ACE Working Group L. Seitz
Internet-Draft Combitech
Intended status: Standards Track G. Selander
Expires: December 25, 2020 Ericsson
E. Wahlstroem
S. Erdtman
Spotify AB
H. Tschofenig
Arm Ltd.
June 23, 2020
Authentication and Authorization for Constrained Environments (ACE)
using the OAuth 2.0 Framework (ACE-OAuth)
draft-ietf-ace-oauth-authz-34
Abstract
This specification defines a framework for authentication and
authorization in Internet of Things (IoT) environments called ACE-
OAuth. The framework is based on a set of building blocks including
OAuth 2.0 and the Constrained Application Protocol (CoAP), thus
transforming a well-known and widely used authorization solution into
a form suitable for IoT devices. Existing specifications are used
where possible, but extensions are added and profiles are defined to
better serve the IoT use cases.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 25, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. OAuth 2.0 . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. CoAP . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Protocol Interactions . . . . . . . . . . . . . . . . . . . . 11
5. Framework . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Discovering Authorization Servers . . . . . . . . . . . . 16
5.1.1. Unauthorized Resource Request Message . . . . . . . . 16
5.1.2. AS Request Creation Hints . . . . . . . . . . . . . . 17
5.1.2.1. The Client-Nonce Parameter . . . . . . . . . . . 19
5.2. Authorization Grants . . . . . . . . . . . . . . . . . . 20
5.3. Client Credentials . . . . . . . . . . . . . . . . . . . 20
5.4. AS Authentication . . . . . . . . . . . . . . . . . . . . 21
5.5. The Authorization Endpoint . . . . . . . . . . . . . . . 21
5.6. The Token Endpoint . . . . . . . . . . . . . . . . . . . 21
5.6.1. Client-to-AS Request . . . . . . . . . . . . . . . . 22
5.6.2. AS-to-Client Response . . . . . . . . . . . . . . . . 25
5.6.3. Error Response . . . . . . . . . . . . . . . . . . . 27
5.6.4. Request and Response Parameters . . . . . . . . . . . 28
5.6.4.1. Grant Type . . . . . . . . . . . . . . . . . . . 28
5.6.4.2. Token Type . . . . . . . . . . . . . . . . . . . 29
5.6.4.3. Profile . . . . . . . . . . . . . . . . . . . . . 29
5.6.4.4. Client-Nonce . . . . . . . . . . . . . . . . . . 30
5.6.5. Mapping Parameters to CBOR . . . . . . . . . . . . . 30
5.7. The Introspection Endpoint . . . . . . . . . . . . . . . 31
5.7.1. Introspection Request . . . . . . . . . . . . . . . . 32
5.7.2. Introspection Response . . . . . . . . . . . . . . . 33
5.7.3. Error Response . . . . . . . . . . . . . . . . . . . 34
5.7.4. Mapping Introspection parameters to CBOR . . . . . . 35
5.8. The Access Token . . . . . . . . . . . . . . . . . . . . 35
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5.8.1. The Authorization Information Endpoint . . . . . . . 36
5.8.1.1. Verifying an Access Token . . . . . . . . . . . . 37
5.8.1.2. Protecting the Authorization Information
Endpoint . . . . . . . . . . . . . . . . . . . . 39
5.8.2. Client Requests to the RS . . . . . . . . . . . . . . 39
5.8.3. Token Expiration . . . . . . . . . . . . . . . . . . 40
5.8.4. Key Expiration . . . . . . . . . . . . . . . . . . . 41
6. Security Considerations . . . . . . . . . . . . . . . . . . . 42
6.1. Protecting Tokens . . . . . . . . . . . . . . . . . . . . 42
6.2. Communication Security . . . . . . . . . . . . . . . . . 43
6.3. Long-Term Credentials . . . . . . . . . . . . . . . . . . 44
6.4. Unprotected AS Request Creation Hints . . . . . . . . . . 44
6.5. Minimal security requirements for communication . 45
6.6. Token Freshness and Expiration . . . . . . . . . . . . . 46
6.7. Combining profiles . . . . . . . . . . . . . . . . . . . 47
6.8. Unprotected Information . . . . . . . . . . . . . . . . . 47
6.9. Identifying audiences . . . . . . . . . . . . . . . . . . 48
6.10. Denial of service against or with Introspection . . 48
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 49
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50
8.1. ACE Authorization Server Request Creation Hints . . . . . 50
8.2. CoRE Resource Type registry . . . . . . . . . . . . . . . 51
8.3. OAuth Extensions Error Registration . . . . . . . . . . . 51
8.4. OAuth Error Code CBOR Mappings Registry . . . . . . . . . 51
8.5. OAuth Grant Type CBOR Mappings . . . . . . . . . . . . . 52
8.6. OAuth Access Token Types . . . . . . . . . . . . . . . . 52
8.7. OAuth Access Token Type CBOR Mappings . . . . . . . . . . 52
8.7.1. Initial Registry Contents . . . . . . . . . . . . . . 53
8.8. ACE Profile Registry . . . . . . . . . . . . . . . . . . 53
8.9. OAuth Parameter Registration . . . . . . . . . . . . . . 54
8.10. OAuth Parameters CBOR Mappings Registry . . . . . . . . . 54
8.11. OAuth Introspection Response Parameter Registration . . . 54
8.12. OAuth Token Introspection Response CBOR Mappings Registry 55
8.13. JSON Web Token Claims . . . . . . . . . . . . . . . . . . 55
8.14. CBOR Web Token Claims . . . . . . . . . . . . . . . . . . 56
8.15. Media Type Registrations . . . . . . . . . . . . . . . . 57
8.16. CoAP Content-Format Registry . . . . . . . . . . . . . . 58
8.17. Expert Review Instructions . . . . . . . . . . . . . . . 58
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 59
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 59
10.1. Normative References . . . . . . . . . . . . . . . . . . 59
10.2. Informative References . . . . . . . . . . . . . . . . . 62
Appendix A. Design Justification . . . . . . . . . . . . . . . . 65
Appendix B. Roles and Responsibilities . . . . . . . . . . . . . 68
Appendix C. Requirements on Profiles . . . . . . . . . . . . . . 71
Appendix D. Assumptions on AS knowledge about C and RS . . . . . 71
Appendix E. Deployment Examples . . . . . . . . . . . . . . . . 72
E.1. Local Token Validation . . . . . . . . . . . . . . . . . 72
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E.2. Introspection Aided Token Validation . . . . . . . . . . 76
Appendix F. Document Updates . . . . . . . . . . . . . . . . . . 80
F.1. Version -21 to 22 . . . . . . . . . . . . . . . . . . . . 81
F.2. Version -20 to 21 . . . . . . . . . . . . . . . . . . . . 81
F.3. Version -19 to 20 . . . . . . . . . . . . . . . . . . . . 81
F.4. Version -18 to -19 . . . . . . . . . . . . . . . . . . . 81
F.5. Version -17 to -18 . . . . . . . . . . . . . . . . . . . 81
F.6. Version -16 to -17 . . . . . . . . . . . . . . . . . . . 81
F.7. Version -15 to -16 . . . . . . . . . . . . . . . . . . . 82
F.8. Version -14 to -15 . . . . . . . . . . . . . . . . . . . 82
F.9. Version -13 to -14 . . . . . . . . . . . . . . . . . . . 82
F.10. Version -12 to -13 . . . . . . . . . . . . . . . . . . . 82
F.11. Version -11 to -12 . . . . . . . . . . . . . . . . . . . 83
F.12. Version -10 to -11 . . . . . . . . . . . . . . . . . . . 83
F.13. Version -09 to -10 . . . . . . . . . . . . . . . . . . . 83
F.14. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 83
F.15. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 83
F.16. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 84
F.17. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 84
F.18. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 84
F.19. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 85
F.20. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 85
F.21. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 85
F.22. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 86
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 86
1. Introduction
Authorization is the process for granting approval to an entity to
access a generic resource [RFC4949]. The authorization task itself
can best be described as granting access to a requesting client, for
a resource hosted on a device, the resource server (RS). This
exchange is mediated by one or multiple authorization servers (AS).
Managing authorization for a large number of devices and users can be
a complex task.
While prior work on authorization solutions for the Web and for the
mobile environment also applies to the Internet of Things (IoT)
environment, many IoT devices are constrained, for example, in terms
of processing capabilities, available memory, etc. For web
applications on constrained nodes, this specification RECOMMENDS the
use of the Constrained Application Protocol (CoAP) [RFC7252] as
replacement for HTTP.
Appendix A gives an overview of the constraints considered in this
design, and a more detailed treatment of constraints can be found in
[RFC7228]. This design aims to accommodate different IoT deployments
and thus a continuous range of device and network capabilities.
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Taking energy consumption as an example: At one end there are energy-
harvesting or battery powered devices which have a tight power
budget, on the other end there are mains-powered devices, and all
levels in between.
Hence, IoT devices may be very different in terms of available
processing and message exchange capabilities and there is a need to
support many different authorization use cases [RFC7744].
This specification describes a framework for authentication and
authorization in constrained environments (ACE) built on re-use of
OAuth 2.0 [RFC6749], thereby extending authorization to Internet of
Things devices. This specification contains the necessary building
blocks for adjusting OAuth 2.0 to IoT environments.
More detailed, interoperable specifications can be found in separate
profile specifications. Implementations may claim conformance with a
specific profile, whereby implementations utilizing the same profile
interoperate while implementations of different profiles are not
expected to be interoperable. Some devices, such as mobile phones
and tablets, may implement multiple profiles and will therefore be
able to interact with a wider range of low end devices. Requirements
on profiles are described at contextually appropriate places
throughout this specification, and also summarized in Appendix C.
2. Terminology
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.
Certain security-related terms such as "authentication",
"authorization", "confidentiality", "(data) integrity", "message
authentication code", and "verify" are taken from [RFC4949].
Since exchanges in this specification are described as RESTful
protocol interactions, HTTP [RFC7231] offers useful terminology.
Terminology for entities in the architecture is defined in OAuth 2.0
[RFC6749] such as client (C), resource server (RS), and authorization
server (AS).
Note that the term "endpoint" is used here following its OAuth
definition, which is to denote resources such as token and
introspection at the AS and authz-info at the RS (see Section 5.8.1
for a definition of the authz-info endpoint). The CoAP [RFC7252]
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definition, which is "An entity participating in the CoAP protocol"
is not used in this specification.
The specifications in this document is called the "framework" or "ACE
framework". When referring to "profiles of this framework" it refers
to additional specifications that define the use of this
specification with concrete transport and communication security
protocols (e.g., CoAP over DTLS).
We use the term "Access Information" for parameters other than the
access token provided to the client by the AS to enable it to access
the RS (e.g. public key of the RS, profile supported by RS).
We use the term "Authorization Information" to denote all
information, including the claims of relevant access tokens, that an
RS uses to determine whether an access request should be granted.
3. Overview
This specification defines the ACE framework for authorization in the
Internet of Things environment. It consists of a set of building
blocks.
The basic block is the OAuth 2.0 [RFC6749] framework, which enjoys
widespread deployment. Many IoT devices can support OAuth 2.0
without any additional extensions, but for certain constrained
settings additional profiling is needed.
Another building block is the lightweight web transfer protocol CoAP
[RFC7252], for those communication environments where HTTP is not
appropriate. CoAP typically runs on top of UDP, which further
reduces overhead and message exchanges. While this specification
defines extensions for the use of OAuth over CoAP, other underlying
protocols are not prohibited from being supported in the future, such
as HTTP/2 [RFC7540], Message Queuing Telemetry Transport (MQTT)
[MQTT5.0], Bluetooth Low Energy (BLE) [BLE] and QUIC
[I-D.ietf-quic-transport]. Note that this document specifies
protocol exchanges in terms of RESTful verbs such as GET and POST.
Future profiles using protocols that do not support these verbs MUST
specify how the corresponding protocol messages are transmitted
instead.
A third building block is the Concise Binary Object Representation
(CBOR) [RFC7049], for encodings where JSON [RFC8259] is not
sufficiently compact. CBOR is a binary encoding designed for small
code and message size, which may be used for encoding of self
contained tokens, and also for encoding payloads transferred in
protocol messages.
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A fourth building block is CBOR Object Signing and Encryption (COSE)
[RFC8152], which enables object-level layer security as an
alternative or complement to transport layer security (DTLS [RFC6347]
or TLS [RFC8446]). COSE is used to secure self-contained tokens such
as proof-of-possession (PoP) tokens, which are an extension to the
OAuth bearer tokens. The default token format is defined in CBOR web
token (CWT) [RFC8392]. Application layer security for CoAP using
COSE can be provided with OSCORE [RFC8613].
With the building blocks listed above, solutions satisfying various
IoT device and network constraints are possible. A list of
constraints is described in detail in [RFC7228] and a description of
how the building blocks mentioned above relate to the various
constraints can be found in Appendix A.
Luckily, not every IoT device suffers from all constraints. The ACE
framework nevertheless takes all these aspects into account and
allows several different deployment variants to co-exist, rather than
mandating a one-size-fits-all solution. It is important to cover the
wide range of possible interworking use cases and the different
requirements from a security point of view. Once IoT deployments
mature, popular deployment variants will be documented in the form of
ACE profiles.
3.1. OAuth 2.0
The OAuth 2.0 authorization framework enables a client to obtain
scoped access to a resource with the permission of a resource owner.
Authorization information, or references to it, is passed between the
nodes using access tokens. These access tokens are issued to clients
by an authorization server with the approval of the resource owner.
The client uses the access token to access the protected resources
hosted by the resource server.
A number of OAuth 2.0 terms are used within this specification:
The token and introspection Endpoints:
The AS hosts the token endpoint that allows a client to request
access tokens. The client makes a POST request to the token
endpoint on the AS and receives the access token in the response
(if the request was successful).
In some deployments, a token introspection endpoint is provided by
the AS, which can be used by the RS if it needs to request
additional information regarding a received access token. The RS
makes a POST request to the introspection endpoint on the AS and
receives information about the access token in the response. (See
"Introspection" below.)
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Access Tokens:
Access tokens are credentials needed to access protected
resources. An access token is a data structure representing
authorization permissions issued by the AS to the client. Access
tokens are generated by the AS and consumed by the RS. The access
token content is opaque to the client.
Access tokens can have different formats, and various methods of
utilization e.g., cryptographic properties) based on the security
requirements of the given deployment.
Refresh Tokens:
Refresh tokens are credentials used to obtain access tokens.
Refresh tokens are issued to the client by the authorization
server and are used to obtain a new access token when the current
access token becomes invalid or expires, or to obtain additional
access tokens with identical or narrower scope (such access tokens
may have a shorter lifetime and fewer permissions than authorized
by the resource owner). Issuing a refresh token is optional at
the discretion of the authorization server. If the authorization
server issues a refresh token, it is included when issuing an
access token (i.e., step (B) in Figure 1).
A refresh token in OAuth 2.0 is a string representing the
authorization granted to the client by the resource owner. The
string is usually opaque to the client. The token denotes an
identifier used to retrieve the authorization information. Unlike
access tokens, refresh tokens are intended for use only with
authorization servers and are never sent to resource servers. In
this framework, refresh tokens are encoded in binary instead of
strings, if used.
Proof of Possession Tokens:
A token may be bound to a cryptographic key, which is then used to
bind the token to a request authorized by the token. Such tokens
are called proof-of-possession tokens (or PoP tokens).
The proof-of-possession (PoP) security concept used here assumes
that the AS acts as a trusted third party that binds keys to
tokens. In the case of access tokens, these so called PoP keys
are then used by the client to demonstrate the possession of the
secret to the RS when accessing the resource. The RS, when
receiving an access token, needs to verify that the key used by
the client matches the one bound to the access token. When this
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specification uses the term "access token" it is assumed to be a
PoP access token token unless specifically stated otherwise.
The key bound to the token (the PoP key) may use either symmetric
or asymmetric cryptography. The appropriate choice of the kind of
cryptography depends on the constraints of the IoT devices as well
as on the security requirements of the use case.
Symmetric PoP key:
The AS generates a random symmetric PoP key. The key is either
stored to be returned on introspection calls or encrypted and
included in the token. The PoP key is also encrypted for the
token recipient and sent to the recipient together with the
token.
Asymmetric PoP key:
An asymmetric key pair is generated on the token's recipient
and the public key is sent to the AS (if it does not already
have knowledge of the recipient's public key). Information
about the public key, which is the PoP key in this case, is
either stored to be returned on introspection calls or included
inside the token and sent back to the requesting party. The
consumer of the token can identify the public key from the
information in the token, which allows the recipient of the
token to use the corresponding private key for the proof of
possession.
The token is either a simple reference, or a structured
information object (e.g., CWT [RFC8392]) protected by a
cryptographic wrapper (e.g., COSE [RFC8152]). The choice of PoP
key does not necessarily imply a specific credential type for the
integrity protection of the token.
