wdiff draft-ietf-ace-oauth-authz-07.txt draft-ietf-ace-oauth-authz-08.txt

ACE Working Group L. Seitz
Internet-Draft RISE SICS
Intended status: Standards Track G. Selander
Expires: February 9, April 22, 2018 Ericsson
E. Wahlstroem
(no affiliation)
S. Erdtman
Spotify AB
H. Tschofenig
ARM Ltd.
August 8,
October 19, 2017

Authentication and Authorization for Constrained Environments (ACE)
draft-ietf-ace-oauth-authz-07
draft-ietf-ace-oauth-authz-08

Abstract

This specification defines a framework for authentication and
authorization in Internet of Things (IoT) environments. The
framework is based on a set of building blocks including OAuth 2.0
and CoAP, thus making a well-known and widely used authorization
solution suitable for IoT devices. Existing specifications are used
where possible, but where the constraints of IoT devices require it,
extensions are added and profiles are defined.

Status of This Memo

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on February 9, April 22, 2018.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. OAuth 2.0 . . . . . . . . . . . . . . . . . . . . . . . . 6 7
3.2. CoAP . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Protocol Interactions . . . . . . . . . . . . . . . . . . . . 9 10
5. Framework . . . . . . . . . . . . . . . . . . . . . . . . . . 13 14
5.1. Discovering Authorization Grants Servers . . . . . . . . . . . . 15
5.1.1. Unauthorized Resource Request Message . . . . . . 14 . . 15
5.1.2. AS Information . . . . . . . . . . . . . . . . . . . 16
5.2. Authorization Grants . . . . . . . . . . . . . . . . . . 17
5.3. Client Credentials . . . . . . . . . . . . . . . . . . . 15
5.3. 18
5.4. AS Authentication . . . . . . . . . . . . . . . . . . . . 15
5.4. 18
5.5. The 'Authorize' Authorization Endpoint . . . . . . . . . . . . . . . . 16
5.5. 18
5.6. The 'Token' Token Endpoint . . . . . . . . . . . . . . . . . . 16
5.5.1. . 19
5.6.1. Client-to-AS Request . . . . . . . . . . . . . . . . 16
5.5.2. 19
5.6.2. AS-to-Client Response . . . . . . . . . . . . . . . . 19
5.5.3. 22
5.6.3. Error Response . . . . . . . . . . . . . . . . . . . 21
5.5.4. 24
5.6.4. Request and Response Parameters . . . . . . . . . . . 22
5.5.4.1. 25
5.6.4.1. Audience . . . . . . . . . . . . . . . . . . . . 22
5.5.4.2. 25
5.6.4.2. Grant Type . . . . . . . . . . . . . . . . . . . 22
5.5.4.3. 25
5.6.4.3. Token Type . . . . . . . . . . . . . . . . . . . 23
5.5.4.4. 26
5.6.4.4. Profile . . . . . . . . . . . . . . . . . . . . . 23
5.5.4.5. 26
5.6.4.5. Confirmation . . . . . . . . . . . . . . . . . . 23
5.5.5. 26
5.6.5. Mapping parameters to CBOR . . . . . . . . . . . . . 26
5.6. 27
5.7. The 'Introspect' Endpoint . . . . . . . . . . . . . . . . 26
5.6.1. 28
5.7.1. RS-to-AS Request . . . . . . . . . . . . . . . . . . 27
5.6.2. 29
5.7.2. AS-to-RS Response . . . . . . . . . . . . . . . . . . 27
5.6.3. 29
5.7.3. Error Response . . . . . . . . . . . . . . . . . . . 28
5.6.4. 30
5.7.4. Client Token . . . . . . . . . . . . . . . . . . . . 29
5.6.5. 31
5.7.5. Mapping Introspection parameters to CBOR . . . . . . 31
5.7. 33
5.8. The Access Token . . . . . . . . . . . . . . . . . . . . 31
5.7.1. 33
5.8.1. The 'Authorization Information' Endpoint . . . . . . 32
5.7.2. 34
5.8.2. Token Expiration . . . . . . . . . . . . . . . . . . 32 35
6. Security Considerations . . . . . . . . . . . . . . . . . . . 33 36
6.1. Unprotected AS Information . . . . . . . . . . . . . . . 37
6.2. Use of Nonces for Replay Protection . . . . . . . . . . . 37
6.3. Combining profiles . . . . . . . . . . . . . . . . . . . 37
6.4. Error responses . . . . . . . . . . . . . . . . . . . . . 37
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 35 38
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 39
8.1. OAuth Introspection Response Parameter Registration . . . 35 39
8.2. OAuth Parameter Registration . . . . . . . . . . . . . . 36 39
8.3. OAuth Access Token Types . . . . . . . . . . . . . . . . 36 40
8.4. OAuth Token Type CBOR Mappings . . . . . . . . . . . . . . . . . . . 36 40
8.4.1. Registration Template . . . . . . . . . . . . . . . . 37 40
8.4.2. Initial Registry Contents . . . . . . . . . . . . . . 37 40
8.5. CBOR Web Token Claims . . . . . . . . . . . . . . . . . . 37 41
8.6. ACE OAuth Profile Registry . . . . . . . . . . . . . . . . . . 38 41
8.6.1. Registration Template . . . . . . . . . . . . . . . . 38 41
8.7. OAuth CBOR Parameter Mappings Registry . . . . . . . . . . . . 38 41
8.7.1. Registration Template . . . . . . . . . . . . . . . . 38 42
8.7.2. Initial Registry Contents . . . . . . . . . . . . . . 39 42
8.8. Introspection Endpoint CBOR Mappings Registry . . . . . . 41 44
8.8.1. Registration Template . . . . . . . . . . . . . . . . 41 44
8.8.2. Initial Registry Contents . . . . . . . . . . . . . . 41 45
8.9. CoAP Option Number Registration . . . . . . . . . . . . . 43
8.10. CWT Confirmation Methods Registry . . . . . . . . . . . . 44
8.10.1. Registration Template . . . . . . . . . . . . . . . 44
8.10.2. Initial Registry Contents . . . . . . . . . . . . . 45 47
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 45 47
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 45 48
10.1. Normative References . . . . . . . . . . . . . . . . . . 45 48
10.2. Informative References . . . . . . . . . . . . . . . . . 46 49
Appendix A. Design Justification . . . . . . . . . . . . . . . . 48 51
Appendix B. Roles and Responsibilities . . . . . . . . . . . . . 50 55
Appendix C. Requirements on Profiles . . . . . . . . . . . . . . 52 57
Appendix D. Assumptions on AS knowledge about C and RS . . . . . 53 58
Appendix E. Deployment Examples . . . . . . . . . . . . . . . . 53 58
E.1. Local Token Validation . . . . . . . . . . . . . . . . . 53 58
E.2. Introspection Aided Token Validation . . . . . . . . . . 57 62
Appendix F. Document Updates . . . . . . . . . . . . . . . . . . 61 66
F.1. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 66
F.2. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 67
F.3. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 61
F.2. 67
F.4. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 61
F.3. 67
F.5. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 61
F.4. 67
F.6. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 62
F.5. 67
F.7. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 62
F.6. 68
F.8. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 62
F.7. 68
F.9. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 63 68
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 63 69

1. Introduction

Authorization is the process for granting approval to an entity to
access a 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 is can be a
complex task.

While prior work on authorization solutions for the Web and for the
mobile environment also applies to the IoT environment Internet of Things (IoT)
environment, many IoT devices are constrained, for example example, in terms
of processing capabilities, available memory, etc. For web
applications on constrained nodes nodes, this specification makes RECOMMENDS the
use of CoAP [RFC7252]. [RFC7252] as replacement for HTTP.

A detailed treatment of constraints can be found in [RFC7228], and
the different IoT deployments present a continuous range of device
and network capabilities. 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 profiles.
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 througout throughout this memo,
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
[RFC2119].

Certain security-related terms such as "authentication",
"authorization", "confidentiality", "(data) integrity", "message
authentication code", and "verify" are taken from [RFC4949].

Since we describe exchanges in this specification are described as RESTful
protocol interactions interactions, HTTP [RFC7231] offers useful terminology.

Terminology for entities in the architecture is defined in OAuth 2.0
[RFC6749] and [I-D.ietf-ace-actors], 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 token and
/introspect
introspection at the AS and /authz-info authz-info at the RS. RS (see Section 5.8.1
for a definition of the authz-info endpoint). The CoAP [RFC7252]
definition, which is "An entity participating in the CoAP protocol"
is not used in this memo. specification.

Since this specification focuses on the problem of access control to
resources, we simplify the actors has been simplified by assuming that the client
authorization server (CAS) functionality is not stand-alone but
subsumed by either the authorization server or the client (see
section 2.2 in [I-D.ietf-ace-actors]).

We call the

The specifications of in this memo document is called the "framework" or "ACE
framework". When referring to "profiles of this framework" we mean it refers
to additional memo's specifications that define the use of this
specification with concrete transport, and communication security
protocols (e.g. (e.g., CoAP over DTLS).

We use the term "RS Information" for parameters describing
characteristics of the RS (e.g. public key) that the AS provides to
the client.

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]
[RFC7252], for those communication environments where HTTP is not
appropriate. CoAP typically runs on top of UDP UDP, which further
reduces overhead and message exchanges. While this specification
defines extensions for the use of OAuth over CoAP, we do envision further other underlying
protocols to be are not prohibited from beeing supported in the future,
such as HTTP/2,
MQTT MQTT, BLE and QUIC.

A third building block is CBOR [RFC7049] [RFC7049], for encodings where JSON
[RFC7159] 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 CoAP POST
parameters and CoAP responses. payload
transferred in protocol messages.

A fourth building block is the compact CBOR-based secure message
format COSE [RFC8152], which enables application layer security as an
alternative or complement to transport layer security (DTLS [RFC6347]
or TLS [RFC5246]). COSE is used to secure self contained self-contained tokens such
as proof-of-possession (PoP) tokens, which is an extension to the
OAuth access tokens, and "client tokens" which are defined in this framework
(see Section 5.6.4). 5.7.4). The default access token format is defined in CBOR web
token (CWT) [I-D.ietf-ace-cbor-web-token]. Application layer
security for CoAP using COSE can be provided with OSCOAP
[I-D.ietf-core-object-security].

