An Authorization Information Format (AIF) for ACE
draft-bormann-core-ace-aif-07

Network Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                         February 24, 2020
Expires: August 27, 2020

           An Authorization Information Format (AIF) for ACE
                     draft-bormann-core-ace-aif-07

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.

   On the ACE mailing list, an activity to create specifications for
   such authenticated authorization for constrained devices is
   contemplated, leading to protocol proposals such as
   [I-D.ietf-ace-dtls-authorize] or [I-D.ietf-ace-oscore-profile].

   One potential work item complementing this protocol work is an
   Authorization Information Format (AIF).

   This document provides a strawman for such a format that should
   enable further discussion of the objectives for its development.

Status of This Memo

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   This Internet-Draft will expire on August 27, 2020.

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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Information Model . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Limitations . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Data Model  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   (See Abstract.)

1.1.  Terminology

   This memo uses terms from [RFC7252] and [RFC4949].

   (Note that this document is itself informational, but it is
   discussing normative statements.)

   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 strawman, 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 on object.

   For the objects, we simply 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).  Similarly, we do not
   concern the AIF format with the subject for which the AIF object is
   issued, focusing the AIF object on a single row in the access matrix
   (such a row traditionally is also called a capability list).

   At the information model level, this leaves a set of pairs of local
   URIs and related permissions.  We simplify the model for the
   permissions to simply giving the subset of the CoAP methods
   permitted.  This model is summarized in Table 1.

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

      Table 1: An authorization instance in the AIF Information Model

2.1.  Limitations

   This simple information model only allows granting permissions for
   static URIs.  It is probably necessary to extend the model towards
   URI templates [RFC6570], however, that requires some considerations
   of the ease and unambiguity of matching a given URI against a set of
   templates in an AIF object.

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   This simple information model also doesn't allow conditionalizing
   access (e.g., "opening a door is allowed if that isn't 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 that were
   specifically created for the subject by others.

3.  Data Model

   For representing the AIF object discussion in Section 2, the
   permission set is reduced to a single number by the following steps:

   o  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

   o  The methods in the permission sets are converted into their CoAP
      method numbers, minus 1

   o  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 numbers.

   This strawman data model could be interchanged in the JSON [RFC8259]
   representation given in Figure 1 (more extensible/more compact
   representations are possible).

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

      Figure 1: An authorization instance encoded in JSON (46 bytes)

   In CDDL [RFC8610], 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:

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   authorization-info = [* authorization]
   authorization = [
     path: tstr,
     permissions: uint .bits methods,
   ]
   methods = &(
     GET: 0
     POST: 1
     PUT: 2
     DELETE: 3
     FETCH: 4
     PATCH: 5
     iPATCH: 6
   )

                           Figure 2: AIF in CDDL

   A representation of this information in CBOR [RFC7049] is given in
   Figure 3; again, several optimizations/improvements are possible.

   83                        # array(3)
      82                     # array(2)
         68                  # text(8)
            2f732f6c69676874 # "/s/light"
         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 3: An authorization instance encoded in CBOR (29 bytes)

4.  IANA Considerations

   This document makes no requirements on IANA.  (This section to be
   removed by RFC editor.)

5.  Security Considerations

   (TBD.  Some issues are already discussed in the security
   considerations of [RFC7252] and in [RFC8576].)

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6.  References

6.1.  Normative References

   [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>.

   [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>.

6.2.  Informative References

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-dtls-
              authorize-09 (work in progress), December 2019.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE profile of the Authentication and Authorization
              for Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-08 (work in progress), July 2019.

   [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>.

   [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>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [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>.

   [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>.

Acknowledgements

   TBD

Author's Address

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

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

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