Scopes and Permissions:
In OAuth 2.0, the client specifies the type of permissions it is
seeking to obtain (via the scope parameter) in the access token
request. In turn, the AS may use the scope response parameter to
inform the client of the scope of the access token issued. As the
client could be a constrained device as well, this specification
defines the use of CBOR encoding, see Section 5, for such requests
and responses.
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The values of the scope parameter in OAuth 2.0 are expressed as a
list of space-delimited, case-sensitive strings, with a semantic
that is well-known to the AS and the RS. More details about the
concept of scopes is found under Section 3.3 in [RFC6749].
Claims:
Information carried in the access token or returned from
introspection, called claims, is in the form of name-value pairs.
An access token may, for example, include a claim identifying the
AS that issued the token (via the "iss" claim) and what audience
the access token is intended for (via the "aud" claim). The
audience of an access token can be a specific resource or one or
many resource servers. The resource owner policies influence what
claims are put into the access token by the authorization server.
While the structure and encoding of the access token varies
throughout deployments, a standardized format has been defined
with the JSON Web Token (JWT) [RFC7519] where claims are encoded
as a JSON object. In [RFC8392], an equivalent format using CBOR
encoding (CWT) has been defined.
Introspection:
Introspection is a method for a resource server to query the
authorization server for the active state and content of a
received access token. This is particularly useful in those cases
where the authorization decisions are very dynamic and/or where
the received access token itself is an opaque reference rather
than a self-contained token. More information about introspection
in OAuth 2.0 can be found in [RFC7662].
3.2. CoAP
CoAP is an application layer protocol similar to HTTP, but
specifically designed for constrained environments. CoAP typically
uses datagram-oriented transport, such as UDP, where reordering and
loss of packets can occur. A security solution needs to take the
latter aspects into account.
While HTTP uses headers and query strings to convey additional
information about a request, CoAP encodes such information into
header parameters called 'options'.
CoAP supports application-layer fragmentation of the CoAP payloads
through blockwise transfers [RFC7959]. However, blockwise transfer
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does not increase the size limits of CoAP options, therefore data
encoded in options has to be kept small.
Transport layer security for CoAP can be provided by DTLS or TLS
[RFC6347][RFC8446] [I-D.ietf-tls-dtls13]. CoAP defines a number of
proxy operations that require transport layer security to be
terminated at the proxy. One approach for protecting CoAP
communication end-to-end through proxies, and also to support
security for CoAP over a different transport in a uniform way, is to
provide security at the application layer using an object-based
security mechanism such as COSE [RFC8152].
One application of COSE is OSCORE [RFC8613], which provides end-to-
end confidentiality, integrity and replay protection, and a secure
binding between CoAP request and response messages. In OSCORE, the
CoAP messages are wrapped in COSE objects and sent using CoAP.
This framework RECOMMENDS the use of CoAP as replacement for HTTP for
use in constrained environments. For communication security this
framework does not make an explicit protocol recommendation, since
the choice depends on the requirements of the specific application.
DTLS [RFC6347], [I-D.ietf-tls-dtls13] and OSCORE [RFC8613] are
mentioned as examples, other protocols fulfilling the requirements
from Section 6.5 are also applicable.
4. Protocol Interactions
The ACE framework is based on the OAuth 2.0 protocol interactions
using the token endpoint and optionally the introspection endpoint.
A client obtains an access token, and optionally a refresh token,
from an AS using the token endpoint and subsequently presents the
access token to an RS to gain access to a protected resource. In
most deployments the RS can process the access token locally, however
in some cases the RS may present it to the AS via the introspection
endpoint to get fresh information. These interactions are shown in
Figure 1. An overview of various OAuth concepts is provided in
Section 3.1.
The OAuth 2.0 framework defines a number of "protocol flows" via
grant types, which have been extended further with extensions to
OAuth 2.0 (such as [RFC7521] and [RFC8628]). What grant types works
best depends on the usage scenario and [RFC7744] describes many
different IoT use cases but there are two preferred grant types,
namely the Authorization Code Grant (described in Section 4.1 of
[RFC7521]) and the Client Credentials Grant (described in Section 4.4
of [RFC7521]). The Authorization Code Grant is a good fit for use
with apps running on smart phones and tablets that request access to
IoT devices, a common scenario in the smart home environment, where
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users need to go through an authentication and authorization phase
(at least during the initial setup phase). The native apps
guidelines described in [RFC8252] are applicable to this use case.
The Client Credential Grant is a good fit for use with IoT devices
where the OAuth client itself is constrained. In such a case, the
resource owner has pre-arranged access rights for the client with the
authorization server, which is often accomplished using a
commissioning tool.
The consent of the resource owner, for giving a client access to a
protected resource, can be provided dynamically as in the traditional
OAuth flows, or it could be pre-configured by the resource owner as
authorization policies at the AS, which the AS evaluates when a token
request arrives. The resource owner and the requesting party (i.e.,
client owner) are not shown in Figure 1.
This framework supports a wide variety of communication security
mechanisms between the ACE entities, such as client, AS, and RS. It
is assumed that the client has been registered (also called enrolled
or onboarded) to an AS using a mechanism defined outside the scope of
this document. In practice, various techniques for onboarding have
been used, such as factory-based provisioning or the use of
commissioning tools. Regardless of the onboarding technique, this
provisioning procedure implies that the client and the AS exchange
credentials and configuration parameters. These credentials are used
to mutually authenticate each other and to protect messages exchanged
between the client and the AS.
It is also assumed that the RS has been registered with the AS,
potentially in a similar way as the client has been registered with
the AS. Established keying material between the AS and the RS allows
the AS to apply cryptographic protection to the access token to
ensure that its content cannot be modified, and if needed, that the
content is confidentiality protected.
The keying material necessary for establishing communication security
between C and RS is dynamically established as part of the protocol
described in this document.
At the start of the protocol, there is an optional discovery step
where the client discovers the resource server and the resources this
server hosts. In this step, the client might also determine what
permissions are needed to access the protected resource. A generic
procedure is described in Section 5.1; profiles MAY define other
procedures for discovery.
In Bluetooth Low Energy, for example, advertisements are broadcasted
by a peripheral, including information about the primary services.
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In CoAP, as a second example, a client can make a request to "/.well-
known/core" to obtain information about available resources, which
are returned in a standardized format as described in [RFC6690].
+--------+ +---------------+
| |---(A)-- Token Request ------->| |
| | | Authorization |
| |<--(B)-- Access Token ---------| Server |
| | + Access Information | |
| | + Refresh Token (optional) +---------------+
| | ^ |
| | Introspection Request (D)| |
| Client | (optional) | |
| | Response | |(E)
| | (optional) | v
| | +--------------+
| |---(C)-- Token + Request ----->| |
| | | Resource |
| |<--(F)-- Protected Resource ---| Server |
| | | |
+--------+ +--------------+
Figure 1: Basic Protocol Flow.
Requesting an Access Token (A):
The client makes an access token request to the token endpoint at
the AS. This framework assumes the use of PoP access tokens (see
Section 3.1 for a short description) wherein the AS binds a key to
an access token. The client may include permissions it seeks to
obtain, and information about the credentials it wants to use
(e.g., symmetric/asymmetric cryptography or a reference to a
specific credential).
Access Token Response (B):
If the AS successfully processes the request from the client, it
returns an access token and optionally a refresh token (note that
only certain grant types support refresh tokens). It can also
return additional parameters, referred to as "Access Information".
In addition to the response parameters defined by OAuth 2.0 and
the PoP access token extension, this framework defines parameters
that can be used to inform the client about capabilities of the
RS, e.g. the profiles the RS supports. More information about
these parameters can be found in Section 5.6.4.
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Resource Request (C):
The client interacts with the RS to request access to the
protected resource and provides the access token. The protocol to
use between the client and the RS is not restricted to CoAP.
HTTP, HTTP/2, QUIC, MQTT, Bluetooth Low Energy, etc., are also
viable candidates.
Depending on the device limitations and the selected protocol,
this exchange may be split up into two parts:
(1) the client sends the access token containing, or
referencing, the authorization information to the RS, that may
be used for subsequent resource requests by the client, and
(2) the client makes the resource access request, using the
communication security protocol and other Access Information
obtained from the AS.
The Client and the RS mutually authenticate using the security
protocol specified in the profile (see step B) and the keys
obtained in the access token or the Access Information. The RS
verifies that the token is integrity protected and originated by
the AS. It then compares the claims contained in the access token
with the resource request. If the RS is online, validation can be
handed over to the AS using token introspection (see messages D
and E) over HTTP or CoAP.
Token Introspection Request (D):
A resource server may be configured to introspect the access token
by including it in a request to the introspection endpoint at that
AS. Token introspection over CoAP is defined in Section 5.7 and
for HTTP in [RFC7662].
Note that token introspection is an optional step and can be
omitted if the token is self-contained and the resource server is
prepared to perform the token validation on its own.
Token Introspection Response (E):
The AS validates the token and returns the most recent parameters,
such as scope, audience, validity etc. associated with it back to
the RS. The RS then uses the received parameters to process the
request to either accept or to deny it.
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Protected Resource (F):
If the request from the client is authorized, the RS fulfills the
request and returns a response with the appropriate response code.
The RS uses the dynamically established keys to protect the
response, according to the communication security protocol used.
5. Framework
The following sections detail the profiling and extensions of OAuth
2.0 for constrained environments, which constitutes the ACE
framework.
Credential Provisioning
For IoT, it cannot be assumed that the client and RS are part of a
common key infrastructure, so the AS provisions credentials or
associated information to allow mutual authentication between
client and RS. The resulting security association between client
and RS may then also be used to bind these credentials to the
access tokens the client uses.
Proof-of-Possession
The ACE framework, by default, implements proof-of-possession for
access tokens, i.e., that the token holder can prove being a
holder of the key bound to the token. The binding is provided by
the "cnf" claim [RFC8747] indicating what key is used for proof-
of-possession. If a client needs to submit a new access token,
e.g., to obtain additional access rights, they can request that
the AS binds this token to the same key as the previous one.
ACE Profiles
The client or RS may be limited in the encodings or protocols it
supports. To support a variety of different deployment settings,
specific interactions between client and RS are defined in an ACE
profile. In ACE framework the AS is expected to manage the
matching of compatible profile choices between a client and an RS.
The AS informs the client of the selected profile using the
"ace_profile" parameter in the token response.
OAuth 2.0 requires the use of TLS both to protect the communication
between AS and client when requesting an access token; between client
and RS when accessing a resource and between AS and RS if
introspection is used. In constrained settings TLS is not always
feasible, or desirable. Nevertheless it is REQUIRED that the
communications named above are encrypted, integrity protected and
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protected against message replay. It is also REQUIRED that the
communicating endpoints perform mutual authentication. Furthermore
it MUST be assured that responses are bound to the requests in the
sense that the receiver of a response can be certain that the
response actually belongs to a certain request. Note that setting up
such a secure communication may require some unprotected messages to
be exchanged first (e.g. sending the token from the client to the
RS).
Profiles MUST specify a communication security protocol that provides
the features required above.
In OAuth 2.0 the communication with the Token and the Introspection
endpoints at the AS is assumed to be via HTTP and may use Uri-query
parameters. When profiles of this framework use CoAP instead, it is
REQUIRED to use of the following alternative instead of Uri-query
parameters: The sender (client or RS) encodes the parameters of its
request as a CBOR map and submits that map as the payload of the POST
request.
Profiles that use CBOR encoding of protocol message parameters at the
outermost encoding layer MUST use the media format 'application/
ace+cbor'. If CoAP is used for communication, the Content-Format
MUST be abbreviated with the ID: 19 (see Section 8.16).
The OAuth 2.0 AS uses a JSON structure in the payload of its
responses both to client and RS. If CoAP is used, it is REQUIRED to
use CBOR [RFC7049] instead of JSON. Depending on the profile, the
CBOR payload MAY be enclosed in a non-CBOR cryptographic wrapper.
5.1. Discovering Authorization Servers
In order to determine the AS in charge of a resource hosted at the
RS, C MAY send an initial Unauthorized Resource Request message to
RS. RS then denies the request and sends the address of its AS back
to C.
Instead of the initial Unauthorized Resource Request message, other
discovery methods may be used, or the client may be pre-provisioned
with an RS-to-AS mapping.
5.1.1. Unauthorized Resource Request Message
An Unauthorized Resource Request message is a request for any
resource hosted by RS for which the client does not have
authorization granted. RSes MUST treat any request for a protected
resource as an Unauthorized Resource Request message when any of the
following hold:
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o The request has been received on an unprotected channel.
o The RS has no valid access token for the sender of the request
regarding the requested action on that resource.
o The RS has a valid access token for the sender of the request, but
that token does not authorize the requested action on the
requested resource.
Note: These conditions ensure that the RS can handle requests
autonomously once access was granted and a secure channel has been
established between C and RS. The authz-info endpoint, as part of
the process for authorizing to protected resources, is not itself a
protected resource and MUST NOT be protected as specified above (cf.
Section 5.8.1).
Unauthorized Resource Request messages MUST be denied with an
"unauthorized_client" error response. In this response, the Resource
Server SHOULD provide proper AS Request Creation Hints to enable the
Client to request an access token from RS's AS as described in
Section 5.1.2.
The handling of all client requests (including unauthorized ones) by
the RS is described in Section 5.8.2.
5.1.2. AS Request Creation Hints
The AS Request Creation Hints message is sent by an RS as a response
to an Unauthorized Resource Request message (see Section 5.1.1) to
help the sender of the Unauthorized Resource Request message acquire
a valid access token. The AS Request Creation Hints message is a
CBOR map, with a MANDATORY element "AS" specifying an absolute URI
(see Section 4.3 of [RFC3986]) that identifies the appropriate AS for
the RS.
The message can also contain the following OPTIONAL parameters:
o A "audience" element containing a suggested audience that the
client should request at the AS.
o A "kid" element containing the key identifier of a key used in an
existing security association between the client and the RS. The
RS expects the client to request an access token bound to this
key, in order to avoid having to re-establish the security
association.
o A "cnonce" element containing a client-nonce. See
Section 5.1.2.1.
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o A "scope" element containing the suggested scope that the client
should request towards the AS.
Figure 2 summarizes the parameters that may be part of the AS Request
Creation Hints.
/-----------+----------+---------------------\
| Name | CBOR Key | Value Type |
|-----------+----------+---------------------|
| AS | 1 | text string |
| kid | 2 | byte string |
| audience | 5 | text string |
| scope | 9 | text or byte string |
| cnonce | 39 | byte string |
\-----------+----------+---------------------/
Figure 2: AS Request Creation Hints
Note that the schema part of the AS parameter may need to be adapted
to the security protocol that is used between the client and the AS.
Thus the example AS value "coap://as.example.com/token" might need to
be transformed to "coaps://as.example.com/token". It is assumed that
the client can determine the correct schema part on its own depending
on the way it communicates with the AS.
Figure 3 shows an example for an AS Request Creation Hints message
payload using CBOR [RFC7049] diagnostic notation, using the parameter
names instead of the CBOR keys for better human readability.
4.01 Unauthorized
Content-Format: application/ace+cbor
Payload :
{
"AS" : "coaps://as.example.com/token",
"audience" : "coaps://rs.example.com"
"scope" : "rTempC",
"cnonce" : h'e0a156bb3f'
}
Figure 3: AS Request Creation Hints payload example
In the example above, the response parameter "AS" points the receiver
of this message to the URI "coaps://as.example.com/token" to request
access tokens. The RS sending this response (i.e., RS) uses an
internal clock that is only loosely synchronized with the clock of
the AS. Therefore it can not reliably verify the expiration time of
access tokens it receives. To ensure a certain level of access token
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freshness nevetheless, the RS has included a "cnonce" parameter (see
Section 5.1.2.1) in the response.
Figure 4 illustrates the mandatory to use binary encoding of the
message payload shown in Figure 3.
a4 # map(4)
01 # unsigned(1) (=AS)
78 1c # text(28)
636f6170733a2f2f61732e657861
6d706c652e636f6d2f746f6b656e # "coaps://as.example.com/token"
05 # unsigned(5) (=audience)
76 # text(22)
636f6170733a2f2f72732e657861
6d706c652e636f6d # "coaps://rs.example.com"
09 # unsigned(9) (=scope)
66 # text(6)
7254656d7043 # "rTempC"
18 27 # unsigned(39) (=cnonce)
45 # bytes(5)
e0a156bb3f #
Figure 4: AS Request Creation Hints example encoded in CBOR
5.1.2.1. The Client-Nonce Parameter
If the RS does not synchronize its clock with the AS, it could be
tricked into accepting old access tokens, that are either expired or
have been compromised. In order to ensure some level of token
freshness in that case, the RS can use the "cnonce" (client-nonce)
parameter. The processing requirements for this parameter are as
follows:
o An RS sending a "cnonce" parameter in an AS Request Creation Hints
message MUST store information to validate that a given cnonce is
fresh. How this is implemented internally is out of scope for
this specification. Expiration of client-nonces should be based
roughly on the time it would take a client to obtain an access
token after receiving the AS Request Creation Hints message, with
some allowance for unexpected delays.
o A client receiving a "cnonce" parameter in an AS Request Creation
Hints message MUST include this in the parameters when requesting
an access token at the AS, using the "cnonce" parameter from
Section 5.6.4.4.