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 RFC 7228 [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 co-exist, rather than
mandating a one-size-fits-all solution. We believe this 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.

In the subsections below we provide further details about the
different building blocks.

3.1. OAuth 2.0

The OAuth 2.0 authorization framework enables a client to obtain
limited
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 introspect introspection Endpoints:

The AS hosts the /token token endpoint that allows a client to request
access tokens. The client makes a POST request to the /token token
endpoint on the AS and receives the access token in the response
(if the request was successful).

The

In some deployments, a token introspection endpoint, /introspect, endpoint is provied by
the AS, which can be used by the RS
when requesting if it needs to request
additional information regarding a received access token. The RS
makes a POST request to /introspect the introspecton endpoint on the AS and
receives information about the access token in the response. (See
"Introspection" below.)

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 authorization server AS and consumed by the resource server. 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.

Proof of Possession Tokens:

An access token may be bound to a cryptographic key, which is then
used by an RS to authenticate requests from a client. Such tokens
are called proof-of-possession access tokens (or PoP access
tokens).

The proof-of-possession (PoP) security concept assumes that the AS
acts as a trusted third party that binds keys to 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 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 access token (aka (the PoP key) may be based on use either
symmetric as well as on or asymmetric cryptography. The appropriate choice of security
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 access token. The PoP key is
also encrypted for the client and sent together with the access
token to the client.

Asymmetric PoP key: An asymmetric key pair is generated on the
client and the public key is sent to the AS (if it does not
already have knowledge of the client'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 access token and sent back to the
requesting client. The RS can identify the client's public key
from the information in the token, which allows the client to
use the corresponding private key for the proof of possession.

The access token is either a simple reference, or a structured
information object (e.g. (e.g., CWT [I-D.ietf-ace-cbor-web-token]),
protected by a cryptographic wrapper (e.g. (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
uses CBOR encoded messages for CoAP, encoding as data formt, defined in Section 5, to request
scopes and to be informed what scopes the access token actually
authorizes.

The values of the scope parameter are expressed as a list of
space- delimited,
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 type-value 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 [I-D.ietf-ace-cbor-web-token] [I-D.ietf-ace-cbor-web-token], 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 a 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 need needs to take the
latter aspects into account.

While HTTP uses headers and query-strings query strings to convey additional
information about a request, CoAP encodes such information in so- into
header parameters called 'options'.

CoAP supports application-layer fragmentation of the CoAP payloads
through blockwise transfers [RFC7959]. However, block-wise blockwise transfer
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 1.2
[RFC6347] or TLS 1.2 [RFC5246]. CoAP defines a number of proxy
operations which requires that require transport layer security to be terminated at
the proxy. One approach for protecting CoAP communication end-to-
end end-to-end
through proxies, and also to support security for CoAP over a
different transport in a uniform way, is to provide security on at the
application layer using an object-based security mechanism such as
COSE [RFC8152].

One application of COSE is OSCOAP [I-D.ietf-core-object-security],
which provides end-to-end confidentiality, integrity and replay
protection, and a secure binding between CoAP request and response
messages. In OSCOAP, the CoAP messages are wrapped in COSE objects
and sent using CoAP.

This framework RECOMMENDS the use of CoAP as replacement for HTTP.

4. Protocol Interactions

The ACE framework is based on the OAuth 2.0 protocol interactions
using the /token token endpoint and /introspect endpoints. optionally the introspection endpoint.
A client obtains an access token from an AS using the /token token endpoint
and subsequently presents the access token to a RS to gain access to
a protected resource. The RS, after receiving an In most deployments the RS can process the
access token, token locally, however in some cases the RS may present it to
the AS via the /introspect introspection endpoint to get information about the
access token. In other deployments the RS may process the access
token locally without the need to contact an AS. 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 RFC 7521 [RFC7521] and
[I-D.ietf-oauth-device-flow]). What grant types works best depends
on the usage scenario and RFC 7744 [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 users need to go
through an authentication and authorization phase (at least during
the initial setup phase). The native apps guidelines described in
[I-D.ietf-oauth-native-apps] 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 case, the resource
owner or another person on his or her behalf have arranged has pre-arranged access rights for the client with the
authorization server out-of-band, 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. (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. We
assume 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
registration
provisioning procedure implies that the client and the AS share
credentials, 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 protocol, there is an optional discovery step
where the client discovers the resource server and the resources this
server hosts. In this step step, the client might also determine what
permissions are needed to access the protected resource. The
detailed A generic
procedure is described in Section 5.1, profiles MAY define other
procedures for this discovery process may be defined in an
ACE profile and depend on the protocols being used and the specific
deployment environment. discovery.

In Bluetooth Low Energy, for example, advertisements are broadcasted
by a peripheral, including information about the primary services.
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 |
| | + RS Information | |
| | +---------------+
| | ^ |
| | Introspection Request (D)| |
| Client | (optional) | |
| | Response + Client Token | |(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 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. It can also returns various return additional
parameters, referred to as "RS Information". In addition to the
response parameters defined by OAuth 2.0 and the PoP access token
extension,
further response parameters, such as information on which profile this framework defines parameters that can be used to
inform the client should use with about capabilities of the resource server(s). RS. More information
about these parameters can be found in Section 5.5.4. 5.6.4.

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 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 RS 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 RS Information or the client
token. The RS verifies that the token is integrity protected by
the AS and 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, in which case the different parts of
step C may be interleaved with introspection.

Token Introspection Request (D):

A resource server may be configured to introspect the access token
by including it in a request to the /introspect introspection endpoint at that
AS. Token introspection over CoAP is defined in Section 5.6 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. The AS can additionally
return information that the RS needs to pass on to the client in
the form of a client token. The latter is used to establish keys
for mutual authentication between client and RS, when the client
has no direct connectivity to the AS, see Section 5.6.4 5.7.4 for
details.

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 used communication security protocol.

5. Framework

The following sections detail the profiling and extensions of OAuth
2.0 for constrained environments environments, which constitutes the ACE
framework.

Credential Provisioning

For IoT we IoT, it cannot generally assume 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. These
credentials need to be provided to the parties before or during
the authentication protocol is executed, and may be re-used for
subsequent token requests.

Proof-of-Possession

The ACE framework framework, by default default, implements proof-of-possession for
access tokens, i.e. 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 [I-D.jones-ace-cwt-proof-of-possession] indicating
what key is used for proof-of-possession. If clients need a client needs to update
submit a token, e.g. new access token e.g., to get obtain additional access
rights, they can request that the AS binds the new access this token to the same
key as the previous token. 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
"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 for
introspection. if
introspection is used. In constrained settings TLS is not always
feasible, or desirable. Nevertheless it is REQUIRED that the data
exchanged with the AS is encrypted and integrity protected. It is
furthermore REQUIRED that the AS and the endpoint communicating with
it (client or RS) perform mutual authentication.

Profiles MUST specify how mutual authentication is done, depending
e.g. on the communication protocol and the credentials used by the
client or the RS.

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. This framework RECOMMENDS to use CoAP instead and
RECOMMENDS the 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. The Content-format depends on the security applied to the
content and MUST be specified by the profile that is used.

The OAuth 2.0 AS uses a JSON structure in the payload of its
responses both to client and RS. This framework RECOMMENDS REQUIRES the use of
CBOR [RFC7049] instead. 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, C MAY
look up the desired resource in a resource directory (cf.
[I-D.ietf-core-resource-directory]).

5.1.1. Unauthorized Resource Request Message

The requesting device can explicitly optional Unauthorized Resource Request message is a request this encoding for a
resource hosted by setting RS for which no proper authorization is granted.
RS MUST treat any request for a protected resource as Unauthorized
Resource Request message when any of the CoAP Accept option in following holds:

o The request has been received on an unprotected channel.
o RS has no valid access token for the sender of the request
regarding the requested action on that resource.
o RS has a valid access token for the sender of the request, but
this does not allow the requested action on the requested
resource.

Note: These conditions ensure that RS can handle requests
autonomously once access was granted and a secure channel has been
established between C and RS. The authz-info endpoint MUST NOT be
protected as specified above, in order to "application/cbor". Depending allow clients to upload
access tokens to RS (cf. Section 5.8.1).

Unauthorized Resource Request messages MUST be denied with a client
error response. In this response, the Resource Server SHOULD provide
proper AS Information to enable the Client to request an access token
from RS's AS as described in Section 5.1.2.

The response code MUST be 4.01 (Unauthorized) in case the sender of
the Unauthorized Resource Request message is not authenticated, or if
RS has no valid access token for C. If RS has an access token for C
but not for the resource that C has requested, RS MUST reject the
request with a 4.03 (Forbidden). If RS has an access token for C but
it does not cover the action C requested on the profile, resource, RS MUST
reject the content 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.

5.1.2. AS Information

The AS Information is sent by RS as a response to an Unauthorized
Resource Request message (see Section 5.1.1) to point the sender of
the Unauthorized Resource Request message to RS's AS. The AS
information is a set of attributes containing an absolute URI (see
Section 4.3 of [RFC3986]) that specifies the AS in charge of RS.

The message MAY arrive also contain a nonce generated by RS to ensure
freshness in case that the RS and AS do not have synchronized clocks.

Figure 2 summarizes the parameters that may be part of the AS
Information.

/----------------+----------+-------------------\
| Parameter name | CBOR Key | Major Type |
|----------------+----------+-------------------|
| AS | 0 | 3 (text string) |
| nonce | 5 | 2 (byte string) |
\----------------+----------+-------------------/

Figure 2: AS Information parameters

Figure 3 shows an example for an AS Information 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
{AS: "coaps://as.example.com/token",
nonce: h'e0a156bb3f'}

Figure 3: AS Information payload example

In this example, the attribute AS points the receiver of this message
to the URI "coaps://as.example.com/token" to request access
permissions. The originator of the AS Information payload (i.e., RS)
uses a different format wrapping local clock that is loosely synchronized with a time scale
common between RS and AS (e.g., wall clock time). Therefore, it has
included a parameter "nonce" for replay attack prevention.

