Internet DRAFT - draft-seitz-ace-oauth-authz
draft-seitz-ace-oauth-authz
ACE Working Group L. Seitz
Internet-Draft SICS
Intended status: Standards Track G. Selander
Expires: April 21, 2016 Ericsson
E. Wahlstroem
S. Erdtman
Nexus Technology
H. Tschofenig
ARM Ltd.
October 19, 2015
Authorization for the Internet of Things using OAuth 2.0
draft-seitz-ace-oauth-authz-00
Abstract
This memo defines how to use OAuth 2.0 as an authorization framework
with Internet of Things (IoT) deployments, thus bringing a well-known
and widely used security solution to IoT devices. Where possible
vanilla OAuth 2.0 is used, but where the limitations of IoT devices
require it, profiles and extensions are provided.
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 April 21, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. OAuth 2.0 . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. CoAP . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Object Security . . . . . . . . . . . . . . . . . . . . . 8
4. Protocol Interactions . . . . . . . . . . . . . . . . . . . . 9
5. OAuth 2.0 Profiling . . . . . . . . . . . . . . . . . . . . . 11
5.1. Communication Security Protocol . . . . . . . . . . . . . 12
5.2. Authorization Information Resource at the Resource Server 12
5.3. Authorization Information Format . . . . . . . . . . . . 13
5.4. CBOR Data Formats . . . . . . . . . . . . . . . . . . . . 13
5.5. CBOR Web Token . . . . . . . . . . . . . . . . . . . . . 13
6. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . 13
6.1. Client and Resource Server are Offline . . . . . . . . . 14
6.2. Resource Server Offline . . . . . . . . . . . . . . . . . 17
6.3. Token Introspection with an Offline Client . . . . . . . 21
6.4. Always-On Connectivity . . . . . . . . . . . . . . . . . 25
6.5. Token-less Authorization . . . . . . . . . . . . . . . . 25
6.6. Securing Group Communication . . . . . . . . . . . . . . 28
7. Security Considerations . . . . . . . . . . . . . . . . . . . 29
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Normative References . . . . . . . . . . . . . . . . . . 30
10.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. Design Justification . . . . . . . . . . . . . . . . 32
Appendix B. Optimizations . . . . . . . . . . . . . . . . . . . 34
Appendix C. CoAP and CBOR profiles for OAuth 2.0 . . . . . . . . 35
C.1. Profile for Token resource . . . . . . . . . . . . . . . 35
C.1.1. Token Request . . . . . . . . . . . . . . . . . . . . 35
C.1.2. Token Response . . . . . . . . . . . . . . . . . . . 37
C.2. CoAP Profile for OAuth Introspection . . . . . . . . . . 38
C.2.1. Introspection Request . . . . . . . . . . . . . . . . 38
C.2.2. Introspection Response . . . . . . . . . . . . . . . 39
Appendix D. CBOR Web Token (CWT) . . . . . . . . . . . . . . . . 41
D.1. Claim Names . . . . . . . . . . . . . . . . . . . . . . . 41
D.1.1. iss (Issuer) Claim . . . . . . . . . . . . . . . . . 41
D.1.2. sub (Subject) Claim . . . . . . . . . . . . . . . . . 42
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D.1.3. aud (Audience) Claim . . . . . . . . . . . . . . . . 42
D.1.4. exp (Expiration Time) Claim . . . . . . . . . . . . . 42
D.1.5. nbf (Not Before) Claim . . . . . . . . . . . . . . . 42
D.1.6. iat (Issued At) Claim . . . . . . . . . . . . . . . . 43
D.1.7. cti (CWT ID) Claim . . . . . . . . . . . . . . . . . 43
D.1.8. cnf (Confirmation) Claim . . . . . . . . . . . . . . 43
D.1.9. cks (COSE Key Structure) Claim . . . . . . . . . . . 43
D.1.10. aif (Authorization Information Format) Claim . . . . 43
D.2. CBOR major types for Claims . . . . . . . . . . . . . . . 43
D.3. CBOR Web Token Example . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
Authorization is the process of deciding what an entity ought to be
allowed to do. Managing authorization information for a large number
of devices and users is often a complex task where dedicated servers
are used.