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o If an AS grants an access token request containing a "cnonce"
parameter, it MUST include this value in the access token, using
the "cnonce" claim specified in Section 5.8.
o An RS that is using the client-nonce mechanism and that receives
an access token MUST verify that this token contains a cnonce
claim, with a client-nonce value that is fresh according to the
information stored at the first step above. If the cnonce claim
is not present or if the cnonce claim value is not fresh, the RS
MUST discard the access token. If this was an interaction with
the authz-info endpoint the RS MUST also respond with an error
message using a response code equivalent to the CoAP code 4.01
(Unauthorized).
5.2. Authorization Grants
To request an access token, the client obtains authorization from the
resource owner or uses its client credentials as a grant. The
authorization is expressed in the form of an authorization grant.
The OAuth framework [RFC6749] defines four grant types. The grant
types can be split up into two groups, those granted on behalf of the
resource owner (password, authorization code, implicit) and those for
the client (client credentials). Further grant types have been added
later, such as [RFC7521] defining an assertion-based authorization
grant.
The grant type is selected depending on the use case. In cases where
the client acts on behalf of the resource owner, the authorization
code grant is recommended. If the client acts on behalf of the
resource owner, but does not have any display or has very limited
interaction possibilities, it is recommended to use the device code
grant defined in [RFC8628]. In cases where the client acts
autonomously the client credentials grant is recommended.
For details on the different grant types, see section 1.3 of
[RFC6749]. The OAuth 2.0 framework provides an extension mechanism
for defining additional grant types, so profiles of this framework
MAY define additional grant types, if needed.
5.3. Client Credentials
Authentication of the client is mandatory independent of the grant
type when requesting an access token from the token endpoint. In the
case of the client credentials grant type, the authentication and
grant coincide.
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Client registration and provisioning of client credentials to the
client is out of scope for this specification.
The OAuth framework defines one client credential type in section
2.3.1 of [RFC6749]: client id and client secret.
[I-D.erdtman-ace-rpcc] adds raw-public-key and pre-shared-key to the
client credentials types. Profiles of this framework MAY extend with
an additional client credentials type using client certificates.
5.4. AS Authentication
The client credential grant does not, by default, authenticate the AS
that the client connects to. In classic OAuth, the AS is
authenticated with a TLS server certificate.
Profiles of this framework MUST specify how clients authenticate the
AS and how communication security is implemented. By default, server
side TLS certificates, as defined by OAuth 2.0, are required.
5.5. The Authorization Endpoint
The OAuth 2.0 authorization endpoint is used to interact with the
resource owner and obtain an authorization grant, in certain grant
flows. The primary use case for the ACE-OAuth framework is for
machine-to-machine interactions that do not involve the resource
owner in the authorization flow; therefore, this endpoint is out of
scope here. Future profiles may define constrained adaptation
mechanisms for this endpoint as well. Non-constrained clients
interacting with constrained resource servers can use the
specification in section 3.1 of [RFC6749] and the attack
countermeasures suggested in section 4.2 of [RFC6819].
5.6. The Token Endpoint
In standard OAuth 2.0, the AS provides the token endpoint for
submitting access token requests. This framework extends the
functionality of the token endpoint, giving the AS the possibility to
help the client and RS to establish shared keys or to exchange their
public keys. Furthermore, this framework defines encodings using
CBOR, as a substitute for JSON.
The endpoint may, however, be exposed over HTTPS as in classical
OAuth or even other transports. A profile MUST define the details of
the mapping between the fields described below, and these transports.
If HTTPS is used, JSON or CBOR payloads may be supported. If JSON
payloads are used, the semantics of Section 4 of the OAuth 2.0
specification MUST be followed (with additions as described below).
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If CBOR payload is supported, the semantics described below MUST be
followed.
For the AS to be able to issue a token, the client MUST be
authenticated and present a valid grant for the scopes requested.
Profiles of this framework MUST specify how the AS authenticates the
client and how the communication between client and AS is protected,
fulfilling the requirements specified in Section 5.
The default name of this endpoint in an url-path is '/token', however
implementations are not required to use this name and can define
their own instead.
The figures of this section use CBOR diagnostic notation without the
integer abbreviations for the parameters or their values for
illustrative purposes. Note that implementations MUST use the
integer abbreviations and the binary CBOR encoding, if the CBOR
encoding is used.
5.6.1. Client-to-AS Request
The client sends a POST request to the token endpoint at the AS. The
profile MUST specify how the communication is protected. The content
of the request consists of the parameters specified in the relevant
subsection of section 4 of the OAuth 2.0 specification [RFC6749],
depending on the grant type, with the following exceptions and
additions:
o The parameter "grant_type" is OPTIONAL in the context of this
framework (as opposed to REQUIRED in RFC6749). If that parameter
is missing, the default value "client_credentials" is implied.
o The "audience" parameter from [RFC8693] is OPTIONAL to request an
access token bound to a specific audience.
o The "cnonce" parameter defined in Section 5.6.4.4 is REQUIRED if
the RS provided a client-nonce in the "AS Request Creation Hints"
message Section 5.1.2
o The "scope" parameter MAY be encoded as a byte string instead of
the string encoding specified in section 3.3 of [RFC6749], in
order allow compact encoding of complex scopes. The syntax of
such a binary encoding is explicitly not specified here and left
to profiles or applications, specifically note that a binary
encoded scope does not necessarily use the space character '0x20'
to delimit scope-tokens.
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o The client can send an empty (null value) "ace_profile" parameter
to indicate that it wants the AS to include the "ace_profile"
parameter in the response. See Section 5.6.4.3.
o A client MUST be able to use the parameters from
[I-D.ietf-ace-oauth-params] in an access token request to the
token endpoint and the AS MUST be able to process these additional
parameters.
The default behavior, is that the AS generates a symmetric proof-of-
possession key for the client. In order to use an asymmetric key
pair or to re-use a key previously established with the RS, the
client is supposed to use the "req_cnf" parameter from
[I-D.ietf-ace-oauth-params].
If CBOR is used then these parameters MUST be encoded as a CBOR map.
When HTTP is used as a transport then the client makes a request to
the token endpoint by sending the parameters using the "application/
x-www-form-urlencoded" format with a character encoding of UTF-8 in
the HTTP request entity-body, as defined in section 3.2 of [RFC6749].
The following examples illustrate different types of requests for
proof-of-possession tokens.
Figure 5 shows a request for a token with a symmetric proof-of-
possession key. The content is displayed in CBOR diagnostic
notation, without abbreviations for better readability.
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"client_id" : "myclient",
"audience" : "tempSensor4711"
}
Figure 5: Example request for an access token bound to a symmetric
key.
Figure 6 shows a request for a token with an asymmetric proof-of-
possession key. Note that in this example OSCORE [RFC8613] is used
to provide object-security, therefore the Content-Format is
"application/oscore" wrapping the "application/ace+cbor" type
content. The OSCORE option has a decoded interpretation appended in
parentheses for the reader's convenience. Also note that in this
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example the audience is implicitly known by both client and AS.
Furthermore note that this example uses the "req_cnf" parameter from
[I-D.ietf-ace-oauth-params].
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
OSCORE: 0x09, 0x05, 0x44, 0x6C
(h=0, k=1, n=001, partialIV= 0x05, kid=[0x44, 0x6C])
Content-Format: "application/oscore"
Payload:
0x44025d1 ... (full payload omitted for brevity) ... 68b3825e
Decrypted payload:
{
"client_id" : "myclient",
"req_cnf" : {
"COSE_Key" : {
"kty" : "EC",
"kid" : h'11',
"crv" : "P-256",
"x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8',
"y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4'
}
}
}
Figure 6: Example token request bound to an asymmetric key.
Figure 7 shows a request for a token where a previously communicated
proof-of-possession key is only referenced using the "req_cnf"
parameter from [I-D.ietf-ace-oauth-params].
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Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"client_id" : "myclient",
"audience" : "valve424",
"scope" : "read",
"req_cnf" : {
"kid" : b64'6kg0dXJM13U'
}
}W
Figure 7: Example request for an access token bound to a key
reference.
Refresh tokens are typically not stored as securely as proof-of-
possession keys in requesting clients. Proof-of-possession based
refresh token requests MUST NOT request different proof-of-possession
keys or different audiences in token requests. Refresh token
requests can only use to request access tokens bound to the same
proof-of-possession key and the same audience as access tokens issued
in the initial token request.
5.6.2. AS-to-Client Response
If the access token request has been successfully verified by the AS
and the client is authorized to obtain an access token corresponding
to its access token request, the AS sends a response with the
response code equivalent to the CoAP response code 2.01 (Created).
If client request was invalid, or not authorized, the AS returns an
error response as described in Section 5.6.3.
Note that the AS decides which token type and profile to use when
issuing a successful response. It is assumed that the AS has prior
knowledge of the capabilities of the client and the RS (see
Appendix D). This prior knowledge may, for example, be set by the
use of a dynamic client registration protocol exchange [RFC7591]. If
the client has requested a specific proof-of-possession key using the
"req_cnf" parameter from [I-D.ietf-ace-oauth-params], this may also
influence which profile the AS selects, as it needs to support the
use of the key type requested the client.
The content of the successful reply is the Access Information. When
using CBOR payloads, the content MUST be encoded as a CBOR map,
containing parameters as specified in Section 5.1 of [RFC6749], with
the following additions and changes:
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ace_profile:
OPTIONAL unless the request included an empty ace_profile
parameter in which case it is MANDATORY. This indicates the
profile that the client MUST use towards the RS. See
Section 5.6.4.3 for the formatting of this parameter. If this
parameter is absent, the AS assumes that the client implicitly
knows which profile to use towards the RS.
token_type:
This parameter is OPTIONAL, as opposed to 'required' in [RFC6749].
By default implementations of this framework SHOULD assume that
the token_type is "PoP". If a specific use case requires another
token_type (e.g., "Bearer") to be used then this parameter is
REQUIRED.
Furthermore [I-D.ietf-ace-oauth-params] defines additional parameters
that the AS MUST be able to use when responding to a request to the
token endpoint.
Figure 8 summarizes the parameters that can currently be part of the
Access Information. Future extensions may define additional
parameters.
/-------------------+-------------------------------\
| Parameter name | Specified in |
|-------------------+-------------------------------|
| access_token | RFC 6749 |
| token_type | RFC 6749 |
| expires_in | RFC 6749 |
| refresh_token | RFC 6749 |
| scope | RFC 6749 |
| state | RFC 6749 |
| error | RFC 6749 |
| error_description | RFC 6749 |
| error_uri | RFC 6749 |
| ace_profile | [this document] |
| cnf | [I-D.ietf-ace-oauth-params] |
| rs_cnf | [I-D.ietf-ace-oauth-params] |
\-------------------+-------------------------------/
Figure 8: Access Information parameters
Figure 9 shows a response containing a token and a "cnf" parameter
with a symmetric proof-of-possession key, which is defined in
[I-D.ietf-ace-oauth-params]. Note that the key identifier 'kid' is
only used to simplify indexing and retrieving the key, and no
assumptions should be made that it is unique in the domains of either
the client or the RS.
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Header: Created (Code=2.01)
Content-Format: "application/ace+cbor"
Payload:
{
"access_token" : b64'SlAV32hkKG ...
(remainder of CWT omitted for brevity;
CWT contains COSE_Key in the "cnf" claim)',
"ace_profile" : "coap_dtls",
"expires_in" : "3600",
"cnf" : {
"COSE_Key" : {
"kty" : "Symmetric",
"kid" : b64'39Gqlw',
"k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
}
}
}
Figure 9: Example AS response with an access token bound to a
symmetric key.
5.6.3. Error Response
The error responses for CoAP-based interactions with the AS are
generally equivalent to the ones for HTTP-based interactions as
defined in Section 5.2 of [RFC6749], with the following exceptions:
o When using CBOR the raw payload before being processed by the
communication security protocol MUST be encoded as a CBOR map.
o A response code equivalent to the CoAP code 4.00 (Bad Request)
MUST be used for all error responses, except for invalid_client
where a response code equivalent to the CoAP code 4.01
(Unauthorized) MAY be used under the same conditions as specified
in Section 5.2 of [RFC6749].
o The Content-Format (for CoAP-based interactions) or media type
(for HTTP-based interactions) "application/ace+cbor" MUST be used
for the error response.
o The parameters "error", "error_description" and "error_uri" MUST
be abbreviated using the codes specified in Figure 12, when a CBOR
encoding is used.
o The error code (i.e., value of the "error" parameter) MUST be
abbreviated as specified in Figure 10, when a CBOR encoding is
used.
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/---------------------------+-------------\
| Name | CBOR Values |
|---------------------------+-------------|
| invalid_request | 1 |
| invalid_client | 2 |
| invalid_grant | 3 |
| unauthorized_client | 4 |
| unsupported_grant_type | 5 |
| invalid_scope | 6 |
| unsupported_pop_key | 7 |
| incompatible_ace_profiles | 8 |
\---------------------------+-------------/
Figure 10: CBOR abbreviations for common error codes
In addition to the error responses defined in OAuth 2.0, the
following behavior MUST be implemented by the AS:
o If the client submits an asymmetric key in the token request that
the RS cannot process, the AS MUST reject that request with a
response code equivalent to the CoAP code 4.00 (Bad Request)
including the error code "unsupported_pop_key" defined in
Figure 10.
o If the client and the RS it has requested an access token for do
not share a common profile, the AS MUST reject that request with a
response code equivalent to the CoAP code 4.00 (Bad Request)
including the error code "incompatible_ace_profiles" defined in
Figure 10.
5.6.4. Request and Response Parameters
This section provides more detail about the new parameters that can
be used in access token requests and responses, as well as
abbreviations for more compact encoding of existing parameters and
common parameter values.
5.6.4.1. Grant Type
The abbreviations specified in the registry defined in Section 8.5
MUST be used in CBOR encodings instead of the string values defined
in [RFC6749], if CBOR payloads are used.
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/--------------------+------------+------------------------\
| Name | CBOR Value | Original Specification |
|--------------------+------------+------------------------|
| password | 0 | [RFC6749] |
| authorization_code | 1 | [RFC6749] |
| client_credentials | 2 | [RFC6749] |
| refresh_token | 3 | [RFC6749] |
\--------------------+------------+------------------------/
Figure 11: CBOR abbreviations for common grant types
5.6.4.2. Token Type
The "token_type" parameter, defined in section 5.1 of [RFC6749],
allows the AS to indicate to the client which type of access token it
is receiving (e.g., a bearer token).
This document registers the new value "PoP" for the OAuth Access
Token Types registry, specifying a proof-of-possession token. How
the proof-of-possession by the client to the RS is performed MUST be
specified by the profiles.
The values in the "token_type" parameter MUST use the CBOR
abbreviations defined in the registry specified by Section 8.7, if a
CBOR encoding is used.
In this framework the "pop" value for the "token_type" parameter is
the default. The AS may, however, provide a different value.
5.6.4.3. Profile
Profiles of this framework MUST define the communication protocol and
the communication security protocol between the client and the RS.
The security protocol MUST provide encryption, integrity and replay
protection. It MUST also provide a binding between requests and
responses. Furthermore profiles MUST define a list of allowed proof-
of-possession methods, if they support proof-of-possession tokens.
A profile MUST specify an identifier that MUST be used to uniquely
identify itself in the "ace_profile" parameter. The textual
representation of the profile identifier is intended for human
readability and for JSON-based interactions, it MUST NOT be used for
CBOR-based interactions. Profiles MUST register their identifier in
the registry defined in Section 8.8.
Profiles MAY define additional parameters for both the token request
and the Access Information in the access token response in order to
support negotiation or signaling of profile specific parameters.
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Clients that want the AS to provide them with the "ace_profile"
parameter in the access token response can indicate that by sending a
ace_profile parameter with a null value (for CBOR-based interactions)
or an empty string (for JSON based interactions) in the access token
request.
5.6.4.4. Client-Nonce
This parameter MUST be sent from the client to the AS, if it
previously received a "cnonce" parameter in the AS Request Creation
Hints Section 5.1.2. The parameter is encoded as a byte string for
CBOR-based interactions, and as a string (Base64 encoded binary) for
JSON-based interactions. It MUST copy the value from the cnonce
parameter in the AS Request Creation Hints.
5.6.5. Mapping Parameters to CBOR
If CBOR encoding is used, all OAuth parameters in access token
requests and responses MUST be mapped to CBOR types as specified in
the registry defined by Section 8.10, using the given integer
abbreviation for the map keys.
Note that we have aligned the abbreviations corresponding to claims
with the abbreviations defined in [RFC8392].