Note: There is an ongoing discussion how freshness of access
tokens
can be achieved in constrained environments. This specification
for now assumes that RS and AS do not have a common understanding
of time that allows RS to achieve its security objectives without
explicitly adding a nonce.

Figure 4 illustrates the mandatory to use binary encoding of the
message payload shown in Figure 3.

a2 # map(2)
00 # unsigned(0) (=AS)
78 1c # text(28)
636f6170733a2f2f61732e657861
6d706c652e636f6d2f746f6b656e # "coaps://as.example.com/token"
05 # unsigned(5) (=nonce)
45 # bytes(5)
e0a156bb3f

Figure 4: AS Information example encoded in CBOR payload.

5.1.

5.2. Authorization Grants

To request an access token, the client obtains authorization from the
resource owner or uses its client credentials as grant. The
authorization is expressed in the form of an authorization grant.

The OAuth framework 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).

The grant type is selected depending on the use case. In cases where
the client acts on behalf of the resource owner, authorization code
grant is recommended. If the client acts on behalf of the resource
owner, but does not have any display or very limited interaction
possibilities it is recommended to use the device code grant defined
in [I-D.ietf-oauth-device-flow]. In cases where the client does not
act on behalf of the resource owner, client credentials grant is
recommended.

For details on the different grant types types, see the OAuth 2.0 framework. framework
[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 types, if needed.

5.2.

5.3. Client Credentials

Authentication of the client is mandatory independent of the grant
type when requesting the access token from the token endpoint. In
the case of client credentials grant type type, the authentication and
grant coincides. coincide.

Client registration and provisioning of client credentials to the
client is out of scope for this specification.

The OAuth framework, [RFC6749], framework [RFC6749] defines one client credential type,
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 additional client credentials such as DTLS pre-shared keys or client
certificates.

5.3.

5.4. AS Authentication

Client credential does not not, by default default, authenticate the AS that the
client connects to. In classic OAuth OAuth, the AS is authenticated with a
TLS server certificate.

Profiles of this framework SHOULD MUST specify how clients authenticate the
AS and how communication security is implemented, otherwise server
side TLS certificates certificates, as defined by OAuth 2.0 is 2.0, are required.

5.4.

5.5. The 'Authorize' Authorization Endpoint

The authorization endpoint is used to interact with the resource
owner and obtain an authorization grant in certain grant flows.
Since it requires the use of a user agent (i.e. (i.e., browser), it is not
expected that these types of grant flow will be used by constrained
clients. This endpoint is therefore out of scope for this
specification. Implementations should use the definition and
recommendations of [RFC6749] and [RFC6819].

If clients involved cannot support HTTP and TLS TLS, profiles MAY define
mappings for the authorization endpoint.

5.5.

5.6. The 'Token' Token Endpoint

In plain standard OAuth 2.0 2.0, the AS provides the /token token endpoint for
submitting access token requests. This framework extends the
functionality of the /token token endpoint, giving the AS the possibility to
help the client and RS to establish shared keys or to exchange their
public keys.
Furthermore Furthermore, this framework defines encodings using CoAP and
CBOR, in
addition to HTTP and as a substitute for JSON.

For the AS to be able to issue a token 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.

The figures of this section uses use CBOR diagnostic notation without the
integer abbreviations for the parameters or their values for better
readability.

5.5.1.
illustrative purposes. Note that implementations MUST use the
integer abbreviations and the binary CBOR encoding.

5.6.1. Client-to-AS Request

The client sends a CoAP POST request to the token endpoint at the AS,
the AS. The
profile MUST specify the Content-Type and wrapping of the payload.
The content of the request consists of the parameters specified in
section 4 of the OAuth 2.0 specification [RFC6749] [RFC6749], encoded as a CBOR
map.

In addition to these parameters, this framework defines the following
parameters for requesting an access token from a /token token endpoint:

aud
OPTIONAL. Specifies the audience for which the client is
requesting an access token. If this parameter is missing missing, it is
assumed that the client and the AS have a pre-established
understanding of the audience that an access token should address.
If a client submits a request for an access token without
specifying an "aud" parameter, and the AS does not have a default an
implicit understanding of the "aud" value for this client, then
the AS MUST respond with an error message with using a response code
equivalent to the CoAP response code 4.00 (Bad Request).

cnf
OPTIONAL. This field contains information about the key the
client would like to bind to the access token for proof-of-
possession. It is RECOMMENDED that an AS reject a request
containing a symmetric key value in the 'cnf' field. field, since the AS
is expected to be able to generate better symmetric keys than a
potentially constrained client. See Section 5.5.4.5 5.6.4.5 for more
details on the formatting of the 'cnf' parameter.

The following examples illustrate different types of requests for
proof-of-possession tokens.

Figure 2 5 shows a request for a token with a symmetric proof-of-
possession key. Note that in this example we assume a DTLS-based it is assumed that
transport layer communication security profile, is used, therefore the
Content-Type is "application/cbor". The content is displayed in CBOR
diagnostic notation, without abbreviations for better readability.

Header: POST (Code=0.02)
Uri-Host: "server.example.com"
Uri-Path: "token"
Content-Type: "application/cbor"
Payload:
{
"grant_type" : "client_credentials",
"client_id" : "myclient",
"aud" : "tempSensor4711", "tempSensor4711"
}

Figure 2: 5: Example request for an access token bound to a symmetric
key.

Figure 3 6 shows a request for a token with an asymmetric proof-of-
possession key. Note that in this example we assume an object
security-based profile, COSE is used to provide
object-security, therefore the Content-Type is "application/
cose". "application/cose".

Header: POST (Code=0.02)
Uri-Host: "server.example.com"
Uri-Path: "token"
Content-Type: "application/cose"
Payload:
"COSE_Encrypted" : {
16(
[ h'a1010a', # protected header: {"alg" : "AES-CCM-16-64-128"}
{5 : b64'ifUvZaHFgJM7UmGnjA'}, # unprotected header, IV
b64'WXThuZo6TMCaZZqi6ef/8WHTjOdGk8kNzaIhIQ' # ciphertext
]
)
}

Decrypted payload:
{
"grant_type" : "client_credentials",
"client_id" : "myclient",
"cnf" : {
"COSE_Key" : {
"kty" : "EC",
"kid" : h'11',
"crv" : "P-256",
"x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8',
"y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4'
}
}
}

Figure 3: 6: Example token request bound to an asymmetric key.

Figure 4 7 shows a request for a token where a previously communicated
proof-of-possession key is only referenced. Note that we assume a
DTLS-based transport
layer based communication security profile for is assumed in this
example, therefore the Content-Type is "application/cbor". Also note
that the client performs a password based authentication in this
example by submitting its client_secret (see section 2.3.1. of
[RFC6749]).

Figure 4: 7: Example request for an access token bound to a key
reference.

5.5.2.

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 5.5.3. 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, 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].

The content of the successful reply is the RS Information. It MUST
be encoded as CBOR map, containing parameters as specified in section
5.1 of [RFC6749]. In addition to these parameters, the following
parameters are also part of a successful response:

profile
REQUIRED.
OPTIONAL. This indicates the profile that the client MUST use
towards the RS. See Section 5.5.4.4 5.6.4.4 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.
cnf
REQUIRED if the token type is 'pop'. OPTIONAL otherwise. If "pop" and a symmetric proof-of-possession algorithms was selected, this key is used.
MUST NOT be present otherwise. This field contains the symmetric
proof-of-possession key. If an asymmetric algorithm
was selected, key the client is supposed to use. See
Section 5.6.4.5 for details on the use of this parameter.
rs_cnf
OPTIONAL if the token type is "pop" and asymmetric keys are used.
MUST NOT be present otherwise. This field contains information
about the public key used by the RS to authenticate. See
Section 5.5.4.5 5.6.4.5 for details on the
formatting use of this parameter. If this
parameter is absent, the AS assumes that the client already knows
the public key of the RS.
token_type
OPTIONAL. By default implementations of this framework SHOULD
assume that the token_type is 'pop'. "pop". If a specific use case
requires another token_type (e.g. 'Bearer') (e.g., "Bearer") to be used then this
parameter is REQUIRED.

Note that if CBOR Web Tokens [I-D.ietf-ace-cbor-web-token] are used,
the access token can also contain a 'cnf' claim. "cnf" claim
[I-D.jones-ace-cwt-proof-of-possession]. This claim is however
consumed by a different party. The access token is created by the AS
and processed by the RS (and opaque to the client) whereas the RS
Information is created by the AS and processed by the client; it is
never forwarded to the resource server.

Figure Figure 5 8 summarizes the parameters that may be part of the RS
Information.

/-------------------+--------------------------\
| 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 |
| profile | [[ this specification ]] |
| cnf | [[ this specification ]] |
| rs_cnf | [[ this specification ]] |
\-------------------+--------------------------/

Figure 5: 8: RS Information parameters

Figure 6 9 shows a response containing a token and a 'cnf' "cnf" parameter
with a symmetric proof-of-possession key. Note that we assume a
DTLS-based communication transport layer
security profile for is assumed in this example, therefore the Content-Type is
"application/cbor".

Header: Created (Code=2.01)
Content-Type: "application/cbor"
Payload:
{
"access_token" : b64'SlAV32hkKG ...
(remainder of CWT omitted for brevity;
CWT contains COSE_Key in the 'cnf' "cnf" claim)',
"profile" : "coap_dtls",
"expires_in" : "3600",
"cnf" : {
"COSE_Key" : {
"kty" : "Symmetric",
"kid" : b64'39Gqlw',
"k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
}
}
}

Figure 6: 9: Example AS response with an access token bound to a
symmetric key.

5.5.3.

