RFC 9237 | ACE AIF | August 2022 |
Bormann | Standards Track | [Page] |
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.¶
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 (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.¶
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.¶
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.¶
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.¶
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).¶
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 |
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.¶
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.¶
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 |
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.¶
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:¶
This data model could be interchanged in the JSON [RFC8259] representation given in Figure 3.¶
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]:¶
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.¶
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].¶
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).¶
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 |
application¶
aif+cbor¶
N/A¶
Toid
:Toid
.
Default value: "URI-local-part" (RFC 9237).¶
Tperm
: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).¶
binary (CBOR)¶
N/A¶
Applications that need to convey structured authorization data for identified resources, conveying sets of permissions.¶
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".)¶
ACE WG mailing list (ace@ietf.org) or IETF Applications and Real-Time Area (art@ietf.org)¶
COMMON¶
N/A¶
IETF¶
no¶
application¶
aif+json¶
N/A¶
Toid
:Toid
.
Default value: "URI-local-part" (RFC 9237).¶
Tperm
:Toid
.
A value from the media-type parameter subregistry for Tperm
.
Default value: "REST-method-set" (RFC 9237).¶
binary (JSON is UTF-8-encoded text)¶
N/A¶
Applications that need to convey structured authorization data for identified resources, conveying sets of permissions.¶
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".)¶
ACE WG mailing list (ace@ietf.org) or IETF Applications and Real-Time Area (art@ietf.org)¶
COMMON¶
N/A¶
IETF¶
no¶
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/Specification | Reference |
---|---|---|---|
Toid | URI-local-part | local-part of URI | RFC 9237 |
Tperm | REST-method-set | set of REST methods represented | RFC 9237 |
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:¶
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 designated experts will develop further criteria and guidelines as needed.¶
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 |
Note that applications that register Toid
and Tperm
values are
encouraged to also register Content-Formats for the relevant
combinations.¶
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:¶
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.¶
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.¶