Note also that abbreviations from -24 to 23 have a 1 byte encoding
size in CBOR. We have thus chosen to assign abbreviations in that
range to parameters we expect to be used most frequently in
constrained scenarios.
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/-------------------+----------+---------------------\
| Name | CBOR Key | Value Type |
|-------------------+----------+---------------------|
| access_token | 1 | byte string |
| expires_in | 2 | unsigned integer |
| audience | 5 | text string |
| scope | 9 | text or byte string |
| client_id | 24 | text string |
| client_secret | 25 | byte string |
| response_type | 26 | text string |
| redirect_uri | 27 | text string |
| state | 28 | text string |
| code | 29 | byte string |
| error | 30 | unsigned integer |
| error_description | 31 | text string |
| error_uri | 32 | text string |
| grant_type | 33 | unsigned integer |
| token_type | 34 | unsigned integer |
| username | 35 | text string |
| password | 36 | text string |
| refresh_token | 37 | byte string |
| ace_profile | 38 | unsigned integer |
| cnonce | 39 | byte string |
\-------------------+----------+---------------------/
Figure 12: CBOR mappings used in token requests and responses
5.7. The Introspection Endpoint
Token introspection [RFC7662] can be OPTIONALLY provided by the AS,
and is then used by the RS and potentially the client to query the AS
for metadata about a given token, e.g., validity or scope. Analogous
to the protocol defined in [RFC7662] for HTTP and JSON, this section
defines adaptations to more constrained environments using CBOR and
leaving the choice of the application protocol to the profile.
Communication between the requesting entity and the introspection
endpoint at the AS MUST be integrity protected and encrypted. The
communication security protocol MUST also provide a binding between
requests and responses. Furthermore the two interacting parties MUST
perform mutual authentication. Finally the AS SHOULD verify that the
requesting entity has the right to access introspection information
about the provided token. Profiles of this framework that support
introspection MUST specify how authentication and communication
security between the requesting entity and the AS is implemented.
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The default name of this endpoint in an url-path is '/introspect',
however implementations are not required to use this name and can
define their own instead.
The figures of this section uses CBOR diagnostic notation without the
integer abbreviations for the parameters or their values for better
readability.
Note that supporting introspection is OPTIONAL for implementations of
this framework.
5.7.1. Introspection Request
The requesting entity sends a POST request to the introspection
endpoint at the AS. The profile MUST specify how the communication
is protected. If CBOR is used, the payload MUST be encoded as a CBOR
map with a "token" entry containing the access token. Further
optional parameters representing additional context that is known by
the requesting entity to aid the AS in its response MAY be included.
For CoAP-based interaction, all messages MUST use the content type
"application/ace+cbor", while for HTTP-based interactions the
equivalent media type "application/ace+cbor" MUST be used.
The same parameters are required and optional as in Section 2.1 of
[RFC7662].
For example, Figure 13 shows an RS calling the token introspection
endpoint at the AS to query about an OAuth 2.0 proof-of-possession
token. Note that object security based on OSCORE [RFC8613] is
assumed in this example, therefore the Content-Format is
"application/oscore". Figure 14 shows the decoded payload.
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "introspect"
OSCORE: 0x09, 0x05, 0x25
Content-Format: "application/oscore"
Payload:
... COSE content ...
Figure 13: Example introspection request.
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{
"token" : b64'7gj0dXJQ43U',
"token_type_hint" : "PoP"
}
Figure 14: Decoded payload.
5.7.2. Introspection Response
If the introspection request is authorized and successfully
processed, the AS sends a response with the response code equivalent
to the CoAP code 2.01 (Created). If the introspection request was
invalid, not authorized or couldn't be processed the AS returns an
error response as described in Section 5.7.3.
In a successful response, the AS encodes the response parameters in a
map including with the same required and optional parameters as in
Section 2.2 of [RFC7662] with the following addition:
ace_profile OPTIONAL. This indicates the profile that the RS MUST
use with the client. See Section 5.6.4.3 for more details on the
formatting of this parameter.
cnonce OPTIONAL. A client-nonce provided to the AS by the client.
The RS MUST verify that this corresponds to the client-nonce
previously provided to the client in the AS Request Creation
Hints. See Section 5.1.2 and Section 5.6.4.4.
exi OPTIONAL. The "expires-in" claim associated to this access
token. See Section 5.8.3.
Furthermore [I-D.ietf-ace-oauth-params] defines more parameters that
the AS MUST be able to use when responding to a request to the
introspection endpoint.
For example, Figure 15 shows an AS response to the introspection
request in Figure 13. Note that this example contains the "cnf"
parameter defined in [I-D.ietf-ace-oauth-params].
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Header: Created (Code=2.01)
Content-Format: "application/ace+cbor"
Payload:
{
"active" : true,
"scope" : "read",
"ace_profile" : "coap_dtls",
"cnf" : {
"COSE_Key" : {
"kty" : "Symmetric",
"kid" : b64'39Gqlw',
"k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
}
}
}
Figure 15: Example introspection response.
5.7.3. Error Response
The error responses for CoAP-based interactions with the AS are
equivalent to the ones for HTTP-based interactions as defined in
Section 2.3 of [RFC7662], with the following differences:
o If content is sent and CBOR is used the payload MUST be encoded as
a CBOR map and the Content-Format "application/ace+cbor" MUST be
used.
o If the credentials used by the requesting entity (usually the RS)
are invalid the AS MUST respond with the response code equivalent
to the CoAP code 4.01 (Unauthorized) and use the required and
optional parameters from Section 5.2 in [RFC6749].
o If the requesting entity does not have the right to perform this
introspection request, the AS MUST respond with a response code
equivalent to the CoAP code 4.03 (Forbidden). In this case no
payload is returned.
o The parameters "error", "error_description" and "error_uri" MUST
be abbreviated using the codes specified in Figure 12.
o The error codes MUST be abbreviated using the codes specified in
the registry defined by Section 8.4.
Note that a properly formed and authorized query for an inactive or
otherwise invalid token does not warrant an error response by this
specification. In these cases, the authorization server MUST instead
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respond with an introspection response with the "active" field set to
"false".
5.7.4. Mapping Introspection parameters to CBOR
If CBOR is used, the introspection request and response parameters
MUST be mapped to CBOR types as specified in the registry defined by
Section 8.12, using the given integer abbreviation for the map key.
Note that we have aligned abbreviations that correspond to a claim
with the abbreviations defined in [RFC8392] and the abbreviations of
parameters with the same name from Section 5.6.5.
/-------------------+----------+-------------------------\
| Parameter name | CBOR Key | Value Type |
|-------------------+----------+-------------------------|
| iss | 1 | text string |
| sub | 2 | text string |
| aud | 3 | text string |
| exp | 4 | integer or |
| | | floating-point number |
| nbf | 5 | integer or |
| | | floating-point number |
| iat | 6 | integer or |
| | | floating-point number |
| cti | 7 | byte string |
| scope | 9 | text or byte string |
| active | 10 | True or False |
| token | 11 | byte string |
| client_id | 24 | text string |
| error | 30 | unsigned integer |
| error_description | 31 | text string |
| error_uri | 32 | text string |
| token_type_hint | 33 | text string |
| token_type | 34 | text string |
| username | 35 | text string |
| ace_profile | 38 | unsigned integer |
| cnonce | 39 | byte string |
| exi | 40 | unsigned integer |
\-------------------+----------+-------------------------/
Figure 16: CBOR Mappings to Token Introspection Parameters.
5.8. The Access Token
This framework RECOMMENDS the use of CBOR web token (CWT) as
specified in [RFC8392].
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In order to facilitate offline processing of access tokens, this
document uses the "cnf" claim from [RFC8747] and the "scope" claim
from [RFC8693] for JWT- and CWT-encoded tokens. In addition to
string encoding specified for the "scope" claim, a binary encoding
MAY be used. The syntax of such an encoding is explicitly not
specified here and left to profiles or applications, specifically
note that a binary encoded scope does not necessarily use the space
character '0x20' to delimit scope-tokens.
If the AS needs to convey a hint to the RS about which profile it
should use to communicate with the client, the AS MAY include an
"ace_profile" claim in the access token, with the same syntax and
semantics as defined in Section 5.6.4.3.
If the client submitted a client-nonce parameter in the access token
request Section 5.6.4.4, the AS MUST include the value of this
parameter in the "cnonce" claim specified here. The "cnonce" claim
uses binary encoding.
5.8.1. The Authorization Information Endpoint
The access token, containing authorization information and
information about the proof-of-possession method used by the client,
needs to be transported to the RS so that the RS can authenticate and
authorize the client request.
This section defines a method for transporting the access token to
the RS using a RESTful protocol such as CoAP. Profiles of this
framework MAY define other methods for token transport.
The method consists of an authz-info endpoint, implemented by the RS.
A client using this method MUST make a POST request to the authz-info
endpoint at the RS with the access token in the payload. The RS
receiving the token MUST verify the validity of the token. If the
token is valid, the RS MUST respond to the POST request with 2.01
(Created). Section Section 5.8.1.1 outlines how an RS MUST proceed
to verify the validity of an access token.
The RS MUST be prepared to store at least one access token for future
use. This is a difference to how access tokens are handled in OAuth
2.0, where the access token is typically sent along with each
request, and therefore not stored at the RS.
This specification RECOMMENDS that an RS stores only one token per
proof-of-possession key, meaning that an additional token linked to
the same key will overwrite any existing token at the RS. The reason
is that this greatly simplifies (constrained) implementations, with
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respect to required storage and resolving a request to the applicable
token.
If the payload sent to the authz-info endpoint does not parse to a
token, the RS MUST respond with a response code equivalent to the
CoAP code 4.00 (Bad Request).
The RS MAY make an introspection request to validate the token before
responding to the POST request to the authz-info endpoint, e.g. if
the token is an opaque reference. Some transport protocols may
provide a way to indicate that the RS is busy and the client should
retry after an interval; this type of status update would be
appropriate while the RS is waiting for an introspection response.
Profiles MUST specify whether the authz-info endpoint is protected,
including whether error responses from this endpoint are protected.
Note that since the token contains information that allow the client
and the RS to establish a security context in the first place, mutual
authentication may not be possible at this point.
The default name of this endpoint in an url-path is '/authz-info',
however implementations are not required to use this name and can
define their own instead.
5.8.1.1. Verifying an Access Token
When an RS receives an access token, it MUST verify it before storing
it. The details of token verification depends on various aspects,
including the token encoding, the type of token, the security
protection applied to the token, and the claims. The token encoding
matters since the security wrapper differs between the token
encodings. For example, a CWT token uses COSE while a JWT token uses
JOSE. The type of token also has an influence on the verification
procedure since tokens may be self-contained whereby token
verification may happen locally at the RS while a token-by-reference
requires further interaction with the authorization server, for
example using token introspection, to obtain the claims associated
with the token reference. Self-contained tokens MUST, at a minimum,
be integrity protected but they MAY also be encrypted.
For self-contained tokens the RS MUST process the security protection
of the token first, as specified by the respective token format. For
CWT the description can be found in [RFC8392] and for JWT the
relevant specification is [RFC7519]. This MUST include a
verification that security protection (and thus the token) was
generated by an AS that has the right to issue access tokens for this
RS.
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In case the token is communicated by reference the RS needs to obtain
the claims first. When the RS uses token introspection the relevant
specification is [RFC7662] with CoAP transport specified in
Section 5.7.
Errors may happen during this initial processing stage:
o If token or claim verification fails, the RS MUST discard the
token and, if this was an interaction with authz-info, return an
error message with a response code equivalent to the CoAP code
4.01 (Unauthorized).
o If the claims cannot be obtained the RS MUST discard the token
and, in case of an interaction via the authz-info endpoint, return
an error message with a response code equivalent to the CoAP code
4.00 (Bad Request).
Next, the RS MUST verify claims, if present, contained in the access
token. Errors are returned when claim checks fail, in the order of
priority of this list:
iss The issuer claim must identify an AS that has the authority to
issue access tokens for the receiving RS. If that is not the case
the RS MUST discard the token. If this was an interaction with
authz-info, the RS MUST also respond with a response code
equivalent to the CoAP code 4.01 (Unauthorized).
exp The expiration date must be in the future. If that is not the
case the RS MUST discard the token. If this was an interaction
with authz-info the RS MUST also respond with a response code
equivalent to the CoAP code 4.01 (Unauthorized). Note that the RS
has to terminate access rights to the protected resources at the
time when the tokens expire.
aud The audience claim must refer to an audience that the RS
identifies with. If that is not the case the RS MUST discard the
token. If this was an interaction with authz-info, the RS MUST
also respond with a response code equivalent to the CoAP code 4.03
(Forbidden).
scope The RS must recognize value of the scope claim. If that is
not the case the RS MUST discard the token. If this was an
interaction with authz-info, the RS MUST also respond with a
response code equivalent to the CoAP code 4.00 (Bad Request). The
RS MAY provide additional information in the error response, to
clarify what went wrong.
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Additional processing may be needed for other claims in a way
specific to a profile or the underlying application.
Note that the Subject (sub) claim cannot always be verified when the
token is submitted to the RS since the client may not have
authenticated yet. Also note that a counter for the expires_in (exi)
claim MUST be initialized when the RS first verifies this token.
Also note that profiles of this framework may define access token
transport mechanisms that do not allow for error responses.
Therefore the error messages specified here only apply if the token
was sent to the authz-info endpoint.
When sending error responses, the RS MAY use the error codes from
Section 3.1 of [RFC6750], to provide additional details to the
client.
5.8.1.2. Protecting the Authorization Information Endpoint
As this framework can be used in RESTful environments, it is
important to make sure that attackers cannot perform unauthorized
requests on the authz-info endpoints, other than submitting access
tokens.
Specifically it SHOULD NOT be possible to perform GET, DELETE or PUT
on the authz-info endpoint and on it's children (if any).
The POST method SHOULD NOT be allowed on children of the authz-info
endpoint.
The RS SHOULD implement rate limiting measures to mitigate attacks
aiming to overload the processing capacity of the RS by repeatedly
submitting tokens. For CoAP-based communication the RS could use the
mechanisms from [RFC8516] to indicate that it is overloaded.
5.8.2. Client Requests to the RS
Before sending a request to an RS, the client MUST verify that the
keys used to protect this communication are still valid. See
Section 5.8.4 for details on how the client determines the validity
of the keys used.
If an RS receives a request from a client, and the target resource
requires authorization, the RS MUST first verify that it has an
access token that authorizes this request, and that the client has
performed the proof-of-possession binding that token to the request.
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The response code MUST be 4.01 (Unauthorized) in case the client has
not performed the proof-of-possession, or if RS has no valid access
token for the client. If RS has an access token for the client but
the token does not authorize access for the resource that was
requested, RS MUST reject the request with a 4.03 (Forbidden). If RS
has an access token for the client but it does not cover the action
that was requested on the resource, RS MUST reject the request with a
4.05 (Method Not Allowed).
Note: The use of the response codes 4.03 and 4.05 is intended to
prevent infinite loops where a dumb Client optimistically tries to
access a requested resource with any access token received from AS.
As malicious clients could pretend to be C to determine C's
privileges, these detailed response codes must be used only when a
certain level of security is already available which can be achieved
only when the Client is authenticated.
Note: The RS MAY use introspection for timely validation of an access
token, at the time when a request is presented.
Note: Matching the claims of the access token (e.g., scope) to a
specific request is application specific.
If the request matches a valid token and the client has performed the
proof-of-possession for that token, the RS continues to process the
request as specified by the underlying application.
5.8.3. Token Expiration
Depending on the capabilities of the RS, there are various ways in
which it can verify the expiration of a received access token. Here
follows a list of the possibilities including what functionality they
require of the RS.
o The token is a CWT and includes an "exp" claim and possibly the
"nbf" claim. The RS verifies these by comparing them to values
from its internal clock as defined in [RFC7519]. In this case the
RS's internal clock must reflect the current date and time, or at
least be synchronized with the AS's clock. How this clock
synchronization would be performed is out of scope for this
specification.
o The RS verifies the validity of the token by performing an
introspection request as specified in Section 5.7. This requires
the RS to have a reliable network connection to the AS and to be
able to handle two secure sessions in parallel (C to RS and RS to
AS).
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o In order to support token expiration for devices that have no
reliable way of synchronizing their internal clocks, this
specification defines the following approach: The claim "exi"
("expires in") can be used, to provide the RS with the lifetime of
the token in seconds from the time the RS first receives the
token. For CBOR-based interaction this parameter is encoded as
unsigned integer, while JSON-based interactions encode this as
JSON number.
o Processing this claim requires that the RS does the following:
* For each token the RS receives, that contains an "exi" claim:
Keep track of the time it received that token and revisit that
list regularly to expunge expired tokens.
* Keep track of the identifiers of tokens containing the "exi"
claim that have expired (in order to avoid accepting them
again). In order to avoid an unbounded memory usage growth,
this MUST be implemented in the following way when the "exi"
claim is used:
+ When creating the token, the AS MUST add a 'cti' claim ( or
'jti' for JWTs) to the access token. The value of this
claim MUST be created as the binary representation of the
concatenation of the identifier of the RS with a sequence
number counting the tokens containing an 'exi' claim, issued
by this AS for the RS.