5.6.3. 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 5.2 of [RFC6749], with the following differences:

o The Content-Type MUST be specified by the communication security
profile used between client and AS. The raw payload before being
processed by the communication security protocol MUST be encoded
as a CBOR map.
o The CoAP 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 response code 4.01
(Unauthorized) MAY be used under the same conditions as specified
in section 5.2 of [RFC6749].
o The parameters "error", "error_description" and "error_uri" MAY MUST
be abbreviated using the codes specified in table Figure 13. 12.
o The error codes MAY code (i.e., value of the "error" parameter) MUST be
abbreviated using the codes as specified in
table Figure 7.

/------------------------+----------+--------------\ 10.

/------------------------+-------------------\
| error code | CBOR Key | Major Type Value (uint) |
|------------------------+----------+--------------|
|------------------------+-------------------|
| invalid_request | 0 | 0 (uint) |
| invalid_client | 1 | 0 |
| invalid_grant | 2 | 0 |
| unauthorized_client | 3 | 0 |
| unsupported_grant_type | 4 | 0 |
| invalid_scope | 5 | 0 |
| unsupported_pop_key | 6 | 0 |
\------------------------+----------+--------------/
\------------------------+-------------------/

Figure 7: 10: CBOR abbreviations for common error codes

In addition to the error responses defined in OAuth 2.0, the
follwoing behaviour
following behavior MUST be implemented by the AS: 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 response code 4.00 (Bad Request) with including the error
code "unsupported_pop_key" defined in figure Figure 7.

5.5.4. 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.

5.5.4.1.

5.6.4.1. Audience

This parameter specifies for which audience the client is requesting
a token. It should be encoded as CBOR text string (major type 3).
The formatting and semantics of these strings are application
specific.

5.5.4.2.

5.6.4.2. Grant Type

The abbreviations in Figure 8 MAY 11 MUST be used in CBOR encodings instead
of the string values defined in [RFC6749].

/--------------------+----------+--------------\

/--------------------+-------------------\
| grant_type | CBOR Key | Major Type Value (uint) |
|--------------------+----------+--------------|
|--------------------+-------------------|
| password | 0 | 0 (uint) |
| authorization_code | 1 | 0 |
| client_credentials | 2 | 0 |
| refresh_token | 3 | 0 |
\--------------------+----------+--------------/
\--------------------+-------------------/

Figure 8: 11: CBOR abbreviations for common grant types

5.5.4.3.

5.6.4.3. Token Type

The token_type parameter is defined in [RFC6749], allowing the AS to
indicate to the client which type of access token it is receiving
(e.g.
(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 is performed MUST be specified by the
profiles.

The values in the 'token_type' "token_type" parameter MUST be CBOR text strings
(major type 3).

In this framework token type 'pop' "pop" MUST be assumed by default if the
AS does not provide a different value.

5.5.4.4.

5.6.4.4. 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. Furthermore profiles MUST define proof-of-possession
methods, if they support proof-of-possession tokens.

A profile MUST specify an identifier that is can be used to uniquely
identify itself in the 'profile' "profile" parameter.

Profiles MAY define additional parameters for both the token request
and the RS Information in the access token response in order to
support negotiation or signalling signaling of profile specific parameters.

5.5.4.5.

5.6.4.5. Confirmation

The "cnf" parameter identifies or provides the key used for proof-of-
possession or for authenticating
possession, while the RS depending on "rs_cnf" parameter provides the proof-of-
possession algorithm and raw public key
of the context cnf is used in. This framework
extends RS. Both parameters use the definition of 'cnf' from [RFC7800] by adding CBOR/COSE
encodings same formatting and semantics as
the "cnf" claim specified in [I-D.jones-ace-cwt-proof-of-possession].

In addition to the use of 'cnf' for transporting keys as a claim in a CWT, the RS
Information.

The "cnf" parameter is
used in the following contexts with the following meaning:

o In the access token, to indicate the proof-of-possession key bound
to this token.
o In the token request C -> AS, to indicate the client's raw public
key, or the key-identifier of a previously established key between
C and RS.
o In the token response AS -> C, to indicate either the symmetric key
generated by the AS for proof-of-possession or the raw public
key used by the RS to authenticate. proof-of-possession.
o In the introspection response AS -> RS, to indicate the proof-of-
possession key bound to the introspected token.
o In the client token AS -> RS -> C, to indicate the proof-of-
possession key bound to the access token.

A CBOR encoded payload MAY contain the 'cnf' parameter with the
following contents:

COSE_Key In this case the 'cnf' parameter contains the proof-of-
possession key to be used by the client. An example is shown in
Figure 9.

"cnf" : {
"COSE_Key" : {
"kty" : "EC",
"kid" : h'11',
"crv" : "P-256",
"x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8',
"y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4'
}
}

Figure 9: Confirmation parameter containing a public key

Note that the COSE_Key structure in a "cnf" claim or parameter may
contain an "alg" or "key_ops" parameter. If such parameters are
present, a client MUST NOT use a key that is not compatible with the
profile or proof-of-
possession proof-of-possession algorithm according to those
parameters.
COSE_Encrypted In this case the 'cnf' parameter contains an
encrypted symmetric key destined for the client. The client is
assumed to be able to decrypt the ciphertext of this parameter.
The parameter is encoded as COSE_Encrypted object wrapping a
COSE_Key object. Figure 10 shows an example of this type of
encoding.

"cnf" : {
"COSE_Encrypted" : {
993(
[ h'a1010a' # protected header : {"alg" : "AES-CCM-16-64-128"}
"iv" : b64'ifUvZaHFgJM7UmGnjA', # unprotected header
b64'WXThuZo6TMCaZZqi6ef/8WHTjOdGk8kNzaIhIQ' # ciphertext
]
)
}
}

Figure 10: Confirmation parameter containing an encrypted symmetric
key

The ciphertext here could e.g. contain a symmetric key as in
Figure 11.

{
"kty" : "Symmetric",
"kid" : b64'39Gqlw',
"k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
}

Figure 11: Example plaintext of an encrypted cnf parameter

Key Identifier In this case the 'cnf' parameter references An RS MUST reject a key
that is assumed to be previously known by the recipient. This
allows clients that perform repeated requests for an access token
for the same audience but e.g. with different scopes to omit key
transport in the access token, token request and token response.
Figure 12 shows proof-of-possession using such an example.

"cnf" : {
"kid" : b64'39Gqlw'
}

Figure 12: A Confirmation parameter with just a key identifier

This specification establishes the IANA "CWT Confirmation Methods"
registry for these types of confirmation methods in Section 8.10 and
registers the methods defined by this specification. Other
specifications can register other methods used for confirmation. The
registry is meant to be analogous to the "JWT Confirmation Methods"
registry defined by [RFC7800].

5.5.5.
key.

5.6.5. Mapping parameters to CBOR

All OAuth parameters in access token requests and responses are MUST be
mapped to CBOR types as follows and are specified in Figure 12, using the given an
integer key value to
save space.

/-------------------+----------+-----------------\ abbreviation for the key.

Note that we have aligned these abbreviations with the claim
abbreviations defined in [I-D.ietf-ace-cbor-web-token].

/-------------------+----------+------------------\
| Parameter name | CBOR Key | Major Type |
|-------------------+----------+-----------------|
|-------------------+----------+------------------|
| aud | 3 | 3 text string |
| client_id | 8 | 3 (text string) text string |
| client_secret | 9 | 2 (byte string) byte string |
| response_type | 10 | 3 text string |
| redirect_uri | 11 | 3 text string |
| scope | 12 | 3 text string |
| state | 13 | 3 text string |
| code | 14 | 2 byte string |
| error | 15 | 3 text string |
| error_description | 16 | 3 text string |
| error_uri | 17 | 3 text string |
| grant_type | 18 | 0 unsigned integer |
| access_token | 19 | 3 text string |
| token_type | 20 | 0 unsigned integer |
| expires_in | 21 | 0 unsigned integer |
| username | 22 | 3 text string |
| password | 23 | 3 text string |
| refresh_token | 24 | 3 text string |
| cnf | 25 | 5 (map) map |
| profile | 26 | 3 text string |
| rs_cnf | 31 | map |
\-------------------+----------+-----------------/
\-------------------+----------+------------------/

Figure 13: 12: CBOR mappings used in token requests

5.6.

5.7. The 'Introspect' 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. e.g., validity or scope. Analogous
to the protocol defined in RFC 7662 [RFC7662] for HTTP and JSON, this
section defines adaptations to more constrained environments using CoAP
CBOR and CBOR. leaving the choice of the application protocol to the
profile.

Communication between the RS and the introspection endpoint at the AS
MUST be integrity protected and encrypted. Furthermore AS and RS
MUST perform mutual authentication. Finally the AS SHOULD verify
that the RS 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 RS and AS is implemented.

The figures of this section uses CBOR diagnostic notation without the
integer abbreviations for the parameters or their values for better
readability.

5.6.1.

Note that supporting introspection is OPTIONAL for implementations of
this framework.

5.7.1. RS-to-AS Request

The RS sends a CoAP POST request to the introspection endpoint at the AS,
the profile MUST specify the Content-Type and wrapping of the
payload. The payload MUST be encoded as a CBOR map with a 'token' "token"
parameter containing either the access token along with or a reference to the
token (e.g., the cti). Further optional parameters representing
additional context that is known by the RS to aid the AS in its response.
response MAY be included.

The same parameters are required and optional as in section 2.1 of
RFC 7662 [RFC7662].

For example, Figure 14 13 shows a RS calling the token introspection
endpoint at the AS to query about an OAuth 2.0 proof-of-possession
token. Note that we assume a object security-based communication security profile for based on COSE is assumed in this
example, therefore the Content-Type is "application/cose+cbor".

Header: POST (Code=0.02)
Uri-Host: "server.example.com"
Uri-Path: "introspect"
Content-Type: "application/cose+cbor"
Payload:
{
"token" : b64'7gj0dXJQ43U',
"token_type_hint" : "pop"
}

Figure 14: 13: Example introspection request.