Managing authorization of users, services and their devices with the
help of dedicated authorization servers (AS) is a common task, found
in enterprise networks as well as on the Web. In its simplest form
the authorization task can be described as granting access to a
resource hosted on a device, the resource server (RS). This exchange
is mediated by one or multiple authorization servers.
We envision that end consumers and enterprises will want to manage
their Internet of Things (IoT) devices in the same style and this
desire will increase with the number of devices that need to be
managed and controlled. The IoT devices may be constrained in
various ways including processing, memory, code, energy, etc., as
defined 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-connected devices which are not constrained in
terms of power, and all levels in between. Thus IoT devices are very
different in terms of available processing and message exchange
capabilities.
This memo describes how to re-use OAuth 2.0 [RFC6749] to extend
authorization to Internet of Things devices with different kinds of
constrainedness. At the time of writing OAuth 2.0 is already used
with certain types of IoT devices and this document will provide
implementers additional guidance for using it in a secure and
privacy-friendly way. Where possible the basic OAuth 2.0 mechanisms
are used; in some circumstances profiles are defined, for example to
support lower the over-the-wire message size and smaller code size.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD 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 as RESTful protocol 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). OAuth 2.0 uses the term
"endpoint" to denote HTTP resources such as /token and /authorize at
the AS, but we will use the term "resource" in this memo to avoid
confusion with the CoAP [RFC7252] term "endpoint".
Since this draft focuses on the problem of access control to
resources, we simplify the actors 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]).
3. Overview
This specification describes a framework for authorization in the
Internet of Things consisting of a set of building blocks.
The basic block is the OAuth 2.0 [RFC6749] framework, which enjoys
widespread deployment. Many IoT devices can support OAuth 2.0
without any additional extensions, but for certain constrained
settings additional profiling is needed.
Another building block is the lightweight web transfer protocol CoAP
[RFC7252] for those communication environments where HTTP is not
appropriate. CoAP typically runs on top of UDP which further reduces
overhead and message exchanges. When CoAP is used over UDP,
transport layer security is provided by DTLS 1.2 [RFC6347] instead of
TLS 1.2 [RFC5246].
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A third building block is CBOR [RFC7049] for encodings where JSON
[RFC7159] is not sufficiently compact. CBOR is a binary encoding
designed for extremely small code size and fairly small message size.
OAuth 2.0 allows access tokens to use different encodings and this
document defines such an alternative encoding. The COSE message
format [I-D.ietf-cose-msg] is also based on CBOR.
A fourth building block is application layer security, which is used
where transport layer security is insufficient. At the time of
writing the preferred approach for securing CoAP at the application
layer is via the use of COSE [I-D.ietf-cose-msg], which adds object
security to CBOR-encoded data. More details about applying COSE to
CoAP can be found in OSCOAP [I-D.selander-ace-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
described framework does, however, takes all these aspects into
account and allows several different deployment variants to co-exist
rather than mandating a one-size-fits-all solution. We believe this
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 form of 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 access to a resource with the permission of a resource owner.
Authorization related information 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 memo:
Access Tokens:
Access tokens are credentials used to access protected resources.
An access token is a data structure representing authorization
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permissions issued to the client. Access tokens are generated by
the authorization server and consumed by the resource server. The
access token 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 tokens (or PoP tokens)
[I-D.ietf-oauth-pop-architecture].
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 included in
the access token. When this memo uses the term "access token" it
is assumed to be a PoP token unless specifically stated otherwise.
The key bound to the access token (aka PoP key) may be based on
symmetric as well as on asymmetrical cryptography. The
appropriate choice of security 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, encrypts it for
the RS and includes it inside an 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 then
included inside the access token and sent back to the
requesting client.
The access token is protected against modifications using a MAC or
a digital signature of the AS. The choice of PoP key does not
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necessarily imply a specific credential type for the integrity
protection of the token. More information about PoP tokens can be
found in [I-D.ietf-oauth-pop-architecture].
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 request.