+ The RS MUST store the highest sequence number of an expired
token containing the "exi" claim that it has seen, and treat
tokens with lower sequence numbers as expired.
If a token that authorizes a long running request such as a CoAP
Observe [RFC7641] expires, the RS MUST send an error response with
the response code equivalent to the CoAP code 4.01 (Unauthorized) to
the client and then terminate processing the long running request.
5.8.4. Key Expiration
The AS provides the client with key material that the RS uses. This
can either be a common symmetric PoP-key, or an asymmetric key used
by the RS to authenticate towards the client. Since there is
currently no expiration metadata associated to those keys, the client
has no way of knowing if these keys are still valid. This may lead
to situations where the client sends requests containing sensitive
information to the RS using a key that is expired and possibly in the
hands of an attacker, or accepts responses from the RS that are not
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properly protected and could possibly have been forged by an
attacker.
In order to prevent this, the client must assume that those keys are
only valid as long as the related access token is. Since the access
token is opaque to the client, one of the following methods MUST be
used to inform the client about the validity of an access token:
o The client knows a default validity time for all tokens it is
using (i.e. how long a token is valid after being issued). This
information could be provisioned to the client when it is
registered at the AS, or published by the AS in a way that the
client can query.
o The AS informs the client about the token validity using the
"expires_in" parameter in the Access Information.
A client that is not able to obtain information about the expiration
of a token MUST NOT use this token.
6. Security Considerations
Security considerations applicable to authentication and
authorization in RESTful environments provided in OAuth 2.0 [RFC6749]
apply to this work. Furthermore [RFC6819] provides additional
security considerations for OAuth which apply to IoT deployments as
well. If the introspection endpoint is used, the security
considerations from [RFC7662] also apply.
The following subsections address issues specific to this document
and it's use in constrained environments.
6.1. Protecting Tokens
A large range of threats can be mitigated by protecting the contents
of the access token by using a digital signature or a keyed message
digest (MAC) or an Authenticated Encryption with Associated Data
(AEAD) algorithm. Consequently, the token integrity protection MUST
be applied to prevent the token from being modified, particularly
since it contains a reference to the symmetric key or the asymmetric
key used for proof-of-possession. If the access token contains the
symmetric key, this symmetric key MUST be encrypted by the
authorization server so that only the resource server can decrypt it.
Note that using an AEAD algorithm is preferable over using a MAC
unless the token needs to be publicly readable.
If the token is intended for multiple recipients (i.e. an audience
that is a group), integrity protection of the token with a symmetric
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key, shared between the AS and the recipients, is not sufficient,
since any of the recipients could modify the token undetected by the
other recipients. Therefore a token with a multi-recipient audience
MUST be protected with an asymmetric signature.
It is important for the authorization server to include the identity
of the intended recipient (the audience), typically a single resource
server (or a list of resource servers), in the token. The same
shared secret MUST NOT be used as proof-of-possession key with
multiple resource servers since the benefit from using the proof-of-
possession concept is then significantly reduced.
If clients are capable of doing so, they should frequently request
fresh access tokens, as this allows the AS to keep the lifetime of
the tokens short. This allows the AS to use shorter proof-of-
possession key sizes, which translate to a performance benefit for
the client and for the resource server. Shorter keys also lead to
shorter messages (particularly with asymmetric keying material).
When authorization servers bind symmetric keys to access tokens, they
SHOULD scope these access tokens to a specific permission.
In certain situations it may be necessary to revoke an access token
that is still valid. Client-initiated revocation is specified in
[RFC7009] for OAuth 2.0. Other revocation mechanisms are currently
not specified, as the underlying assumption in OAuth is that access
tokens are issued with a relatively short lifetime. This may not
hold true for disconnected constrained devices, needing access tokens
with relatively long lifetimes, and would therefore necessitate
further standardization work that is out of scope for this document.
6.2. Communication Security
Communication with the authorization server MUST use confidentiality
protection. This step is extremely important since the client or the
RS may obtain the proof-of-possession key from the authorization
server for use with a specific access token. Not using
confidentiality protection exposes this secret (and the access token)
to an eavesdropper thereby completely negating proof-of-possession
security. Profiles MUST specify how communication security according
to the requirements in Section 5 is provided.
Additional protection for the access token can be applied by
encrypting it, for example encryption of CWTs is specified in
Section 5.1 of [RFC8392]. Such additional protection can be
necessary if the token is later transferred over an insecure
connection (e.g. when it is sent to the authz-info endpoint).
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Developers MUST ensure that the ephemeral credentials (i.e., the
private key or the session key) are not leaked to third parties. An
adversary in possession of the ephemeral credentials bound to the
access token will be able to impersonate the client. Be aware that
this is a real risk with many constrained environments, since
adversaries can often easily get physical access to the devices.
This risk can also be mitigated to some extent by making sure that
keys are refreshed more frequently.
6.3. Long-Term Credentials
Both clients and RSs have long-term credentials that are used to
secure communications, and authenticate to the AS. These credentials
need to be protected against unauthorized access. In constrained
devices, deployed in publicly accessible places, such protection can
be difficult to achieve without specialized hardware (e.g. secure key
storage memory).
If credentials are lost or compromised, the operator of the affected
devices needs to have procedures to invalidate any access these
credentials give and to revoke tokens linked to such credentials.
The loss of a credential linked to a specific device MUST NOT lead to
a compromise of other credentials not linked to that device,
therefore secret keys used for authentication MUST NOT be shared
between more than two parties.
Operators of clients or RS SHOULD have procedures in place to replace
credentials that are suspected to have been compromised or that have
been lost.
Operators also SHOULD have procedures for decommissioning devices,
that include securely erasing credentials and other security critical
material in the devices being decommissioned.
6.4. Unprotected AS Request Creation Hints
Initially, no secure channel exists to protect the communication
between C and RS. Thus, C cannot determine if the AS Request
Creation Hints contained in an unprotected response from RS to an
unauthorized request (see Section 5.1.2) are authentic. It is
therefore advisable to provide C with a (possibly hard-coded) list of
trustworthy authorization servers, possibly including information
used to authenticate the AS, such as a public key or certificate
fingerprint. AS Request Creation Hints referring to a URI not listed
there would be ignored.
A compromised RS may use the hints to trick a client into contacting
an AS that is not supposed to be in charge of that RS. Since this AS
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must be in the hard-coded list of trusted AS no violation of
privileges and or exposure of credentials should happen, since a
trusted AS is expected to refuse requestes for which it is not
applicable and render a corresponding error response. However a
compromised RS may use this to perform a denial of service against a
specific AS, by redirecting a large number of client requests to that
AS.
A compromised client can be made to contact any AS, including
compromised ones. This should not affect the RS, since it is
supposed to keep track of which AS are trusted and have corresponding
credentials to verify the source of access tokens it receives.
6.5. Minimal security requirements for communication
This section summarizes the minimal requirements for the
communication security of the different protocol interactions.
C-AS All communication between the client and the Authorization
Server MUST be encrypted, integrity and replay protected.
Furthermore responses from the AS to the client MUST be bound to
the client's request to avoid attacks where the attacker swaps the
intended response for an older one valid for a previous request.
This requires that the client and the Authorization Server have
previously exchanged either a shared secret or their public keys
in order to negotiate a secure communication. Furthermore the
client MUST be able to determine whether an AS has the authority
to issue access tokens for a certain RS. This can for example be
done through pre-configured lists, or through an online lookup
mechanism that in turn also must be secured.
RS-AS The communication between the Resource Server and the
Authorization Server via the introspection endpoint MUST be
encrypted, integrity and replay protected. Furthermore responses
from the AS to the RS MUST be bound to the RS's request. This
requires that the RS and the Authorization Server have previously
exchanged either a shared secret, or their public keys in order to
negotiate a secure communication. Furthermore the RS MUST be able
to determine whether an AS has the authority to issue access
tokens itself. This is usually configured out of band, but could
also be performed through an online lookup mechanism provided that
it is also secured in the same way.
C-RS The initial communication between the client and the Resource
Server can not be secured in general, since the RS is not in
possession of on access token for that client, which would carry
the necessary parameters. If both parties support DTLS without
client authentication it is RECOMMEND to use this mechanism for
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protecting the initial communication. After the client has
successfully transmitted the access token to the RS, a secure
communication protocol MUST be established between client and RS
for the actual resource request. This protocol MUST provide
confidentiality, integrity and replay protection as well as a
binding between requests and responses. This requires that the
client learned either the RS's public key or received a symmetric
proof-of-possession key bound to the access token from the AS.
The RS must have learned either the client's public key or a
shared symmetric key from the claims in the token or an
introspection request. Since ACE does not provide profile
negotiation between C and RS, the client MUST have learned what
profile the RS supports (e.g. from the AS or pre-configured) and
initiate the communication accordingly.
6.6. Token Freshness and Expiration
An RS that is offline faces the problem of clock drift. Since it
cannot synchronize its clock with the AS, it may be tricked into
accepting old access tokens that are no longer valid or have been
compromised. In order to prevent this, an RS may use the nonce-based
mechanism defined in Section 5.1.2 to ensure freshness of an Access
Token subsequently presented to this RS.
Another problem with clock drift is that evaluating the standard
token expiration claim "exp" can give unpredictable results.
Acceptable ranges of clock drift are highly dependent on the concrete
application. Important factors are how long access tokens are valid,
and how critical timely expiration of access token is.
The expiration mechanism implemented by the "exi" claim, based on the
first time the RS sees the token was defined to provide a more
predictable alternative. The "exi" approach has some drawbacks that
need to be considered:
A malicious client may hold back tokens with the "exi" claim in
order to prolong their lifespan.
If an RS loses state (e.g. due to an unscheduled reboot), it may
loose the current values of counters tracking the "exi" claims of
tokens it is storing.
The first drawback is inherent to the deployment scenario and the
"exi" solution. It can therefore not be mitigated without requiring
the the RS be online at times. The second drawback can be mitigated
by regularly storing the value of "exi" counters to persistent
memory.
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6.7. Combining profiles
There may be use cases were different profiles of this framework are
combined. For example, an MQTT-TLS profile is used between the
client and the RS in combination with a CoAP-DTLS profile for
interactions between the client and the AS. The security of a
profile MUST NOT depend on the assumption that the profile is used
for all the different types of interactions in this framework.
6.8. Unprotected Information
Communication with the authz-info endpoint, as well as the various
error responses defined in this framework, all potentially include
sending information over an unprotected channel. These messages may
leak information to an adversary, or may be manipulated by active
attackers to induce incorrect behavior. For example error responses
for requests to the Authorization Information endpoint can reveal
information about an otherwise opaque access token to an adversary
who has intercepted this token.
As far as error messages are concerned, this framework is written
under the assumption that, in general, the benefits of detailed error
messages outweigh the risk due to information leakage. For
particular use cases, where this assessment does not apply, detailed
error messages can be replaced by more generic ones.
In some scenarios it may be possible to protect the communication
with the authz-info endpoint (e.g. through DTLS with only server-side
authentication). In cases where this is not possible this framework
RECOMMENDS to use encrypted CWTs or tokens that are opaque references
and need to be subjected to introspection by the RS.
If the initial unauthorized resource request message (see
Section 5.1.1) is used, the client MUST make sure that it is not
sending sensitive content in this request. While GET and DELETE
requests only reveal the target URI of the resource, POST and PUT
requests would reveal the whole payload of the intended operation.
Since the client is not authenticated at the point when it is
submitting an access token to the authz-info endpoint, attackers may
be pretending to be a client and trying to trick an RS to use an
obsolete profile that in turn specifies a vulnerable security
mechanism via the authz-info endpoint. Such an attack would require
a valid access token containing an "ace_profile" claim requesting the
use of said obsolete profile. Resource Owners should update the
configuration of their RS's to prevent them from using such obsolete
profiles.
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6.9. Identifying audiences
The audience claim as defined in [RFC7519] and the equivalent
"audience" parameter from [RFC8693] are intentionally vague on how to
match the audience value to a specific RS. This is intended to allow
application specific semantics to be used. This section attempts to
give some general guidance for the use of audiences in constrained
environments.
URLs are not a good way of identifying mobile devices that can switch
networks and thus be associated with new URLs. If the audience
represents a single RS, and asymmetric keys are used, the RS can be
uniquely identified by a hash of its public key. If this approach is
used this framework RECOMMENDS to apply the procedure from section 3
of [RFC6920].
If the audience addresses a group of resource servers, the mapping of
group identifier to individual RS has to be provisioned to each RS
before the group-audience is usable. Managing dynamic groups could
be an issue, if any RS is not always reachable when the groups'
memberships change. Furthermore, issuing access tokens bound to
symmetric proof-of-possession keys that apply to a group-audience is
problematic, as an RS that is in possession of the access token can
impersonate the client towards the other RSs that are part of the
group. It is therefore NOT RECOMMENDED to issue access tokens bound
to a group audience and symmetric proof-of possession keys.
Even the client must be able to determine the correct values to put
into the "audience" parameter, in order to obtain a token for the
intended RS. Errors in this process can lead to the client
inadvertently obtaining a token for the wrong RS. The correct values
for "audience" can either be provisioned to the client as part of its
configuration, or dynamically looked up by the client in some
directory. In the latter case the integrity and correctness of the
directory data must be assured. Note that the "audience" hint
provided by the RS as part of the "AS Request Creation Hints"
Section 5.1.2 is not typically source authenticated and integrity
protected, and should therefore not be treated a trusted value.
6.10. Denial of service against or with Introspection
The optional introspection mechanism provided by OAuth and supported
in the ACE framework allows for two types of attacks that need to be
considered by implementers.
First, an attacker could perform a denial of service attack against
the introspection endpoint at the AS in order to prevent validation
of access tokens. To maintain the security of the system, an RS that
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is configured to use introspection MUST NOT allow access based on a
token for which it couldn't reach the introspection endpoint.
Second, an attacker could use the fact that an RS performs
introspection to perform a denial of service attack against that RS
by repeatedly sending tokens to its authz-info endpoint that require
an introspection call. RS can mitigate such attacks by implementing
rate limits on how many introspection requests they perform in a
given time interval for a certain client IP address submitting tokens
to /authz-info. When that limit has been reached, incoming requests
from that address are rejected for a certain amount of time. A
general rate limit on the introspection requests should also be
considered, to mitigate distributed attacks.
7. Privacy Considerations
Implementers and users should be aware of the privacy implications of
the different possible deployments of this framework.
The AS is in a very central position and can potentially learn
sensitive information about the clients requesting access tokens. If
the client credentials grant is used, the AS can track what kind of
access the client intends to perform. With other grants this can be
prevented by the Resource Owner. To do so, the resource owner needs
to bind the grants it issues to anonymous, ephemeral credentials that
do not allow the AS to link different grants and thus different
access token requests by the same client.
The claims contained in a token can reveal privacy sensitive
information about the client and the RS to any party having access to
them (whether by processing the content of a self-contained token or
by introspection). The AS SHOULD be configured to minimize the
information about clients and RSs disclosed in the tokens it issues.
If tokens are only integrity protected and not encrypted, they may
reveal information to attackers listening on the wire, or able to
acquire the access tokens in some other way. In the case of CWTs the
token may, e.g., reveal the audience, the scope and the confirmation
method used by the client. The latter may reveal the identity of the
device or application running the client. This may be linkable to
the identity of the person using the client (if there is a person and
not a machine-to-machine interaction).
Clients using asymmetric keys for proof-of-possession should be aware
of the consequences of using the same key pair for proof-of-
possession towards different RSs. A set of colluding RSs or an
attacker able to obtain the access tokens will be able to link the
requests, or even to determine the client's identity.
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An unprotected response to an unauthorized request (see
Section 5.1.2) may disclose information about RS and/or its existing
relationship with C. It is advisable to include as little
information as possible in an unencrypted response. If means of
encrypting communication between C and RS already exist, more
detailed information may be included with an error response to
provide C with sufficient information to react on that particular
error.
8. IANA Considerations
This document creates several registries with a registration policy
of "Expert Review"; guidelines to the experts are given in
Section 8.17.
8.1. ACE Authorization Server Request Creation Hints
This specification establishes the IANA "ACE Authorization Server
Request Creation Hints" registry. The registry has been created to
use the "Expert Review" registration procedure [RFC8126]. It should
be noted that, in addition to the expert review, some portions of the
registry require a specification, potentially a Standards Track RFC,
be supplied as well.
The columns of the registry are:
Name The name of the parameter
CBOR Key CBOR map key for the parameter. Different ranges of values
use different registration policies [RFC8126]. Integer values
from -256 to 255 are designated as Standards Action. Integer
values from -65536 to -257 and from 256 to 65535 are designated as
Specification Required. Integer values greater than 65535 are
designated as Expert Review. Integer values less than -65536 are
marked as Private Use.
