Internet Engineering Task Force (IETF) C. Bormann

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)

Abstract

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
path.

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
https://www.rfc-editor.org/info/rfc9237.

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
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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
Acknowledgements
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
devices.

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",
"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.

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
devices).

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
others.

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.

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

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
[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 "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:

Toid:
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).

Tperm:
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 ([email protected]) or IETF Applications and Real-Time
Area ([email protected])

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:

Toid:
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).

Tperm:
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 ([email protected]) or IETF Applications and Real-Time
Area ([email protected])

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
languages.

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
combinations.

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
available.

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>.

[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>.

[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.

[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>.

[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[RFC9165] Bormann, C., "Additional Control Operators for the Concise
Data Definition Language (CDDL)", RFC 9165,
DOI 10.17487/RFC9165, December 2021,
<https://www.rfc-editor.org/info/rfc9165>.

7.2. Informative References

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

[IANA.media-type-sub-parameters]
IANA, "MIME Media Type Sub-Parameter Registries",
<https://www.iana.org/assignments/media-type-sub-
parameters>.

[KebabCase]
"Kebab Case", 29 August 2014,
<http://wiki.c2.com/?KebabCase>.

[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>.

[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>.

[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>.

[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,
<https://www.rfc-editor.org/info/rfc9200>.

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. 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
document.

Author's Address

Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: [email protected]