5.6.2.

5.7.2. AS-to-RS Response

If the introspection request is authorized and successfully
processed, the AS sends a response with the CoAP 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 5.6.3. 5.7.3.

In a successful response, the AS encodes the response parameters in a
CBOR map including with the same required and optional parameters as
in section 2.2. of RFC 7662 [RFC7662] with the following additions:

cnf
OPTIONAL. This field contains information about the proof-of-
possession key that binds the client to the access token. See
Section 5.5.4.5 5.6.4.5 for more details on the formatting use of the 'cnf' "cnf"
parameter.

profile
OPTIONAL. This indicates the profile that the RS MUST use with
the client. See Section 5.5.4.4 5.6.4.4 for more details on the
formatting of this parameter.

client_token
OPTIONAL. This parameter contains information that the RS MUST
pass on to the client. See Section 5.6.4 5.7.4 for more details.

For example, Figure 15 14 shows an AS response to the introspection
request in Figure 14. 13. Note that we assume a DTLS-based communication transport layer security profile for is assumed
in this example, therefore the Content-Type is "application/cbor".

Header: Created Code=2.01)
Content-Type: "application/cbor"
Payload:
{
"active" : true,
"scope" : "read",
"profile" : "coap_dtls",
"client_token" : b64'2QPhg0OhAQo ...
(remainder of client token omitted for brevity)',
"cnf" : {
"COSE_Key" : {
"kty" : "Symmetric",
"kid" : b64'39Gqlw',
"k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
}
}
}

Figure 15: 14: Example introspection response.

5.6.3.

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, the Content-Type MUST be set according to the
specification of the communication security profile, and the
content payload MUST be encoded as a CBOR map.

o If the credentials used by the RS are invalid the AS MUST respond
with the CoAP response code equivalent to the CoAP code 4.01
(Unauthorized) and use the required and optional parameters from
section 5.2 in RFC 6749 [RFC6749].
o If the RS does not have the right to perform this introspection
request, the AS MUST respond with a response code equivalent to
the CoAP response code 4.03 (Forbidden). In this case no payload is
returned.
o The parameters "error", "error_description" and "error_uri" MAY MUST
be abbreviated using the codes specified in table Figure 13. 12.
o The error codes MAY MUST be abbreviated using the codes specified in
table
Figure 7. 10.

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
respond with an introspection response with the "active" field set to
"false".

5.6.4.

5.7.4. Client Token

EDITORIAL NOTE: We have tentatively introduced this concept and would
specifically like feedback whether this is viewed as a useful
addition to the framework.

In cases where the client has limited connectivity and needs to get
access to a previously unknown resource servers, this framework
suggests the following OPTIONAL approach: The client is pre-configured pre-
configured with a
generic, long-term access token, which is not self-contained
(i.e. it is only a reference to a token at the AS) when it is
commissioned. When the client then tries to access a RS it transmits
this access token. The RS then performs token introspection to learn
what access this token grants. In the introspection response, the AS
also relays information for the client, such as the proof-of-possession proof-of-
possession key, through the RS. The RS passes on this Client Token
to the client in response to the submission of the token.

The client_token parameter is designed to carry such information, and
is intended to be used as described in Figure 16. 15.

Figure 16: 15: Use of the client_token parameter.

The client token is a COSE_Encrypted object, containing as payload a
CBOR map with the following claims:

cnf
REQUIRED if the token type is 'pop', "pop", OPTIONAL otherwise. Contains
information about the proof-of-possession key the client is to use
with its access token. See Section 5.5.4.5. 5.6.4.5.

token_type
OPTIONAL. See Section 5.5.4.3. 5.6.4.3.

profile
REQUIRED. See Section 5.5.4.4. 5.6.4.4.

rs_cnf
OPTIONAL. Contains information about the key that the RS uses to
authenticate towards the client. If the key is symmetric then
this claim MUST NOT be part of the Client Token, since this is the
same key as the one specified through the 'cnf' "cnf" claim. This claim
uses the same encoding as the 'cnf' "cnf" parameter. See
Section 5.5.4.4. 5.6.4.4.

The AS encrypts this token using a key shared between the AS and the
client, so that only the client can decrypt it and access its
payload. How this key is established is out of scope of this
framework, however it can be established at the same time at which
the client's long term token is created.

5.6.5.

An RS that is configured to perform introspection, MUST do so
immediately after receiving an access token, in order to be able to
return a potential client token to the client. This does not
preclude the RS to perform additional introspection asynchronously,
e.g., when the token is later used.

5.7.5. Mapping Introspection parameters to CBOR

The introspection request and response parameters are MUST be mapped to
CBOR types as follows and are specified in Figure 16, using the given an integer key value to save space.
abbreviation for the key.

Note that we have aligned these abbreviatations with the claim
abbreviations defined in [I-D.ietf-ace-cbor-web-token].

/-----------------+----------+-----------------\
| Parameter name | CBOR Key | Major Type |
|-----------------+----------+-----------------|
| iss | 1 | 3 (text string) |
| sub | 2 | 3 |
| aud | 3 | 3 |
| exp | 4 | 6 tag value 1 |
| nbf | 5 | 6 tag value 1 |
| iat | 6 | 6 tag value 1 |
| cti | 7 | 2 (byte string) |
| client_id | 8 | 3 |
| scope | 12 | 3 |
| token_type | 20 | 3 |
| username | 22 | 3 |
| cnf | 25 | 5 (map) |
| profile | 26 | 0 (uint) |
| token | 27 | 3 |
| token_type_hint | 28 | 3 |
| active | 29 | 0 |
| client_token | 30 | 3 |
| rs_cnf | 31 | 5 |
\-----------------+----------+-----------------/

Figure 17: 16: CBOR Mappings to Token Introspection Parameters.

5.7.

5.8. The Access Token

This framework RECOMMENDS the use of CBOR web token (CWT) as
specified in [I-D.ietf-ace-cbor-web-token].

In order to facilitate offline processing of access tokens, this
draft specifies uses the "cnf" claim from

[I-D.jones-ace-cwt-proof-of-possession] and specifies the "scope" claims
claim for CBOR web tokens.

The "scope" claim explicitly encodes the scope of a given access
token. This claim follows the same encoding rules as defined in
section 3.3 of [RFC6749]. The meaning of a specific scope value is
application specific and expected to be known to the RS running that
application.

The "cnf" claim follows the same rules as specified for JOSE web
token in RFC7800 [RFC7800], except that it is encoded in COSE in the
same way as specified for the "cnf" parameter in Section 5.5.4.5.

5.7.1.

5.8.1. The 'Authorization Information' Endpoint

The access token, containing authorization information and
information about the key 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 authz-info endpoint, implemented by the RS.
A client using this method MUST make a POST request to /authz-
info 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). This response MAY contain the an identifier of the token
(e.g.
(e.g., the cti for a CWT) as a payload. payload, in order to allow the client
to refer to the 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.

If the token is not valid, the RS MUST respond with a response code
equivalent to the CoAP response code 4.01 (Unauthorized). If the token is
valid but the audience of the token does not match the RS, the RS
MUST respond with a response code equivalent to the CoAP
response code 4.03
(Forbidden). If the token is valid but is associated to claims that
the RS cannot process (e.g. (e.g., an unknown scope) the RS MUST respond
with a response code equivalent to the CoAP response code 4.00 (Bad Request).
In the latter case the RS MAY provide additional information in the
error response, in order to clarify what went wrong.

The RS MAY make an introspection request to validate the token before
responding to the POST /authz-info request. request to the authz-info endpoint. If the
introspection response contains a client token (Section 5.6.4) 5.7.4) then
this token SHALL be included in the payload of the 2.01 (Created)
response.

Profiles MUST specify how the /authz-info authz-info endpoint is 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 RS MUST be prepared to store more than one token for each client,
and MUST apply the combined permissions granted by all applicable,
valid tokens to client requests.

5.7.2.

5.8.2. Token Expiration

Depending on the capabilities of the RS, there are various ways in
which it can verify the validity of a received access token. We Here
follows a list of the possibilities here including what functionality they
require of the RS.

o The token is a CWT/JWT CWT and includes a 'exp' an "exp" claim and possibly the
'nbf'
"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 memo.
specification.
o The RS verifies the validity of the token by performing an
introspection request as specified in Section 5.6. 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 AS to
RS).
o The RS and the AS both store a sequence number linked to their
common security association. The AS increments this number for
each access token it issues and includes it in the access token,
which is a CWT. The RS keeps track of the most recently received
sequence number, and only accepts tokens as valid, that are in a
certain range around this number. This method does only require
the RS to keep track of the sequence number. The method does not
provide timely expiration, but it makes sure that older tokens
cease to be valid after a certain number of newer ones got issued.
For a constrained RS with no network connectivity and no means of
reliably measuring time, this is the best that can be achieved.

If a token, token that authorizes a long running request such as e.g. a CoAP
Observe [RFC7641], [RFC7641] expires, the RS MUST send an error response with
the response code 4.01 Unauthorized to the client and then terminate
processing the long running request.

6. Security Considerations

Security considerations applicable to authentication and
authorization in RESTful environments provided in OAuth 2.0 [RFC6749]
apply to this work, as well as the security considerations from
[I-D.ietf-ace-actors]. Furthermore [RFC6819] provides additional
security considerations for OAuth which apply to IoT deployments as
well.

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 AEAD 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. 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 preferrable
preferable over using a MAC unless the message needs to be publicly
readable.

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. Using a single
shared secret with multiple resource servers to simplify key
management is NOT RECOMMENDED since the benefit from using the proof-
of-possession concept is significantly reduced.

The authorization server MUST offer confidentiality protection for
any interactions with the client. This step is extremely important
since the client may obtion 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 confidentiality protection is
provided, and additional protection can be applied by encrypting the
token, for example encryption of CWTs is specified in section 5.1 of
[I-D.ietf-ace-cbor-web-token].

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.