Value Type The CBOR data types allowable for the values of this
parameter.
Reference This contains a pointer to the public specification of the
request creation hint abbreviation, if one exists.
This registry will be initially populated by the values in Figure 2.
The Reference column for all of these entries will be this document.
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8.2. CoRE Resource Type registry
IANA is requested to register a new Resource Type (rt=) Link Target
Attribute in the "Resource Type (rt=) Link Target Attribute Values"
subregistry under the "Constrained RESTful Environments (CoRE)
Parameters" [IANA.CoreParameters] registry:
rt="ace.ai". This resource type describes an ACE-OAuth authz-info
endpoint resource.
Specific ACE-OAuth profiles can use this common resource type for
defining their profile-specific discovery processes.
8.3. OAuth Extensions Error Registration
This specification registers the following error values in the OAuth
Extensions Error registry [IANA.OAuthExtensionsErrorRegistry].
o Error name: "unsupported_pop_key"
o Error usage location: token error response
o Related protocol extension: [this document]
o Change Controller: IESG
o Specification document(s): Section 5.6.3 of [this document]
o Error name: "incompatible_ace_profiles"
o Error usage location: token error response
o Related protocol extension: [this document]
o Change Controller: IESG
o Specification document(s): Section 5.6.3 of [this document]
8.4. OAuth Error Code CBOR Mappings Registry
This specification establishes the IANA "OAuth Error Code CBOR
Mappings" registry. The registry has been created to use the "Expert
Review" registration procedure [RFC8126], except for the value range
designated for private use.
The columns of the registry are:
Name The OAuth Error Code name, refers to the name in Section 5.2.
of [RFC6749], e.g., "invalid_request".
CBOR Value CBOR abbreviation for this error code. Integer values
less than -65536 are marked as "Private Use", all other values use
the registration policy "Expert Review" [RFC8126].
Reference This contains a pointer to the public specification of the
error code abbreviation, if one exists.
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This registry will be initially populated by the values in Figure 10.
The Reference column for all of these entries will be this document.
8.5. OAuth Grant Type CBOR Mappings
This specification establishes the IANA "OAuth Grant Type CBOR
Mappings" registry. The registry has been created to use the "Expert
Review" registration procedure [RFC8126], except for the value range
designated for private use.
The columns of this registry are:
Name The name of the grant type as specified in Section 1.3 of
[RFC6749].
CBOR Value CBOR abbreviation for this grant type. Integer values
less than -65536 are marked as "Private Use", all other values use
the registration policy "Expert Review" [RFC8126].
Reference This contains a pointer to the public specification of the
grant type abbreviation, if one exists.
Original Specification This contains a pointer to the public
specification of the grant type, if one exists.
This registry will be initially populated by the values in Figure 11.
The Reference column for all of these entries will be this document.
8.6. OAuth Access Token Types
This section registers the following new token type in the "OAuth
Access Token Types" registry [IANA.OAuthAccessTokenTypes].
o Type name: "PoP"
o Additional Token Endpoint Response Parameters: "cnf", "rs_cnf" see
section 3.3 of [I-D.ietf-ace-oauth-params].
o HTTP Authentication Scheme(s): N/A
o Change Controller: IETF
o Specification document(s): [this document]
8.7. OAuth Access Token Type CBOR Mappings
This specification established the IANA "OAuth Access Token Type CBOR
Mappings" registry. The registry has been created to use the "Expert
Review" registration procedure [RFC8126], except for the value range
designated for private use.
The columns of this registry are:
Name The name of token type as registered in the OAuth Access Token
Types registry, e.g., "Bearer".
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CBOR Value CBOR abbreviation for this token type. Integer values
less than -65536 are marked as "Private Use", all other values use
the registration policy "Expert Review" [RFC8126].
Reference This contains a pointer to the public specification of the
OAuth token type abbreviation, if one exists.
Original Specification This contains a pointer to the public
specification of the OAuth token type, if one exists.
8.7.1. Initial Registry Contents
o Name: "Bearer"
o Value: 1
o Reference: [this document]
o Original Specification: [RFC6749]
o Name: "PoP"
o Value: 2
o Reference: [this document]
o Original Specification: [this document]
8.8. ACE Profile Registry
This specification establishes the IANA "ACE Profile" registry. The
registry has been created to use the "Expert Review" registration
procedure [RFC8126]. It should be noted that, in addition to the
expert review, some portions of the registry require a specification,
potentially a Standards Track RFC, be supplied as well.
The columns of this registry are:
Name The name of the profile, to be used as value of the profile
attribute.
Description Text giving an overview of the profile and the context
it is developed for.
CBOR Value CBOR abbreviation for this profile name. Different
ranges of values use different registration policies [RFC8126].
Integer values from -256 to 255 are designated as Standards
Action. Integer values from -65536 to -257 and from 256 to 65535
are designated as Specification Required. Integer values greater
than 65535 are designated as "Expert Review". Integer values less
than -65536 are marked as Private Use.
Reference This contains a pointer to the public specification of the
profile abbreviation, if one exists.
This registry will be initially empty and will be populated by the
registrations from the ACE framework profiles.
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8.9. OAuth Parameter Registration
This specification registers the following parameter in the "OAuth
Parameters" registry [IANA.OAuthParameters]:
o Name: "ace_profile"
o Parameter Usage Location: token response
o Change Controller: IESG
o Reference: Section 5.6.2 and Section 5.6.4.3 of [this document]
8.10. OAuth Parameters CBOR Mappings Registry
This specification establishes the IANA "OAuth Parameters CBOR
Mappings" registry. The registry has been created to use the "Expert
Review" registration procedure [RFC8126], except for the value range
designated for private use.
The columns of this registry are:
Name The OAuth Parameter name, refers to the name in the OAuth
parameter registry, e.g., "client_id".
CBOR Key CBOR map key for this parameter. Integer values less than
-65536 are marked as "Private Use", all other values use the
registration policy "Expert Review" [RFC8126].
Value Type The allowable CBOR data types for values of this
parameter.
Reference This contains a pointer to the public specification of the
OAuth parameter abbreviation, if one exists.
This registry will be initially populated by the values in Figure 12.
The Reference column for all of these entries will be this document.
8.11. OAuth Introspection Response Parameter Registration
This specification registers the following parameters in the OAuth
Token Introspection Response registry
[IANA.TokenIntrospectionResponse].
o Name: "ace_profile"
o Description: The ACE profile used between client and RS.
o Change Controller: IESG
o Reference: Section 5.7.2 of [this document]
o Name: "cnonce"
o Description: "client-nonce". A nonce previously provided to the
AS by the RS via the client. Used to verify token freshness when
the RS cannot synchronize its clock with the AS.
o Change Controller: IESG
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o Reference: Section 5.7.2 of [this document]
o Name: "exi"
o Description: "Expires in". Lifetime of the token in seconds from
the time the RS first sees it. Used to implement a weaker from of
token expiration for devices that cannot synchronize their
internal clocks.
o Change Controller: IESG
o Reference: Section 5.7.2 of [this document]
8.12. OAuth Token Introspection Response CBOR Mappings Registry
This specification establishes the IANA "OAuth Token Introspection
Response CBOR Mappings" registry. The registry has been created to
use the "Expert Review" registration procedure [RFC8126], except for
the value range designated for private use.
The columns of this registry are:
Name The OAuth Parameter name, refers to the name in the OAuth
parameter registry, e.g., "client_id".
CBOR Key CBOR map key for this parameter. Integer values less than
-65536 are marked as "Private Use", all other values use the
registration policy "Expert Review" [RFC8126].
Value Type The allowable CBOR data types for values of this
parameter.
Reference This contains a pointer to the public specification of the
introspection response parameter abbreviation, if one exists.
This registry will be initially populated by the values in Figure 16.
The Reference column for all of these entries will be this document.
Note that the mappings of parameters corresponding to claim names
intentionally coincide with the CWT claim name mappings from
[RFC8392].
8.13. JSON Web Token Claims
This specification registers the following new claims in the JSON Web
Token (JWT) registry of JSON Web Token Claims
[IANA.JsonWebTokenClaims]:
o Claim Name: "ace_profile"
o Claim Description: The ACE profile a token is supposed to be used
with.
o Change Controller: IESG
o Reference: Section 5.8 of [this document]
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o Claim Name: "cnonce"
o Claim Description: "client-nonce". A nonce previously provided to
the AS by the RS via the client. Used to verify token freshness
when the RS cannot synchronize its clock with the AS.
o Change Controller: IESG
o Reference: Section 5.8 of [this document]
o Claim Name: "exi"
o Claim Description: "Expires in". Lifetime of the token in seconds
from the time the RS first sees it. Used to implement a weaker
from of token expiration for devices that cannot synchronize their
internal clocks.
o Change Controller: IESG
o Reference: Section 5.8.3 of [this document]
8.14. CBOR Web Token Claims
This specification registers the following new claims in the "CBOR
Web Token (CWT) Claims" registry [IANA.CborWebTokenClaims].
o Claim Name: "ace_profile"
o Claim Description: The ACE profile a token is supposed to be used
with.
o JWT Claim Name: ace_profile
o Claim Key: TBD (suggested: 38)
o Claim Value Type(s): integer
o Change Controller: IESG
o Specification Document(s): Section 5.8 of [this document]
o Claim Name: "cnonce"
o Claim Description: The client-nonce sent to the AS by the RS via
the client.
o JWT Claim Name: cnonce
o Claim Key: TBD (suggested: 39)
o Claim Value Type(s): byte string
o Change Controller: IESG
o Specification Document(s): Section 5.8 of [this document]
o Claim Name: "exi"
o Claim Description: The expiration time of a token measured from
when it was received at the RS in seconds.
o JWT Claim Name: exi
o Claim Key: TBD (suggested: 40)
o Claim Value Type(s): integer
o Change Controller: IESG
o Specification Document(s): Section 5.8.3 of [this document]
o Claim Name: "scope"
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o Claim Description: The scope of an access token as defined in
[RFC6749].
o JWT Claim Name: scope
o Claim Key: TBD (suggested: 42)
o Claim Value Type(s): byte string or text string
o Change Controller: IESG
o Specification Document(s): Section 4.2 of [RFC8693]
8.15. Media Type Registrations
This specification registers the 'application/ace+cbor' media type
for messages of the protocols defined in this document carrying
parameters encoded in CBOR. This registration follows the procedures
specified in [RFC6838].
Type name: application
Subtype name: ace+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Must be encoded as CBOR map containing the
protocol parameters defined in [this document].
Security considerations: See Section 6 of [this document]
Interoperability considerations: N/A
Published specification: [this document]
Applications that use this media type: The type is used by
authorization servers, clients and resource servers that support the
ACE framework as specified in [this document].
Fragment identifier considerations: N/A
Additional information: N/A
Person & email address to contact for further information:
<iesg@ietf.org>
Intended usage: COMMON
Restrictions on usage: none
Author: Ludwig Seitz <ludwig.seitz@combitech.se>
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Change controller: IESG
8.16. CoAP Content-Format Registry
This specification registers the following entry to the "CoAP
Content-Formats" registry:
Media Type: application/ace+cbor
Encoding: -
ID: TBD (suggested: 19)
Reference: [this document]
8.17. Expert Review Instructions
All of the IANA registries established in this document are defined
to use a registration policy of Expert Review. This section gives
some general guidelines for what the experts should be looking for,
but they are being designated as experts for a reason, so they should
be given substantial latitude.
Expert reviewers should take into consideration the following points:
o Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate one that is already
registered, and that the point is likely to be used in
deployments. The zones tagged as private use are intended for
testing purposes and closed environments; code points in other
ranges should not be assigned for testing.
o Specifications are needed for the first-come, first-serve range if
they are expected to be used outside of closed environments in an
interoperable way. When specifications are not provided, the
description provided needs to have sufficient information to
identify what the point is being used for.
o Experts should take into account the expected usage of fields when
approving point assignment. The fact that there is a range for
standards track documents does not mean that a standards track
document cannot have points assigned outside of that range. The
length of the encoded value should be weighed against how many
code points of that length are left, the size of device it will be
used on.
o Since a high degree of overlap is expected between these
registries and the contents of the OAuth parameters
[IANA.OAuthParameters] registries, experts should require new
registrations to maintain alignment with parameters from OAuth
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that have comparable functionality. Deviation from this alignment
should only be allowed if there are functional differences, that
are motivated by the use case and that cannot be easily or
efficiently addressed by comparable OAuth parameters.
9. Acknowledgments
This document is a product of the ACE working group of the IETF.
Thanks to Eve Maler for her contributions to the use of OAuth 2.0 and
UMA in IoT scenarios, Robert Taylor for his discussion input, and
Malisa Vucinic for his input on the predecessors of this proposal.
Thanks to the authors of draft-ietf-oauth-pop-key-distribution, from
where large parts of the security considerations where copied.
Thanks to Stefanie Gerdes, Olaf Bergmann, and Carsten Bormann for
contributing their work on AS discovery from draft-gerdes-ace-dcaf-
authorize (see Section 5.1).
Thanks to Jim Schaad and Mike Jones for their comprehensive reviews.
Thanks to Benjamin Kaduk for his input on various questions related
to this work.
Thanks to Cigdem Sengul for some very useful review comments.
Thanks to Carsten Bormann for contributing the text for the CoRE
Resource Type registry.
Ludwig Seitz and Goeran Selander worked on this document as part of
the CelticPlus project CyberWI, with funding from Vinnova. Ludwig
Seitz was also received further funding for this work by Vinnova in
the context of the CelticNext project Critisec.
10. References
10.1. Normative References
[I-D.ietf-ace-oauth-params]
Seitz, L., "Additional OAuth Parameters for Authorization
in Constrained Environments (ACE)", draft-ietf-ace-oauth-
params-13 (work in progress), April 2020.
[IANA.CborWebTokenClaims]
IANA, "CBOR Web Token (CWT) Claims",
<https://www.iana.org/assignments/cwt/cwt.xhtml#claims-
registry>.
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[IANA.CoreParameters]
IANA, "Constrained RESTful Environments (CoRE)
Parameters", <https://www.iana.org/assignments/core-
parameters/core-parameters.xhtml>.
[IANA.JsonWebTokenClaims]
IANA, "JSON Web Token Claims",
<https://www.iana.org/assignments/jwt/jwt.xhtml#claims>.
[IANA.OAuthAccessTokenTypes]
IANA, "OAuth Access Token Types",
<https://www.iana.org/assignments/oauth-parameters/oauth-
parameters.xhtml#token-types>.
[IANA.OAuthExtensionsErrorRegistry]
IANA, "OAuth Extensions Error Registry",
<https://www.iana.org/assignments/oauth-parameters/oauth-
parameters.xhtml#extensions-error>.
[IANA.OAuthParameters]
IANA, "OAuth Parameters",
<https://www.iana.org/assignments/oauth-parameters/oauth-
parameters.xhtml#parameters>.
[IANA.TokenIntrospectionResponse]
IANA, "OAuth Token Introspection Response",
<https://www.iana.org/assignments/oauth-parameters/oauth-
parameters.xhtml#token-introspection-response>.
[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>.
[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>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
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[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
[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>.
[RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
Keranen, A., and P. Hallam-Baker, "Naming Things with
Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
<https://www.rfc-editor.org/info/rfc6920>.
[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>.
[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>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[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>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[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>.
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[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
[RFC8693] Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J.,
and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693,
DOI 10.17487/RFC8693, January 2020,
<https://www.rfc-editor.org/info/rfc8693>.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.
10.2. Informative References
[BLE] Bluetooth SIG, "Bluetooth Core Specification v5.1",
Section 4.4, January 2019,
<https://www.bluetooth.com/specifications/bluetooth-core-
specification/>.
[I-D.erdtman-ace-rpcc]
Seitz, L. and S. Erdtman, "Raw-Public-Key and Pre-Shared-
Key as OAuth client credentials", draft-erdtman-ace-
rpcc-02 (work in progress), October 2017.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-29 (work
in progress), June 2020.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-38 (work in progress), May
2020.
[Margi10impact]
Margi, C., de Oliveira, B., de Sousa, G., Simplicio Jr,
M., Barreto, P., Carvalho, T., Naeslund, M., and R. Gold,
"Impact of Operating Systems on Wireless Sensor Networks
(Security) Applications and Testbeds", Proceedings of
the 19th International Conference on Computer
Communications and Networks (ICCCN), August 2010.
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[MQTT5.0] Banks, A., Briggs, E., Borgendale, K., and R. Gupta, "MQTT
Version 5.0", OASIS Standard, March 2019,
<https://docs.oasis-open.org/mqtt/mqtt/v5.0/mqtt-
v5.0.html>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
DOI 10.17487/RFC6819, January 2013,
<https://www.rfc-editor.org/info/rfc6819>.
[RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
August 2013, <https://www.rfc-editor.org/info/rfc7009>.
[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>.