In turn, the AS may use the "scope" response parameter to inform
the client of the scope of the access token issued. This memo
uses CBOR encoded messages defined in Appendix C to request scopes
and to be informed what scopes the access token was actually
authorized for by the AS.
The values of the scope parameter are expressed as a list of
space- delimited, case-sensitive strings, with a semantic that is
well-known to the AS and the RS. More details about the concept
of scopes is found under Section 3.3 in [RFC6749].
Claims:
The information carried in the access token in the form of type-
value pairs is called claims. An access token may for example
include a claim about the AS that issued the token (the "iss"
claim) and what audience the access token is intended for (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 the 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 Appendix D we define a CBOR version of JWT
that we call CBOR Web Token (CWT).
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 reference rather than a
self-contained token. More information about introspection in
OAuth 2.0 can be found in [I-D.ietf-oauth-introspection].
3.2. CoAP
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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 HTTP uses headers and query-strings to convey additional
information about a request, CoAP encodes such information in so-
called 'options'.
CoAP supports application-layer fragmentation of the CoAP payloads
through blockwise transfers [I-D.ietf-core-block]. However, this
method does not allow the fragmentation of large CoAP options,
therefore data encoded in options has to be kept small.
3.3. Object Security
Transport layer security is not always sufficient and application
layer security has to be provided. COSE [I-D.ietf-cose-msg] defines
a message format for cryptographic protection of data using CBOR
encoding. There are two main approaches for application layer
security:
Object Security of CoAP (OSCOAP)
OSCOAP [I-D.selander-ace-object-security] is a method for
protecting CoAP request/response message exchanges, including CoAP
payloads, CoAP header fields as well as CoAP options. OSCOAP
provides end-to-end confidentiality, integrity and replay
protection, and a secure binding between CoAP request and response
messages.
A CoAP message protected with OSCOAP contains the CoAP option
"Object-Security" which signals that the CoAP message carries a
COSE message ([I-D.ietf-cose-msg]). OSCOAP defines a profile of
COSE which includes replay protection.
Object Security of Content (OSCON)
For the case of wrapping of application layer payload data
("content") only, such as resource representations or claims of
access tokens, the same COSE profile can be applied to obtain end-
to-end confidentiality, integrity and replay protection.
[I-D.selander-ace-object-security] defines this functionality as
Object Security of Content (OSCON).
In this case, the message is not bound to the underlying
application layer protocol and can therefore be used with HTTP,
CoAP, Bluetooth Smart, etc. Whereas OSCOAP integrity protects
specific CoAP message meta-data like request/response code, and
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binds a response to a specific request, since OSCON protects only
payload/content, those security features are lost. The advantages
are that an OSCON message can be passed across different
protocols, from request to response, and used to secure group
communications.
4. Protocol Interactions
This framework is based on the same protocol interactions as OAuth
2.0: A client obtains an access token from an AS and presents the
token to an RS to gain access to a protected resource. These
interactions are shown in Figure 1. An overview of various OAuth
concepts is provided in Section 3.1.
The consent of the resource owner, for giving a client access to a
protected resource, can be pre-configured authorization policies or
dynamically at the time when the request is sent. The resource owner
and the requesting party (= client owner) are not shown in Figure 1.
For the description in this document we assume that the client has
been registered to an AS. Registration means that the two share
credentials, configuration parameters and that some form of
authorization has taken place. These credentials are used to protect
the token request by the client and the transport of access tokens
and client information from AS to the client.
It is also assumed that the RS 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
the content cannot be modified, and if needed, that the content is
confidentiality protected.
The keying material necessary for establishing communication security
between C and RS is dynamically established as part of the protocol
described in this document.
At the start of the protocol there is an optional discovery step
where the client discovers the resource server and the resources this
server hosts. In this step the client might also determine what
permissions are needed to access the protected resource. The exact
procedure depends on the protocols being used and the specific
deployment environment. In Bluetooth Smart, for example,
advertisements are broadcasted by a peripheral, including information
about the supported services. In CoAP, as a second example, a client
can makes a request to "/.well-known/core" to obtain information
about available resources, which are returned in a standardized
format as described in [RFC6690].