Clients can at any time request a new proof-of-possession capable
access token. If clients have that capability, the AS can keep the
lifetime of the access token and the associated proof-of-possesion proof-of-possession
key short and therefore 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 permissions.
Furthermore access tokens using symmetric keys for proof-of-
possession SHOULD NOT be targeted at an audience that contains more
than one RS, since otherwise any RS in the audience that receives
that access token can impersonate the client towards the other
members of the audience.

6.1. Unprotected AS Information

Initially, no secure channel exists to protect the communication
between C and RS. Thus, C cannot determine if the AS information
contained in an unprotected response from RS to an unauthorized
request (c.f. Section 5.1.2) is authentic. It is therefore
advisable to provide C with a (possibly hard-coded) list of
trustworthy authorization servers. AS information responses
referring to a URI not listed there would be ignored.

6.2. Use of Nonces for Replay Protection

RS may add a nonce to the AS Information message sent as a response
to an unauthorized request to ensure freshness of an Access Token
subsequently presented to RS. While a timestamp of some granularity
would be sufficient to protect against replay attacks, using
randomized nonce is preferred to prevent disclosure of information
about RS's internal clock characteristics.

6.3. Combining profiles

There may exist reasonable use cases where implementers want to
combine different profiles of this framework, e.g., using an MQTT
profile between client and RS, while using a DTLS profile for
interactions between client and AS. Profiles should be designed in a
way that the security of a protocol interaction does not depend on
the specific security mechanisms used in other protocol interactions.

6.4. Error responses

The various error responses defined in this framework may leak
information to an adversary. For example errors 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. 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.

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 so, the resource owner needs
to bind the grants it issues to anonymous, ephemeral credentials, credentials that
do not allow the AS to link different grants and thus different
access token requests by the same client.

If access 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 or
JWTs the
token may e.g. e.g., reveal the audience, the scope and the confirmation
method used by the client. The latter may reveal the
client's identity. 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 RS. RSs. A set of colluding RS 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.

An unprotected response to an unauthorized request (c.f.
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. 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 specification registers new parameters for OAuth and establishes
registries for mappings to CBOR.

8.1. OAuth Introspection Response Parameter Registration

This specification registers the following parameters in the OAuth
introspection response parameters

o Name: "cnf"
o Description: Key to prove the right to use an access token,
formatted as
defined in [RFC7800].
o Change Controller: IESG
o Specification Document(s): this document

o Name: "aud"
o Description: Reference to intended receiving RS, as defined specified in PoP
token specification. [I-D.jones-ace-cwt-proof-of-possession].
o Change Controller: IESG
o Specification Document(s): this document

o Name: "profile"
o Description: The communication and communication security profile
used between client and RS, as defined in ACE profiles.
o Change Controller: IESG
o Specification Document(s): this document

o Name: "client_token"
o Description: Information that the RS MUST pass to the client e.g. e.g.,
about the proof-of-possession keys.
o Change Controller: IESG
o Specification Document(s): this document

o Name: "rs_cnf"
o Description: Describes the public key the RS uses to authenticate.
o Change Controller: IESG
o Specification Document(s): this document

8.2. OAuth Parameter Registration

This specification registers the following parameters in the OAuth
Parameters Registry

o Parameter name: "profile"
o Parameter usage location: token request, and token response
o Change Controller: IESG
o Specification Document(s): this document

o Name: "cnf"
o Description: Key to prove the right to use an access token,
formatted as defined in [RFC7800]. [I-D.jones-ace-cwt-proof-of-possession].
o Change Controller: IESG
o Specification Document(s): this document

8.3. OAuth Access Token Types

This specification registers the following new token type in the
OAuth Access Token Types Registry

o Name: "PoP"
o Description: A proof-of-possession token.
o Change Controller: IESG
o Specification Document(s): this document

8.4. OAuth Token Type CBOR Mappings

A new registry will be requested from IANA, entitled "Token Type
Mappings". The registry is to be created as Expert Review Required.

8.4.1. Registration Template

Token Type:
Name of token type as registered in the OAuth token type registry
e.g.
e.g., "Bearer".
Mapped value:
Integer representation for the token type value. The key value
MUST be an integer in the range of 1 integer. Integer values from -65536 to 65536. 65535 are
designated as Specification Required. Integer values of greater
than 65535 designated as expert review. Integer values less than
-65536 are marked as private use.
Change Controller:
For Standards Track RFCs, list the "IESG". For others, give the
name of the responsible party. Other details (e.g., postal
address, email address, home page URI) may also be included.
Specification Document(s):
Reference to the document or documents that specify the
parameter,preferably including URIs that can be used to retrieve
copies of the documents. An indication of the relevant sections
may also be included but is not required.

8.4.2. Initial Registry Contents

o Parameter name: "Bearer"
o Mapped value: 1
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "pop"
o Mapped value: 2
o Change Controller: IESG
o Specification Document(s): this document

8.5. CBOR Web Token Claims

This specification registers the following new claims in the CBOR Web
Token (CWT) registry:

o Claim Name: "scope"
o Claim Description: The scope of an access token as defined in
[RFC6749].
o Change Controller: IESG
o Specification Document(s): this document

o Claim Name: "cnf"
o Claim Description: The proof-of-possession key of an access token
as defined in [RFC7800].
o Change Controller: IESG
o Specification Document(s): this document

8.6. ACE OAuth Profile Registry

A new registry will be requested from IANA, entitled "ACE Profile
Registry". The registry is to be created as Expert Review Required.

8.6.1. Registration Template

Profile name:
Name of the profile to be included in the profile attribute.
Profile description:
Text giving an overview of the profile and the context it is
developed for.
Profile ID:
Integer value to identify the profile. The value MUST be an
integer in the range of 1 Integer values from -65536
to 65536. 65535 are designated as Specification Required. Integer values
of greater than 65535 designated as expert review. Integer values
less than -65536 are marked as private use.
Change Controller:
For Standards Track RFCs, list the "IESG". For others, give the
name of the responsible party. Other details (e.g., postal
address, email address, home page URI) may also be included.
Specification Document(s):
Reference to the document or documents that specify the
parameter,preferably including URIs that can be used to retrieve
copies of the documents. An indication of the relevant sections
may also be included but is not required.

8.7. OAuth CBOR Parameter Mappings Registry

A new registry will be requested from IANA, entitled "Token Endpoint
CBOR Mappings Registry". The registry is to be created as Expert
Review Required.

8.7.1. Registration Template

Parameter name:
OAuth Parameter name, refers to the name in the OAuth parameter
registry e.g. e.g., "client_id".
CBOR key value:
Key value for the claim. The key value MUST be an integer in the
range of 1 integer.
Integer values from -65536 to 65536. 65535 are designated as
Specification Required. Integer values of greater than 65535
designated as expert review. Integer values less than -65536 are
marked as private use.
Change Controller:
For Standards Track RFCs, list the "IESG". For others, give the
name of the responsible party. Other details (e.g., postal
address, email address, home page URI) may also be included.
Specification Document(s):
Reference to the document or documents that specify the
parameter,preferably including URIs that can be used to retrieve
copies of the documents. An indication of the relevant sections
may also be included but is not required.

8.7.2. Initial Registry Contents

o Parameter name: "aud"
o CBOR key value: 3
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "client_id"
o CBOR key value: 8
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "client_secret"
o CBOR key value: 9
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "response_type"
o CBOR key value: 10
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "redirect_uri"
o CBOR key value: 11
o Change Controller: IESG
o Specification Document(s): this document
o Parameter name: "scope"
o CBOR key value: 12
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "state"
o CBOR key value: 13
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "code"
o CBOR key value: 14
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "error"
o CBOR key value: 15
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "error_description"
o CBOR key value: 16
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "error_uri"
o CBOR key value: 17
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "grant_type"
o CBOR key value: 18
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "access_token"
o CBOR key value: 19
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "token_type"
o CBOR key value: 20
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "expires_in"
o CBOR key value: 21
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "username"
o CBOR key value: 22
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "password"
o CBOR key value: 23
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "refresh_token"
o CBOR key value: 24
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "cnf"
o CBOR key value: 25
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "profile"
o CBOR key value: 26
o Change Controller: IESG
o Specification Document(s): this document

8.8. Introspection Endpoint CBOR Mappings Registry

A new registry will be requested from IANA, entitled "Introspection
Endpoint CBOR Mappings Registry". The registry is to be created as
Expert Review Required.

8.8.1. Registration Template

Response parameter name:
Name of the response parameter as defined in the "OAuth Token
Introspection Response" registry e.g. e.g., "active".
CBOR key value:
Key value for the claim. The key value MUST be an integer in the
range of 1 integer.
Integer values from -65536 to 65536. 65535 are designated as
Specification Required. Integer values of greater than 65535
designated as expert review. Integer values less than -65536 are
marked as private use.
Change Controller:
For Standards Track RFCs, list the "IESG". For others, give the
name of the responsible party. Other details (e.g., postal
address, email address, home page URI) may also be included.

Specification Document(s):
Reference to the document or documents that specify the
parameter,preferably including URIs that can be used to retrieve
copies of the documents. An indication of the relevant sections
may also be included but is not required.

8.8.2. Initial Registry Contents

o Response parameter name: "iss"
o CBOR key value: 1
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "sub"
o CBOR key value: 2
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "aud"
o CBOR key value: 3
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "exp"
o CBOR key value: 4
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "nbf"
o CBOR key value: 5
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "iat"
o CBOR key value: 6
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "cti"
o CBOR key value: 7
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "client_id"
o CBOR key value: 8
o Change Controller: IESG
o Specification Document(s): this document
o Response parameter name: "scope"
o CBOR key value: 12
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "token_type"
o CBOR key value: 20
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "username"
o CBOR key value: 22
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "cnf"
o CBOR key value: 25
o Change Controller: IESG
o Specification Document(s): this document

o Parameter name: "profile"
o CBOR key value: 26
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "token"
o CBOR key value: 27
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "token_type_hint"
o CBOR key value: 28
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "active"
o CBOR key value: 29
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "client_token"
o CBOR key value: 30
o Change Controller: IESG
o Specification Document(s): this document

o Response parameter name: "rs_cnf"
o CBOR key value: 31
o Change Controller: IESG
o Specification Document(s): this document

8.9. CoAP Option Number Registration

This section registers the "Access-Token" CoAP Option Number in the
"CoRE Parameters" sub-registry "CoAP Option Numbers" in the manner
described in [RFC7252].