[RFC7521] Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
"Assertion Framework for OAuth 2.0 Client Authentication
and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
May 2015, <https://www.rfc-editor.org/info/rfc7521>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<https://www.rfc-editor.org/info/rfc7591>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
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[RFC7744] Seitz, L., Ed., Gerdes, S., Ed., Selander, G., Mani, M.,
and S. Kumar, "Use Cases for Authentication and
Authorization in Constrained Environments", RFC 7744,
DOI 10.17487/RFC7744, January 2016,
<https://www.rfc-editor.org/info/rfc7744>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
<https://www.rfc-editor.org/info/rfc8252>.
[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>.
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8516] Keranen, A., ""Too Many Requests" Response Code for the
Constrained Application Protocol", RFC 8516,
DOI 10.17487/RFC8516, January 2019,
<https://www.rfc-editor.org/info/rfc8516>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
[RFC8628] Denniss, W., Bradley, J., Jones, M., and H. Tschofenig,
"OAuth 2.0 Device Authorization Grant", RFC 8628,
DOI 10.17487/RFC8628, August 2019,
<https://www.rfc-editor.org/info/rfc8628>.
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Appendix A. Design Justification
This section provides further insight into the design decisions of
the solution documented in this document. Section 3 lists several
building blocks and briefly summarizes their importance. The
justification for offering some of those building blocks, as opposed
to using OAuth 2.0 as is, is given below.
Common IoT constraints are:
Low Power Radio:
Many IoT devices are equipped with a small battery which needs to
last for a long time. For many constrained wireless devices, the
highest energy cost is associated to transmitting or receiving
messages (roughly by a factor of 10 compared to AES)
[Margi10impact]. It is therefore important to keep the total
communication overhead low, including minimizing the number and
size of messages sent and received, which has an impact of choice
on the message format and protocol. By using CoAP over UDP and
CBOR encoded messages, some of these aspects are addressed.
Security protocols contribute to the communication overhead and
can, in some cases, be optimized. For example, authentication and
key establishment may, in certain cases where security
requirements allow, be replaced by provisioning of security
context by a trusted third party, using transport or application
layer security.
Low CPU Speed:
Some IoT devices are equipped with processors that are
significantly slower than those found in most current devices on
the Internet. This typically has implications on what timely
cryptographic operations a device is capable of performing, which
in turn impacts, e.g., protocol latency. Symmetric key
cryptography may be used instead of the computationally more
expensive public key cryptography where the security requirements
so allow, but this may also require support for trusted-third-
party-assisted secret key establishment using transport- or
application-layer security.
Small Amount of Memory:
Microcontrollers embedded in IoT devices are often equipped with
only a small amount of RAM and flash memory, which places
limitations on what kind of processing can be performed and how
much code can be put on those devices. To reduce code size, fewer
and smaller protocol implementations can be put on the firmware of
such a device. In this case, CoAP may be used instead of HTTP,
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symmetric-key cryptography instead of public-key cryptography, and
CBOR instead of JSON. An authentication and key establishment
protocol, e.g., the DTLS handshake, in comparison with assisted
key establishment, also has an impact on memory and code
footprints.
User Interface Limitations:
Protecting access to resources is both an important security as
well as privacy feature. End users and enterprise customers may
not want to give access to the data collected by their IoT device
or to functions it may offer to third parties. Since the
classical approach of requesting permissions from end users via a
rich user interface does not work in many IoT deployment
scenarios, these functions need to be delegated to user-controlled
devices that are better suitable for such tasks, such as smart
phones and tablets.
Communication Constraints:
In certain constrained settings an IoT device may not be able to
communicate with a given device at all times. Devices may be
sleeping, or just disconnected from the Internet because of
general lack of connectivity in the area, for cost reasons, or for
security reasons, e.g., to avoid an entry point for Denial-of-
Service attacks.
The communication interactions this framework builds upon (as
shown graphically in Figure 1) may be accomplished using a variety
of different protocols, and not all parts of the message flow are
used in all applications due to the communication constraints.
Deployments making use of CoAP are expected, but this framework is
not limited to them. Other protocols such as HTTP, or even
protocols such as Bluetooth Smart communication that do not
necessarily use IP, could also be used. The latter raises the
need for application layer security over the various interfaces.
In the light of these constraints we have made the following design
decisions:
CBOR, COSE, CWT:
This framework RECOMMENDS the use of CBOR [RFC7049] as data
format. Where CBOR data needs to be protected, the use of COSE
[RFC8152] is RECOMMENDED. Furthermore, where self-contained
tokens are needed, this framework RECOMMENDS the use of CWT
[RFC8392]. These measures aim at reducing the size of messages
sent over the wire, the RAM size of data objects that need to be
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kept in memory and the size of libraries that devices need to
support.
CoAP:
This framework RECOMMENDS the use of CoAP [RFC7252] instead of
HTTP. This does not preclude the use of other protocols
specifically aimed at constrained devices, like, e.g., Bluetooth
Low Energy (see Section 3.2). This aims again at reducing the
size of messages sent over the wire, the RAM size of data objects
that need to be kept in memory and the size of libraries that
devices need to support.
Access Information:
This framework defines the name "Access Information" for data
concerning the RS that the AS returns to the client in an access
token response (see Section 5.6.2). This aims at enabling
scenarios where a powerful client, supporting multiple profiles,
needs to interact with an RS for which it does not know the
supported profiles and the raw public key.
Proof-of-Possession:
This framework makes use of proof-of-possession tokens, using the
"cnf" claim [RFC8747]. A request parameter "cnf" and a Response
parameter "cnf", both having a value space semantically and
syntactically identical to the "cnf" claim, are defined for the
token endpoint, to allow requesting and stating confirmation keys.
This aims at making token theft harder. Token theft is
specifically relevant in constrained use cases, as communication
often passes through middle-boxes, which could be able to steal
bearer tokens and use them to gain unauthorized access.
Authz-Info endpoint:
This framework introduces a new way of providing access tokens to
an RS by exposing a authz-info endpoint, to which access tokens
can be POSTed. This aims at reducing the size of the request
message and the code complexity at the RS. The size of the
request message is problematic, since many constrained protocols
have severe message size limitations at the physical layer (e.g.,
in the order of 100 bytes). This means that larger packets get
fragmented, which in turn combines badly with the high rate of
packet loss, and the need to retransmit the whole message if one
packet gets lost. Thus separating sending of the request and
sending of the access tokens helps to reduce fragmentation.
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Client Credentials Grant:
This framework RECOMMENDS the use of the client credentials grant
for machine-to-machine communication use cases, where manual
intervention of the resource owner to produce a grant token is not
feasible. The intention is that the resource owner would instead
pre-arrange authorization with the AS, based on the client's own
credentials. The client can then (without manual intervention)
obtain access tokens from the AS.
Introspection:
This framework RECOMMENDS the use of access token introspection in
cases where the client is constrained in a way that it can not
easily obtain new access tokens (i.e. it has connectivity issues
that prevent it from communicating with the AS). In that case
this framework RECOMMENDS the use of a long-term token, that could
be a simple reference. The RS is assumed to be able to
communicate with the AS, and can therefore perform introspection,
in order to learn the claims associated with the token reference.
The advantage of such an approach is that the resource owner can
change the claims associated to the token reference without having
to be in contact with the client, thus granting or revoking access
rights.
Appendix B. Roles and Responsibilities
Resource Owner
* Make sure that the RS is registered at the AS. This includes
making known to the AS which profiles, token_type, scopes, and
key types (symmetric/asymmetric) the RS supports. Also making
it known to the AS which audience(s) the RS identifies itself
with.
* Make sure that clients can discover the AS that is in charge of
the RS.
* If the client-credentials grant is used, make sure that the AS
has the necessary, up-to-date, access control policies for the
RS.
Requesting Party
* Make sure that the client is provisioned the necessary
credentials to authenticate to the AS.
* Make sure that the client is configured to follow the security
requirements of the Requesting Party when issuing requests
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(e.g., minimum communication security requirements, trust
anchors).
* Register the client at the AS. This includes making known to
the AS which profiles, token_types, and key types (symmetric/
asymmetric) the client.
Authorization Server
* Register the RS and manage corresponding security contexts.
* Register clients and authentication credentials.
* Allow Resource Owners to configure and update access control
policies related to their registered RSs.
* Expose the token endpoint to allow clients to request tokens.
* Authenticate clients that wish to request a token.
* Process a token request using the authorization policies
configured for the RS.
* Optionally: Expose the introspection endpoint that allows RS's
to submit token introspection requests.
* If providing an introspection endpoint: Authenticate RSs that
wish to get an introspection response.
* If providing an introspection endpoint: Process token
introspection requests.
* Optionally: Handle token revocation.
* Optionally: Provide discovery metadata. See [RFC8414]
* Optionally: Handle refresh tokens.
Client
* Discover the AS in charge of the RS that is to be targeted with
a request.
* Submit the token request (see step (A) of Figure 1).
+ Authenticate to the AS.
+ Optionally (if not pre-configured): Specify which RS, which
resource(s), and which action(s) the request(s) will target.
+ If raw public keys (rpk) or certificates are used, make sure
the AS has the right rpk or certificate for this client.
* Process the access token and Access Information (see step (B)
of Figure 1).
+ Check that the Access Information provides the necessary
security parameters (e.g., PoP key, information on
communication security protocols supported by the RS).
+ Safely store the proof-of-possession key.
+ If provided by the AS: Safely store the refresh token.
* Send the token and request to the RS (see step (C) of
Figure 1).
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+ Authenticate towards the RS (this could coincide with the
proof of possession process).
+ Transmit the token as specified by the AS (default is to the
authz-info endpoint, alternative options are specified by
profiles).
+ Perform the proof-of-possession procedure as specified by
the profile in use (this may already have been taken care of
through the authentication procedure).
* Process the RS response (see step (F) of Figure 1) of the RS.
Resource Server
* Expose a way to submit access tokens. By default this is the
authz-info endpoint.
* Process an access token.
+ Verify the token is from a recognized AS.
+ Check the token's integrity.
+ Verify that the token applies to this RS.
+ Check that the token has not expired (if the token provides
expiration information).
+ Store the token so that it can be retrieved in the context
of a matching request.
Note: The order proposed here is not normative, any process
that arrives at an equivalent result can be used. A noteworthy
consideration is whether one can use cheap operations early on
to quickly discard non-applicable or invalid tokens, before
performing expensive cryptographic operations (e.g. doing an
expiration check before verifying a signature).
* Process a request.
+ Set up communication security with the client.
+ Authenticate the client.
+ Match the client against existing tokens.
+ Check that tokens belonging to the client actually authorize
the requested action.
+ Optionally: Check that the matching tokens are still valid,
using introspection (if this is possible.)
* Send a response following the agreed upon communication
security mechanism(s).
* Safely store credentials such as raw public keys for
authentication or proof-of-possession keys linked to access
tokens.
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Appendix C. Requirements on Profiles
This section lists the requirements on profiles of this framework,
for the convenience of profile designers.
o Optionally define new methods for the client to discover the
necessary permissions and AS for accessing a resource, different
from the one proposed in Section 5.1. Section 4
o Optionally specify new grant types. Section 5.2
o Optionally define the use of client certificates as client
credential type. Section 5.3
o Specify the communication protocol the client and RS the must use
(e.g., CoAP). Section 5 and Section 5.6.4.3
o Specify the security protocol the client and RS must use to
protect their communication (e.g., OSCORE or DTLS). This must
provide encryption, integrity and replay protection.
Section 5.6.4.3
o Specify how the client and the RS mutually authenticate.
Section 4
o Specify the proof-of-possession protocol(s) and how to select one,
if several are available. Also specify which key types (e.g.,
symmetric/asymmetric) are supported by a specific proof-of-
possession protocol. Section 5.6.4.2
o Specify a unique ace_profile identifier. Section 5.6.4.3
o If introspection is supported: Specify the communication and
security protocol for introspection. Section 5.7
o Specify the communication and security protocol for interactions
between client and AS. This must provide encryption, integrity
protection, replay protection and a binding between requests and
responses. Section 5 and Section 5.6
o Specify how/if the authz-info endpoint is protected, including how
error responses are protected. Section 5.8.1
o Optionally define other methods of token transport than the authz-
info endpoint. Section 5.8.1
Appendix D. Assumptions on AS knowledge about C and RS
This section lists the assumptions on what an AS should know about a
client and an RS in order to be able to respond to requests to the
token and introspection endpoints. How this information is
established is out of scope for this document.
o The identifier of the client or RS.
o The profiles that the client or RS supports.
o The scopes that the RS supports.
o The audiences that the RS identifies with.
o The key types (e.g., pre-shared symmetric key, raw public key, key
length, other key parameters) that the client or RS supports.
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o The types of access tokens the RS supports (e.g., CWT).
o If the RS supports CWTs, the COSE parameters for the crypto
wrapper (e.g., algorithm, key-wrap algorithm, key-length) that the
RS supports.
o The expiration time for access tokens issued to this RS (unless
the RS accepts a default time chosen by the AS).
o The symmetric key shared between client and AS (if any).
o The symmetric key shared between RS and AS (if any).
o The raw public key of the client or RS (if any).
o Whether the RS has synchronized time (and thus is able to use the
'exp' claim) or not.
Appendix E. Deployment Examples
There is a large variety of IoT deployments, as is indicated in
Appendix A, and this section highlights a few common variants. This
section is not normative but illustrates how the framework can be
applied.
For each of the deployment variants, there are a number of possible
security setups between clients, resource servers and authorization
servers. The main focus in the following subsections is on how
authorization of a client request for a resource hosted by an RS is
performed. This requires the security of the requests and responses
between the clients and the RS to be considered.
Note: CBOR diagnostic notation is used for examples of requests and
responses.
E.1. Local Token Validation
In this scenario, the case where the resource server is offline is
considered, i.e., it is not connected to the AS at the time of the
access request. This access procedure involves steps A, B, C, and F
of Figure 1.
Since the resource server must be able to verify the access token
locally, self-contained access tokens must be used.
This example shows the interactions between a client, the
authorization server and a temperature sensor acting as a resource
server. Message exchanges A and B are shown in Figure 17.
A: The client first generates a public-private key pair used for
communication security with the RS.
The client sends a CoAP POST request to the token endpoint at the
AS. The security of this request can be transport or application
layer. It is up the the communication security profile to define.
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In the example it is assumed that both client and AS have
performed mutual authentication e.g. via DTLS. The request
contains the public key of the client and the Audience parameter
set to "tempSensorInLivingRoom", a value that the temperature
sensor identifies itself with. The AS evaluates the request and
authorizes the client to access the resource.
B: The AS responds with a 2.05 Content response containing the
Access Information, including the access token. The PoP access
token contains the public key of the client, and the Access
Information contains the public key of the RS. For communication
security this example uses DTLS RawPublicKey between the client
and the RS. The issued token will have a short validity time,
i.e., "exp" close to "iat", in order to mitigate attacks using
stolen client credentials. The token includes the claim such as
"scope" with the authorized access that an owner of the
temperature device can enjoy. In this example, the "scope" claim,
issued by the AS, informs the RS that the owner of the token, that
can prove the possession of a key is authorized to make a GET
request against the /temperature resource and a POST request on
the /firmware resource. Note that the syntax and semantics of the
scope claim are application specific.
Note: In this example it is assumed that the client knows what
resource it wants to access, and is therefore able to request
specific audience and scope claims for the access token.
Authorization
Client Server
| |
|<=======>| DTLS Connection Establishment
| | and mutual authentication
| |
A: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"token"
| | Content-Format: application/ace+cbor
| | Payload: <Request-Payload>
| |
B: |<--------+ Header: 2.05 Content
| 2.05 | Content-Format: application/ace+cbor
| | Payload: <Response-Payload>
| |
Figure 17: Token Request and Response Using Client Credentials.
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 18 Note that the parameter "rs_cnf" from
[I-D.ietf-ace-oauth-params] is used to inform the client about the
resource server's public key.
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Request-Payload :
{
"audience" : "tempSensorInLivingRoom",
"client_id" : "myclient",
"req_cnf" : {
"COSE_Key" : {
"kid" : b64'1Bg8vub9tLe1gHMzV76e8',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
"y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
}
}
}
Response-Payload :
{
"access_token" : b64'0INDoQEKoQVNKkXfb7xaWqMTf6 ...',
"rs_cnf" : {
"COSE_Key" : {
"kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4',
"y" : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM'
}
}
}
Figure 18: Request and Response Payload Details.
The content of the access token is shown in Figure 19.
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{
"aud" : "tempSensorInLivingRoom",
"iat" : "1563451500",
"exp" : "1563453000",
"scope" : "temperature_g firmware_p",
"cnf" : {
"COSE_Key" : {
"kid" : b64'1Bg8vub9tLe1gHMzV76e8',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
"y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
}
}
}
Figure 19: Access Token including Public Key of the Client.
Messages C and F are shown in Figure 20 - Figure 21.
C: The client then sends the PoP access token to the authz-info
endpoint at the RS. This is a plain CoAP POST request, i.e., no
transport or application layer security is used between client and
RS since the token is integrity protected between the AS and RS.
