifier is specialized as a text string
   giving a relative URI (local-part as absolute path on the server
   serving as enforcement point).  The permission set is specialized to
   a single number by the following steps:

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   *  The entries in the table that specify the same 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 chosen as 32 (e.g., 35 for
      Dynamic-DELETE).

   *  The set of numbers is converted into a single number by taking
      each number to the power of two 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.

   [["/s/temp", 1], ["/a/led", 5], ["/dtls", 2]]

       Figure 3: An authorization instance encoded in JSON (46 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]:

   AIF-REST = AIF-Generic<path, permissions>
   path = tstr   ; URI relative to enforcement point
   permissions = 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
   )

                           Figure 4: AIF in CDDL

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   A representation of this information in CBOR [RFC8949] is given in
   Figure 5; again, several optimizations/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 choosing 32 as Dynamic-Offset means that all future CoAP
   methods that can be registered can be represented both as themselves
   and in the Dynamic-X variant, but only the dynamic forms of methods 1
   to 21 are typically usable in a JSON form [RFC7493].

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 "local-uri"
   for Toid and "REST-method-set" for Tperm.

   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 is requested to add the following Media-Types to the "Media
   Types" registry.

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   +==========+======================+=====================+
   | Name     | Template             | Reference           |
   +==========+======================+=====================+
   | aif+cbor | application/aif+cbor | RFC XXXX, Section 4 |
   +----------+----------------------+---------------------+
   | aif+json | application/aif+json | RFC XXXX, Section 4 |
   +----------+----------------------+---------------------+

                            Table 3

   // RFC Ed.: please replace RFC XXXX with this RFC number and remove
   this note.

   For application/aif+cbor:

   Type name:  application
   Subtype name:  aif+cbor
   Required parameters:
      *  Toid: the identifier for the object for which permissions are
         supplied.  A value from the subregistry for Toid.  Default
         value: "local-uri" (RFC XXXX).

      *  Tperm: the data type of a permission set for the object
         identified via a Toid.  A value from the subregistry for Tperm.
         Default value: "REST-method-set" (RFC XXXX).
   Optional parameters:  none
   Encoding considerations:  binary (CBOR)
   Security considerations:  Section 6 of RFC XXXX
   Interoperability considerations:  none
   Published specification:  Section 4 of RFC XXXX
   Applications that use this media type:  No known applications
      currently use this media type.
   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers is as specified for "application/cbor".  (At
      publication of RFC XXXX, 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:  none
   Author/Change controller:  IETF
   Provisional registration:  no

   For application/aif+json:

   Type name:  application
   Subtype name:  aif+json

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   Required parameters:
      *  Toid: the identifier for the object for which permissions are
         supplied.  A value from the subregistry for Toid.  Default
         value: "local-uri" (RFC XXXX).

      *  Tperm: the data type of a permission set for the object
         identified via a Toid.  A value from the subregistry for Tperm.
         Default value: "REST-method-set" (RFC XXXX).
   Optional parameters:  none
   Encoding considerations:  binary (JSON is UTF-8-encoded text)
   Security considerations:  Section 6 of RFC XXXX
   Interoperability considerations:  none
   Published specification:  Section 4 of RFC XXXX
   Applications that use this media type:  No known applications
      currently use this media type.
   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers is as specified for "application/json".  (At
      publication of RFC XXXX, 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:  none
   Author/Change controller:  IETF
   Provisional registration:  no

5.2.  Registries

   IANA is requested to create a registry for AIF with two sub-
   registries for Toid and Tperm, populated with:

    +=============+=================+=================================+
    | Subregistry | name            | Description/Specification       |
    +=============+=================+=================================+
    | Toid        | local-part      | local-part of URI as specified  |
    |             |                 | in RFC XXXX                     |
    +-------------+-----------------+---------------------------------+
    | Tperm       | REST-method-set | set of REST methods represented |
    |             |                 | as specified in RFC XXXX        |
    +-------------+-----------------+---------------------------------+

                                  Table 4

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

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   // RFC Ed.: please replace RFC XXXX with this RFC number and remove
   this note.

5.3.  Content-Format

   IANA is requested to register Content-Format numbers in the "CoAP
   Content-Formats" subregistry, within the "Constrained RESTful
   Environments (CoRE) Parameters" Registry [IANA.core-parameters], as
   follows:

   +======================+================+======+===========+
   | Media Type           | Content Coding | ID   | Reference |
   +======================+================+======+===========+
   | application/aif+cbor | -              | TBD1 | RFC XXXX  |
   +----------------------+----------------+------+-----------+
   | application/aif+json | -              | TBD2 | RFC XXXX  |
   +----------------------+----------------+------+-----------+

                             Table 5

   // RFC Ed.: please replace TBD1 and TBD2 with assigned IDs and remove
   this note. // RFC Ed.: please replace RFC XXXX with this RFC number
   and remove this note.

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

6.  Security Considerations

   The security considerations of [RFC7252] apply.  Some wider issues
   are discussed in [RFC8576].

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

   *  ensure that the cryptographic armor employed around this format
      fulfills the security objectives, and that the armor or some
      additional information included in it with the AIF information
      unambiguously identifies the subject to which the authorizations
      shall apply, and

   *  ensure that the types used for Toid and Tperm provide the
      appropriate granularity so that application requirements on the
      precision of the authorization information are fulfilled, and that
      all parties understand Toid/Tperm pairs to signify the same
      operations.

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

   A generic 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 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 usual principle "everything is
   denied until it is explicitly allowed" needs to hold here as well.

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>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [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,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

7.2.  Informative References

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   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
              draft-ietf-ace-oauth-authz-46, 8 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-46.txt>.

   [IANA.core-parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              <https://www.iana.org/assignments/core-parameters>.

   [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,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/info/rfc6570>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,
              <https://www.rfc-editor.org/info/rfc7493>.

   [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,
              <https://www.rfc-editor.org/info/rfc8132>.

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   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [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,
              <https://www.rfc-editor.org/info/rfc8576>.

   [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,
              <https://www.rfc-editor.org/info/rfc8881>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

Acknowledgements

   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.

Author's Address

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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