Name

Access-Token
Number

TBD
Reference

[This document].
Meaning in Request

Contains an Access Token according to [This document] containing
access permissions of the client.
Meaning in Response

Not used in response
Safe-to-Forward

Yes
Format

Based on the observer the format is perceived differently. Opaque
data to the client and CWT or reference token to the RS.
Length

Less then 255 bytes

8.10. CWT Confirmation Methods Registry

This specification establishes the IANA "CWT Confirmation Methods"
registry for CWT "cnf" member values. The registry records the
confirmation method member and a reference to the specification that
defines it.

8.10.1. Registration Template

Confirmation Method Name:
The name requested (e.g., "kid").

9. Acknowledgments

This name is intended to be
human readable and be used for debugging purposes. It is case
sensitive. Names may not match other registered names in a case-
insensitive manner unless the Designated Experts state that there document is a compelling reason to allow an exception.

Confirmation Method Value:
Integer representation for the confirmation method value.
Intended for use to uniquely identify the confirmation method.
The value MUST be an integer in the range of 1 to 65536.

Confirmation Method Description:
Brief description of the confirmation method (e.g. "Key
Identifier").

Change Controller:
For Standards Track RFCs, list the "IESG". For others, give the
name product of the responsible party. Other details (e.g., postal
address, email address, home page URI) may also be included.

Specification Document(s):
Reference to the document or documents that specify the parameter,
preferably including URIs that can be used to retrieve copies of
the documents. An indication ACE working group of the relevant sections may also be
included but is not required.

8.10.2. Initial Registry Contents

o Confirmation Method Name: "COSE_Key"
o Confirmation Method Value: 1
o Confirmation Method Description: A COSE_Key that is either a
public key or a symmetric key.
o Change Controller: IESG
o Specification Document(s): this document

o Confirmation Method Name: "COSE_Encrypted"
o Confirmation Method Value: 2
o Confirmation Method Description: A COSE_Encrypted structure that
wraps a COSE_Key containing a symmetric key.
o Change Controller: IESG
o Specification Document(s): this document

o Confirmation Method Name: "Key Identifier"
o Confirmation Method Value: 3
o Confirmation Method Description: A key identifier.
o Change Controller: IESG
o Specification Document(s): this document

9. Acknowledgments

We would like IETF.

Thanks to thank 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. Finally, we would like

Thanks to thank the ACE working group in
general for their feedback.

We would like to thank the authors of draft-ietf-oauth-pop-key-
distribution, draft-ietf-oauth-pop-key-distribution, from
where we copied large parts of our the security
considerations. 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).

Ludwig Seitz and Goeran Selander worked on this document as part of
the CelticPlus project CyberWI, with funding from Vinnova.

10. References

10.1. Normative References

[I-D.ietf-ace-cbor-web-token]
Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", draft-ietf-ace-cbor-web-token-08
(work in progress), August 2017.

[I-D.jones-ace-cwt-proof-of-possession]
Jones, M., Seitz, L., Selander, G., Wahlstroem, E.,
Erdtman, S., and H. Tschofenig, "Proof-of-Possession Key
Semantics for CBOR Web Tokens (CWTs)", draft-jones-ace-
cwt-proof-of-possession-01 (work in progress), June 2017.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <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>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>. <https://www.rfc-editor.org/info/rfc6347>.

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

[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<http://www.rfc-editor.org/info/rfc7662>.

[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<http://www.rfc-editor.org/info/rfc7800>.
<https://www.rfc-editor.org/info/rfc7662>.

[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<http://www.rfc-editor.org/info/rfc8152>.
<https://www.rfc-editor.org/info/rfc8152>.

10.2. Informative References

[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-01 (work in progress), August 2017.

[I-D.ietf-ace-actors]
Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
architecture for authorization in constrained
environments", draft-ietf-ace-actors-05 (work in
progress), March 2017.

[I-D.ietf-ace-cbor-web-token]
Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", draft-ietf-ace-cbor-web-token-07
(work in progress), July 2017.

[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-05 (work in
progress), September 2017.

[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
object-security-04
resource-directory-11 (work in progress), July 2017.

[I-D.ietf-oauth-device-flow]
Denniss, W., Bradley, J., Jones, M., and H. Tschofenig,
"OAuth 2.0 Device Flow for Browserless and Input
Constrained Devices", draft-ietf-oauth-device-flow-06
(work in progress), May 2017.

[I-D.ietf-oauth-discovery]
Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", draft-ietf-oauth-
discovery-07 (work in progress), September 2017.

[I-D.ietf-oauth-native-apps]
Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
draft-ietf-oauth-native-apps-12 (work in progress), June
2017.

[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), 2010 August.

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

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>. <https://www.rfc-
editor.org/info/rfc5246>.

[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
<https://www.rfc-editor.org/info/rfc6690>.

[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.
<https://www.rfc-editor.org/info/rfc6749>.

[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,
<http://www.rfc-editor.org/info/rfc6819>. <https://www.rfc-
editor.org/info/rfc6819>.

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

[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>. <https://www.rfc-editor.org/info/rfc7159>.

[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>. <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,
<http://www.rfc-editor.org/info/rfc7231>. <https://www.rfc-
editor.org/info/rfc7231>.

[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<http://www.rfc-editor.org/info/rfc7519>.
<https://www.rfc-editor.org/info/rfc7519>.

[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, <http://www.rfc-editor.org/info/rfc7521>. <https://www.rfc-editor.org/info/rfc7521>.

[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,
<http://www.rfc-editor.org/info/rfc7591>.
<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,
<http://www.rfc-editor.org/info/rfc7641>. <https://www.rfc-
editor.org/info/rfc7641>.

[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,
<http://www.rfc-editor.org/info/rfc7744>. <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,
<http://www.rfc-editor.org/info/rfc7959>. <https://www.rfc-
editor.org/info/rfc7959>.

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 devices, the
highest energy cost is associated to transmitting or receiving
messages.
messages (roughly by a factor of 10 compared to e.g. 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, UDP and
CBOR encoded messages messages, some of these aspects are addressed.
Security protocols contribute to the communication overhead and
can
can, in some cases cases, be optimized. For example example, authentication and
key establishment may may, in certain cases where security
requirements
so allows 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 to perform, of performing, which
in turn impacts e.g. e.g., protocol latency. Symmetric key
cryptography may be used instead of the computationally more
expensive public key cryptography where the security requirements
so allows, 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
small amount of RAM and flash memory, which places limitations
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, symmetric
key cryptography instead of public key cryptography, and CBOR
instead of JSON. Authentication and key establishment protocol,
e.g.
e.g., the DTLS handshake, in comparison with assisted key
establishment also has an impact on memory and code.

User Interface Limitations:

Protecting access to resources is both an important security as
well as privacy feature. End users and enterprise customers do 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
scenarios, these functions need to be delegated to user controlled 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. 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.
While we envision deployments to make
Deployments making use of CoAP we explicitly
want to support are expected, but not limited to,
other protocols such as HTTP, HTTP/2 or other specific protocols,
such as Bluetooth Smart communication, which does that do not necessarily use IP.
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 REQUIRES 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
[I-D.ietf-ace-cbor-web-token]. These measures aim 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.

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.

RS Information:

This framework defines the name "RS Information" for data
concerning the RS that the AS returns to the client in an access
token response (see Section 5.6.2). This includes the "profile"
and the "rs_cnf" parameters. This aims at enabling scenarios,
where a powerful client, supporting multiple profiles, needs to
interact with a 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 [I-D.jones-ace-cwt-proof-of-possession]. A
semantically and syntactically identical request and response
parameter is 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.

Auth-Info endpoint:

This framework introduces a new way of providing access tokens to
a 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.

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 the (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.

Client Token:

In cases where the client is constrained and does not have
connectivity to the AS, and furthermore does not have a previous
security relation to the RS that it needs to communicate with,
this framework proposes the use of "client tokens". A client
token is a data object obtained from the AS by the RS, during
access token introspection. The RS passes the client token on to
the client. It contains information that allows the client to
perform the proof of possession for its access token and to
authenticate the RS (e.g. with it's public key).

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_types, 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 which 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, Party when issuing requests
(e.g.
(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 including authentication credentials.
* Allow Resource Owners to configure and update access control
policies related to their registered RS' RSs.
* Expose the /token token endpoint to allow clients to request tokens.
* Authenticate clients that wish to request a token.
* Process a token request against using the authorization policies
configured for the RS.

* Optionally: Expose the /introspection introspection endpoint that allows RS's
to submit token introspection requests.
* If providing an introspection endpoint: Authenticate RS's RSs that
wish to get an introspection response.
* If providing an introspection endpoint: Process token
introspection requests.
* Optionally: Handle token revocation.
* Optionally: Provide discovery metadta. See
[I-D.ietf-oauth-discovery]

Client

* Discover the AS in charge of the RS that is to be targeted with
a request.
* Submit the token request (A). (see step (A) of Figure 1).

+ Authenticate towards 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 key keys (rpk) or certificate is certificates are used, make sure
the AS has the right rpk or certificate for this client.
* Process the access token and RS Information (see step (B) of
Figure 1).

+ Check that the RS Information provides the necessary
security parameters (e.g. (e.g., PoP key, information on
communication security protocols supported by the RS).
* Send the token and request to the RS (see step (C) of
Figure 1).

+ 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
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) requirements of Figure 1) of the Requesting
Party, when issuing requests (e.g. minimum communication
security requirements, trust anchors).
* Register the client at the AS. RS.

Resource Server

* Expose a way to submit access tokens. By default this is the
/authz-info
authz-info endpoint.
* Process an access token.

+ Verify the token is from the right a recognized AS.
+ Verify that the token applies to this RS.