The RS verifies that the PoP access token was created by a known
and trusted AS, that it applies to this RS, and that it is valid.
The RS caches the security context together with authorization
information about this client contained in the PoP access token.
Resource
Client Server
| |
C: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"authz-info"
| | Payload: 0INDoQEKoQVN ...
| |
|<--------+ Header: 2.04 Changed
| 2.04 |
| |
Figure 20: Access Token provisioning to RS
The client and the RS runs the DTLS handshake using the raw public
keys established in step B and C.
The client sends a CoAP GET request to /temperature on RS over
DTLS. The RS verifies that the request is authorized, based on
previously established security context.
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F: The RS responds over the same DTLS channel with a CoAP 2.05
Content response, containing a resource representation as payload.
Resource
Client Server
| |
|<=======>| DTLS Connection Establishment
| | using Raw Public Keys
| |
+-------->| Header: GET (Code=0.01)
| GET | Uri-Path: "temperature"
| |
| |
| |
F: |<--------+ Header: 2.05 Content
| 2.05 | Payload: <sensor value>
| |
Figure 21: Resource Request and Response protected by DTLS.
E.2. Introspection Aided Token Validation
In this deployment scenario it is assumed that a client is not able
to access the AS at the time of the access request, whereas the RS is
assumed to be connected to the back-end infrastructure. Thus the RS
can make use of token introspection. This access procedure involves
steps A-F of Figure 1, but assumes steps A and B have been carried
out during a phase when the client had connectivity to AS.
Since the client is assumed to be offline, at least for a certain
period of time, a pre-provisioned access token has to be long-lived.
Since the client is constrained, the token will not be self contained
(i.e. not a CWT) but instead just a reference. The resource server
uses its connectivity to learn about the claims associated to the
access token by using introspection, which is shown in the example
below.
In the example interactions between an offline client (key fob), an
RS (online lock), and an AS is shown. It is assumed that there is a
provisioning step where the client has access to the AS. This
corresponds to message exchanges A and B which are shown in
Figure 22.
Authorization consent from the resource owner can be pre-configured,
but it can also be provided via an interactive flow with the resource
owner. An example of this for the key fob case could be that the
resource owner has a connected car, he buys a generic key that he
wants to use with the car. To authorize the key fob he connects it
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to his computer that then provides the UI for the device. After that
OAuth 2.0 implicit flow can used to authorize the key for his car at
the the car manufacturers AS.
Note: In this example the client does not know the exact door it will
be used to access since the token request is not send at the time of
access. So the scope and audience parameters are set quite wide to
start with, while tailored values narrowing down the claims to the
specific RS being accessed can be provided to that RS during an
introspection step.
A: The client sends a CoAP POST request to the token endpoint at
AS. The request contains the Audience parameter set to "PACS1337"
(PACS, Physical Access System), a value the that identifies the
physical access control system to which the individual doors are
connected. The AS generates an access token as an opaque string,
which it can match to the specific client and the targeted
audience. It furthermore generates a symmetric proof-of-
possession key. The communication security and authentication
between client and AS is assumed to have been provided at
transport layer (e.g. via DTLS) using a pre-shared security
context (psk, rpk or certificate).
B: The AS responds with a CoAP 2.05 Content response, containing
as playload the Access Information, including the access token and
the symmetric proof-of-possession key. Communication security
between C and RS will be DTLS and PreSharedKey. The PoP key is
used as the PreSharedKey.
Note: In this example we are using a symmetric key for a multi-RS
audience, which is not recommended normally (see Section 6.9).
However in this case the risk is deemed to be acceptable, since all
the doors are part of the same physical access control system, and
therefore the risk of a malicious RS impersonating the client towards
another RS is low.
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Authorization
Client Server
| |
|<=======>| DTLS Connection Establishment
| | and mutual authentication
| |
A: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"token"
| | Content-Format: application/ace+cbor
| | Payload: <Request-Payload>
| |
B: |<--------+ Header: 2.05 Content
| | Content-Format: application/ace+cbor
| 2.05 | Payload: <Response-Payload>
| |
Figure 22: Token Request and Response using Client Credentials.
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 23.
Request-Payload:
{
"client_id" : "keyfob",
"audience" : "PACS1337"
}
Response-Payload:
{
"access_token" : b64'VGVzdCB0b2tlbg==',
"cnf" : {
"COSE_Key" : {
"kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
"kty" : "oct",
"alg" : "HS256",
"k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE'
}
}
}
Figure 23: Request and Response Payload for C offline
The access token in this case is just an opaque byte string
referencing the authorization information at the AS.
C: Next, the client POSTs the access token to the authz-info
endpoint in the RS. This is a plain CoAP request, i.e., no DTLS
between client and RS. Since the token is an opaque string, the
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RS cannot verify it on its own, and thus defers to respond the
client with a status code until after step E.
D: The RS sends the token to the introspection endpoint on the AS
using a CoAP POST request. In this example RS and AS are assumed
to have performed mutual authentication using a pre shared
security context (psk, rpk or certificate) with the RS acting as
DTLS client.
E: The AS provides the introspection response (2.05 Content)
containing parameters about the token. This includes the
confirmation key (cnf) parameter that allows the RS to verify the
client's proof of possession in step F. Note that our example in
Figure 25 assumes a pre-established key (e.g. one used by the
client and the RS for a previous token) that is now only
referenced by its key-identifier 'kid'.
After receiving message E, the RS responds to the client's POST in
step C with the CoAP response code 2.01 (Created).
Resource
Client Server
| |
C: +-------->| Header: POST (T=CON, Code=0.02)
| POST | Uri-Path:"authz-info"
| | Payload: b64'VGVzdCB0b2tlbg=='
| |
| | Authorization
| | Server
| | |
| D: +--------->| Header: POST (Code=0.02)
| | POST | Uri-Path: "introspect"
| | | Content-Format: "application/ace+cbor"
| | | Payload: <Request-Payload>
| | |
| E: |<---------+ Header: 2.05 Content
| | 2.05 | Content-Format: "application/ace+cbor"
| | | Payload: <Response-Payload>
| | |
| |
|<--------+ Header: 2.01 Created
| 2.01 |
| |
Figure 24: Token Introspection for C offline
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 25.
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Request-Payload:
{
"token" : b64'VGVzdCB0b2tlbg==',
"client_id" : "FrontDoor",
}
Response-Payload:
{
"active" : true,
"aud" : "lockOfDoor4711",
"scope" : "open, close",
"iat" : 1563454000,
"cnf" : {
"kid" : b64'c29tZSBwdWJsaWMga2V5IGlk'
}
}
Figure 25: Request and Response Payload for Introspection
The client uses the symmetric PoP key to establish a DTLS
PreSharedKey secure connection to the RS. The CoAP request PUT is
sent to the uri-path /state on the RS, changing the state of the
door to locked.
F: The RS responds with a appropriate over the secure DTLS
channel.
Resource
Client Server
| |
|<=======>| DTLS Connection Establishment
| | using Pre Shared Key
| |
+-------->| Header: PUT (Code=0.03)
| PUT | Uri-Path: "state"
| | Payload: <new state for the lock>
| |
F: |<--------+ Header: 2.04 Changed
| 2.04 | Payload: <new state for the lock>
| |
Figure 26: Resource request and response protected by OSCORE
Appendix F. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
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F.1. Version -21 to 22
o Provided section numbers in references to OAuth RFC.
o Updated IANA mapping registries to only use "Private Use" and
"Expert Review".
o Made error messages optional for RS at token submission since it
may not be able to send them depending on the profile.
o Corrected errors in examples.
F.2. Version -20 to 21
o Added text about expiration of RS keys.
F.3. Version -19 to 20
o Replaced "req_aud" with "audience" from the OAuth token exchange
draft.
o Updated examples to remove unnecessary elements.
F.4. Version -18 to -19
o Added definition of "Authorization Information".
o Explicitly state that ACE allows encoding refresh tokens in binary
format in addition to strings.
o Renamed "AS Information" to "AS Request Creation Hints" and added
the possibility to specify req_aud and scope as hints.
o Added the "kid" parameter to AS Request Creation Hints.
o Added security considerations about the integrity protection of
tokens with multi-RS audiences.
o Renamed IANA registries mapping OAuth parameters to reflect the
mapped registry.
o Added JWT claim names to CWT claim registrations.
o Added expert review instructions.
o Updated references to TLS from 1.2 to 1.3.
F.5. Version -17 to -18
o Added OSCORE options in examples involving OSCORE.
o Removed requirement for the client to send application/cwt, since
the client has no way to know.
o Clarified verification of tokens by the RS.
o Added exi claim CWT registration.
F.6. Version -16 to -17
o Added references to (D)TLS 1.3.
o Added requirement that responses are bound to requests.
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o Specify that grant_type is OPTIONAL in C2AS requests (as opposed
to REQUIRED in OAuth).
o Replaced examples with hypothetical COSE profile with OSCORE.
o Added requirement for content type application/ace+cbor in error
responses for token and introspection requests and responses.
o Reworked abbreviation space for claims, request and response
parameters.
o Added text that the RS may indicate that it is busy at the authz-
info resource.
o Added section that specifies how the RS verifies an access token.
o Added section on the protection of the authz-info endpoint.
o Removed the expiration mechanism based on sequence numbers.
o Added reference to RFC7662 security considerations.
o Added considerations on minimal security requirements for
communication.
o Added security considerations on unprotected information sent to
authz-info and in the error responses.
F.7. Version -15 to -16
o Added text the RS using RFC6750 error codes.
o Defined an error code for incompatible token request parameters.
o Removed references to the actors draft.
o Fixed errors in examples.
F.8. Version -14 to -15
o Added text about refresh tokens.
o Added text about protection of credentials.
o Rephrased introspection so that other entities than RS can do it.
o Editorial improvements.
F.9. Version -13 to -14
o Split out the 'aud', 'cnf' and 'rs_cnf' parameters to
[I-D.ietf-ace-oauth-params]
o Introduced the "application/ace+cbor" Content-Type.
o Added claim registrations from 'profile' and 'rs_cnf'.
o Added note on schema part of AS Information Section 5.1.2
o Realigned the parameter abbreviations to push rarely used ones to
the 2-byte encoding size of CBOR integers.
F.10. Version -12 to -13
o Changed "Resource Information" to "Access Information" to avoid
confusion.
o Clarified section about AS discovery.
o Editorial changes
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F.11. Version -11 to -12
o Moved the Request error handling to a section of its own.
o Require the use of the abbreviation for profile identifiers.
o Added rs_cnf parameter in the introspection response, to inform
RS' with several RPKs on which key to use.
o Allowed use of rs_cnf as claim in the access token in order to
inform an RS with several RPKs on which key to use.
o Clarified that profiles must specify if/how error responses are
protected.
o Fixed label number range to align with COSE/CWT.
o Clarified the requirements language in order to allow profiles to
specify other payload formats than CBOR if they do not use CoAP.
F.12. Version -10 to -11
o Fixed some CBOR data type errors.
o Updated boilerplate text
F.13. Version -09 to -10
o Removed CBOR major type numbers.
o Removed the client token design.
o Rephrased to clarify that other protocols than CoAP can be used.
o Clarifications regarding the use of HTTP
F.14. Version -08 to -09
o Allowed scope to be byte strings.
o Defined default names for endpoints.
o Refactored the IANA section for briefness and consistency.
o Refactored tables that define IANA registry contents for
consistency.
o Created IANA registry for CBOR mappings of error codes, grant
types and Authorization Server Information.
o Added references to other document sections defining IANA entries
in the IANA section.
F.15. Version -07 to -08
o Moved AS discovery from the DTLS profile to the framework, see
Section 5.1.
o Made the use of CBOR mandatory. If you use JSON you can use
vanilla OAuth.
o Made it mandatory for profiles to specify C-AS security and RS-AS
security (the latter only if introspection is supported).
o Made the use of CBOR abbreviations mandatory.
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o Added text to clarify the use of token references as an
alternative to CWTs.
o Added text to clarify that introspection must not be delayed, in
case the RS has to return a client token.
o Added security considerations about leakage through unprotected AS
discovery information, combining profiles and leakage through
error responses.
o Added privacy considerations about leakage through unprotected AS
discovery.
o Added text that clarifies that introspection is optional.
o Made profile parameter optional since it can be implicit.
o Clarified that CoAP is not mandatory and other protocols can be
used.
o Clarified the design justification for specific features of the
framework in appendix A.
o Clarified appendix E.2.
o Removed specification of the "cnf" claim for CBOR/COSE, and
replaced with references to [RFC8747]
F.16. Version -06 to -07
o Various clarifications added.
o Fixed erroneous author email.
F.17. Version -05 to -06
o Moved sections that define the ACE framework into a subsection of
the framework Section 5.
o Split section on client credentials and grant into two separate
sections, Section 5.2, and Section 5.3.
o Added Section 5.4 on AS authentication.
o Added Section 5.5 on the Authorization endpoint.
F.18. Version -04 to -05
o Added RFC 2119 language to the specification of the required
behavior of profile specifications.
o Added Section 5.3 on the relation to the OAuth2 grant types.
o Added CBOR abbreviations for error and the error codes defined in
OAuth2.
o Added clarification about token expiration and long-running
requests in Section 5.8.3
o Added security considerations about tokens with symmetric PoP keys
valid for more than one RS.
o Added privacy considerations section.
o Added IANA registry mapping the confirmation types from RFC 7800
to equivalent COSE types.
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o Added appendix D, describing assumptions about what the AS knows
about the client and the RS.
F.19. Version -03 to -04
o Added a description of the terms "framework" and "profiles" as
used in this document.
o Clarified protection of access tokens in section 3.1.
o Clarified uses of the "cnf" parameter in section 6.4.5.
o Clarified intended use of Client Token in section 7.4.
F.20. Version -02 to -03
o Removed references to draft-ietf-oauth-pop-key-distribution since
the status of this draft is unclear.
o Copied and adapted security considerations from draft-ietf-oauth-
pop-key-distribution.
o Renamed "client information" to "RS information" since it is
information about the RS.
o Clarified the requirements on profiles of this framework.
o Clarified the token endpoint protocol and removed negotiation of
"profile" and "alg" (section 6).
o Renumbered the abbreviations for claims and parameters to get a
consistent numbering across different endpoints.
o Clarified the introspection endpoint.
o Renamed token, introspection and authz-info to "endpoint" instead
of "resource" to mirror the OAuth 2.0 terminology.
o Updated the examples in the appendices.
F.21. Version -01 to -02
o Restructured to remove communication security parts. These shall
now be defined in profiles.
o Restructured section 5 to create new sections on the OAuth
endpoints token, introspection and authz-info.
o Pulled in material from draft-ietf-oauth-pop-key-distribution in
order to define proof-of-possession key distribution.
o Introduced the "cnf" parameter as defined in RFC7800 to reference
or transport keys used for proof of possession.
o Introduced the "client-token" to transport client information from
the AS to the client via the RS in conjunction with introspection.
o Expanded the IANA section to define parameters for token request,
introspection and CWT claims.
o Moved deployment scenarios to the appendix as examples.
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o Changed 5.1. from "Communication Security Protocol" to "Client
Information".
o Major rewrite of 5.1 to clarify the information exchanged between
C and AS in the PoP access token request profile for IoT.
* Allow the client to indicate preferences for the communication
security protocol.
* Defined the term "Client Information" for the additional
information returned to the client in addition to the access
token.
* Require that the messages between AS and client are secured,
either with (D)TLS or with COSE_Encrypted wrappers.
* Removed dependency on OSCOAP and added generic text about
object security instead.
* Defined the "rpk" parameter in the client information to
transmit the raw public key of the RS from AS to client.
* (D)TLS MUST use the PoP key in the handshake (either as PSK or
as client RPK with client authentication).
* Defined the use of x5c, x5t and x5tS256 parameters when a
client certificate is used for proof of possession.
* Defined "tktn" parameter for signaling for how to transfer the
access token.
o Added 5.2. the CoAP Access-Token option for transferring access
tokens in messages that do not have payload.
o 5.3.2. Defined success and error responses from the RS when
receiving an access token.
o 5.6.:Added section giving guidance on how to handle token
expiration in the absence of reliable time.
o Appendix B Added list of roles and responsibilities for C, AS and
RS.
Authors' Addresses
Ludwig Seitz
Combitech
Djaeknegatan 31
Malmoe 211 35
Sweden
Email: ludwig.seitz@combitech.se
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Goeran Selander
Ericsson
Faroegatan 6
Kista 164 80
Sweden
Email: goran.selander@ericsson.com
Erik Wahlstroem
Sweden
Email: erik@wahlstromstekniska.se
Samuel Erdtman
Spotify AB
Birger Jarlsgatan 61, 4tr
Stockholm 113 56
Sweden
Email: erdtman@spotify.com
Hannes Tschofenig
Arm Ltd.
Absam 6067
Austria
Email: Hannes.Tschofenig@arm.com
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