+ Check that the token has not expired (if the token provides
expiration information).
+ Check the token's integrity.
+ Store the token so that it can be retrieved in the context
of a matching request.
* 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.

Appendix C. Requirements on Profiles

This section lists the requirements on profiles of this framework,
for the convenience of a profile designer.

o Optionally Specify the discovery process of how the client finds
the right AS for an RS it wants to send a request to. Section 4 designers.

o Specify the communication protocol the client and RS the must use
(e.g.
(e.g., CoAP). Section 5 and Section 5.5.4.4 5.6.4.4
o Specify the security protocol the client and RS must use to
protect their communication (e.g. (e.g., OSCOAP or DTLS over CoAP).
This must provide encryption and encryption, integrity and replay protection.
Section 5.5.4.4 5.6.4.4
o Specify how the client and the RS mutually authenticate.
Section 4
o Specify the Content-format of the protocol messages (e.g. (e.g.,
"application/cbor" or "application/cose+cbor"). 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. (e.g.,
symmetric/asymmetric) are supported by a specific proof-of-
possession protocol. Section 5.5.4.3 5.6.4.3
o Specify a unique profile identifier. Section 5.5.4.4 5.6.4.4
o Optionally specify how the RS talks to If introspection is supported: Specify the AS communication and
security protocol for introspection.Section 5.6 5.7
o Optionally specify how the client talks to Specify the AS communication and security protocol for requesting a
token. interactions
between client and AS. Section 5.5 5.6
o Specify how/if the /authz-info authz-info endpoint is protected.
Section 5.7.1 5.8.1
o Optionally define other methods of token transport than the
/authz-info authz-
info endpoint. Section 5.7.1 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 a RS in order to be able to respond to requests to the
/token
token and /introspect 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. (e.g., pre-shared symmetric key, raw public key, key
length, other key parameters) that the client or RS supports.
o The types of access tokens the RS supports (e.g. (e.g., CWT).
o If the RS supports CWTs, the COSE parameters for the crypto
wrapper (e.g. (e.g., algorithm, key-wrap algorithm, key-length).
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 or RS and AS (if any).
o The raw public key of the client or RS (if any).

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 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 a RS is
performed. This requires the the security of the requests and responses
between the clients and the RS to consider.

Note: CBOR diagnostic notation is used for examples of requests and
responses.

E.1. Local Token Validation

In this scenario we consider scenario, the case where the resource server is
offline, i.e. 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 18. 17.

A: The client first generates a public-private key pair used for
communication security with the RS.
The client sends the POST request to /token the token endpoint at the AS.
The security of this request can be transport or application layer, it
layer. It is up the the communication security profile to define.
In the example transport layer identification of the AS is done
and the client identifies with client_id and client_secret as in
classic OAuth. 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 PoP access token and RS Information.
The PoP access token contains the public key of the client, and
the RS 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' i.e., "exp" close to 'iat', "iat", to protect the RS from
replay attacks. 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' "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 we assume 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.

Figure 18: 17: Token Request and Response Using Client Credentials.

The information contained in the Request-Payload and the Response-
Payload is shown in Figure 19. 18. Note that we assume a DTLS-based transport layer security
based communication security profile for is used in this example,
therefore the Content-Type is "application/cbor".

Request-Payload :
{
"grant_type" : "client_credentials",
"aud" : "tempSensorInLivingRoom",
"client_id" : "myclient",
"client_secret" : "qwerty"
}

Response-Payload :
{
"access_token" : b64'SlAV32hkKG ...',
"token_type" : "pop",
"csp" : "DTLS",
"cnf"
"rs_cnf" : {
"COSE_Key" : {
"kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4',
"y" : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM'
}
}
}

Figure 19: 18: Request and Response Payload Details.

The content of the access token is shown in Figure 20. 19.

{
"aud" : "tempSensorInLivingRoom",
"iat" : "1360189224",
"exp" : "1360289224",
"scope" : "temperature_g firmware_p",
"cnf" : {
"jwk"
"COSE_Key" : {
"kid" : b64'1Bg8vub9tLe1gHMzV76e8',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
"y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
}
}
}

Figure 20: 19: Access Token including Public Key of the Client.

Messages C and F are shown in Figure 21 20 - Figure 22. 21.

C: The client then sends the PoP access token to the /authz-info authz-info
endpoint at the RS. This is a plain CoAP request, i.e. i.e., no
transport or application layer security 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, is valid, and responds to the client. The RS caches
the security context together with authorization information about
this client contained in the PoP access token.

Figure 21: 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 the CoAP request GET to /temperature on RS over
DTLS. The RS verifies that the request is authorized, based on
previously established security context.
F: The RS responds with a resource representation over DTLS.

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 22: 21: Resource Request and Response protected by DTLS.

E.2. Introspection Aided Token Validation

In this deployment scenario we assume it is assumed that a client is not able
to access the AS at the time of the access request. Since request, whereas the RS is,
however, is
assumed to be connected to the back-end infrastructure it 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 may use
uses its online connectivity to validate learn about the claims assoicated to the
access token with the authorization server, by using introspection, which is shown in the example
below.

In the example interactions between an offline client (key fob), a RS
(online lock), and an AS is shown. We assume 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 23. 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
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 is are set quite wide to
start with and new values different form the original once can be
returned from introspection later on.

A: The client sends the request using POST to /token the token endpoint
at AS. The request contains the Audience parameter set to
"PACS1337" (PACS, Physical Access System), a value the that the
online door in question identifies itself with. The AS generates
an access token as on an opaque string, which it can match to the
specific client, a targeted audience and a symmetric key. The
security is provided by identifying the AS on transport layer
using a pre shared security context (psk, rpk or certificate) and
then the client is identified using client_id and client_secret as
in classic OAuth OAuth.
B: The AS responds with the an access token and RS Information,
the latter containing a symmetric key. Communication security
between C and RS will be DTLS and PreSharedKey. The PoP key being is
used as the PreSharedKey.

Figure 23: 22: Token Request and Response using Client Credentials.

The information contained in the Request-Payload and the Response-
Payload is shown in Figure 24. 23.

Request-Payload:
{
"grant_type" : "client_credentials",
"aud" : "lockOfDoor4711",
"client_id" : "keyfob",
"client_secret" : "qwerty"
}

Response-Payload:
{
"access_token" : b64'SlAV32hkKG ...'
"token_type" : "pop",
"csp" : "DTLS",
"cnf" : {
"COSE_Key" : {
"kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
"kty" : "oct",
"alg" : "HS256",
"k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE'
}
}
}

Figure 24: 23: Request and Response Payload for C offline

The access token in this case is just an opaque string referencing
the authorization information at the AS.

C: Next, the client POSTs the access token to the /authz-info authz-info
endpoint in the RS. This is a plain CoAP request, i.e. i.e., no DTLS
between client and RS. Since the token is an opaque string, the
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 forwards the token to the /introspect introspection endpoint on the
AS. Introspection assumes a secure connection between the AS and
the RS, e.g. e.g., using transport of application layer security. In
the example AS is identified using pre shared security context
(psk, rpk or certificate) while RS is acting as client and is
identified with client_id and client_secret.
E: The AS provides the introspection response 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.
After receiving message E, the RS responds to the client's POST in
step C with the CoAP response code 2.01 (Created).

Figure 25: 24: Token Introspection for C offline
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 26. 25.

Request-Payload:
{
"token" : b64'SlAV32hkKG...',
"client_id" : "FrontDoor",
"client_secret" : "ytrewq"
}

Response-Payload:
{
"active" : true,
"aud" : "lockOfDoor4711",
"scope" : "open, close",
"iat" : 1311280970,
"cnf" : {
"kid" : b64'JDLUhTMjU2IiwiY3R5Ijoi ...'
}
}

Figure 26: 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 RS the RS, changing the state of the
door to locked.
F: The RS responds with a appropriate over the secure DTLS
channel.

Figure 27: 26: Resource request and response protected by OSCOAP

Appendix F. Document Updates

F.1. Version -08 to -09

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.
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.

F.2. Version -07 to -08

o Removed specification of the "cnf" claim for CBOR/COSE, and
replaced with references to
[I-D.jones-ace-cwt-proof-of-possession]

F.3. Version -06 to -07

o Various clarifications added.
o Fixed erroneous author email.

F.2.

F.4. 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 5.1, 5.2, and Section 5.2. 5.3.
o Added Section 5.3 5.4 on AS authentication.
o Added Section 5.4 5.5 on the Authorize Authorization endpoint.

F.3.

F.5. Version -04 to -05

o Added RFC 2119 language to the specification of the required
behavior of profile specifications.
o Added Section 5.2 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 5.7.2 5.8.2
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.
o Added appendix D, describing assumptions about what the AS knows
about the client and the RS.

F.4.

F.6. 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' "cnf" parameter in section 6.4.5.
o Clarified intended use of Client Token in section 7.4.

F.5.

F.7. 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'
"profile" and 'alg' "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' "endpoint" instead
of 'resource' "resource" to mirror the OAuth 2.0 terminology.
o Updated the examples in the appendices.

F.6.

F.8. 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, /introspect token, introspection and /authz-info. 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' "cnf" parameter as defined in RFC7800 to reference
or transport keys used for proof of possession.
o Introduced the 'client-token' "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.

F.7.

F.9. Version -00 to -01

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
RISE SICS
Scheelevaegen 17
Lund 223 70
SWEDEN
Sweden

Email: ludwig.seitz@ri.se

Goeran Selander
Ericsson
Faroegatan 6
Kista 164 80
SWEDEN
Sweden

Email: goran.selander@ericsson.com

Erik Wahlstroem
(no affiliation)
Sweden

Email: erik@wahlstromtekniska.se
Samuel Erdtman
Spotify AB
Birger Jarlsgatan 61, 4tr
Stockholm 113 56
Sweden

Email: erdtman@spotify.com

Hannes Tschofenig
ARM Ltd.
Hall in Tirol 6060
Austria

Email: Hannes.Tschofenig@arm.com