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+--------+ +---------------+
| |---(A)-- Token Request ------------->| |
| | | Authorization |
| |<--(B)-- Access Token ---------------| Server |
| | + Client Information | |
| | +---------------+
| | ^ |
| | Introspection Request & Response (D)| |(E)
| Client | | v
| | +--------------+
| |---(C)-- Token + Request ----------->| |
| | | Resource |
| |<--(F)-- Protected Resource ---------| Server |
| | | |
+--------+ +--------------+
Figure 1: Overview of the basic protocol flow
Requesting an Access Token (A):
The client makes an access token request to the AS. This memo
assumes the use of PoP 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 type of credentials it wants to use (i.e., symmetric or
asymmetric cryptography).
Access Token Response (B):
If the AS successfully processes the request from the client, it
returns an access token. It also includes various parameters,
which we call "Client Information". In addition to the response
parameters defined by OAuth 2.0 and the PoP token extension, we
consider new kinds of response parameters in Section 5, including
information on which security protocol the client should use with
the resource server(s) that it has just been authorized to access.
Communication security between client and RS may be based on pre-
provisioned keys/security contexts or dynamically established to
the RS via the PoP token; and to the client via the client
information as described in Section 5.1.
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, Bluetooth Smart etc., are also possible candidates.
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Depending on the device limitations and the selected protocol this
exchange may be split up into two phases:
(1) the client sends the access token to a newly defined
authorization endpoint at the RS (see Section 5.2) , which
conveys authorization information to the RS that may be used
for subsequent resource requests, and
(2) the client makes the resource access request, using the
communication security protocol and other client information
obtained from the AS.
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 use token introspection to
interact with the AS to obtain the most recent claims, such as
scope, audience, validity etc. associated with a specific access
token. Token introspection over CoAP is defined in
[I-D.wahlstroem-ace-oauth-introspection] and for HTTP in
[I-D.ietf-oauth-introspection].
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 claims associated with
it back to the RS. The RS then uses the received claims to
process the request to either accept or to deny it.
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. OAuth 2.0 Profiling
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This section describes profiles of OAuth 2.0 adjusting it to
constrained environments for use cases where this is necessary.
5.1. Communication Security Protocol
OAuth 2.0 using bearer tokens, as described in RFC 6749 and in RFC
6750, requires TLS for all communication interactions between client,
authorization server, and resource server. This is possible in the
scope where OAuth 2.0 was originally developed, web and mobile
applications. In these environments resources like computational
power and bandwidth are not scarce and operating systems as well as
browser platforms are pre-provisioned with trust anchors that enable
clients to authenticate servers based on the Web PKI. In a more
heterogeneous IoT environment a wider range of use cases needs to be
supported. Therefore, this document suggests extensions to OAuth 2.0
that enable the AS to inform the client on how to communicate
securely with a RS.
The client and the RS might not have any prior knowledge about each
other, therefore the AS needs to help them to establish a security
context or at least a key. The AS does this by indicating
communication security protocol ("csp") and additional key parameters
in the client information.
The "csp" parameter specifies how client and RS communication is
going to be secured based on returned keys. Currently defined values
are "TLS", "DTLS", "OSCOAP" and "OSCON". Depending on the value
different additional parameters become mandatory.
TLS with certificates may make use of pre-established trust anchors
or configured more tightly with additional client information
parameters, like x5c, x5t or x5t#S256.
CoAP specifies three security "modes" of DTLS: PreSharedKey,
RawPublicKey and Certificate. In case of PreSharedKey and
RawPublicKey DTLS is based on the use keys distributed in the PoP
token and via the client information. Additional certificate
information may also be added, for example using the parameter x5c,
x5t or x5t#S256.
To use OSCOAP and OSCON requires security context to be established,
which can be provisioned with PoP token and client information, or
derived from keys provisioned in this way.
5.2. Authorization Information Resource at the Resource Server
A consequence of allowing the use of CoAP as web transfer protocol is
that we cannot rely on HTTP specific mechanisms, such as transferring
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information elements in HTTP headers since those are not necessarily
gracefully mapped to CoAP. In case the access token is larger then
255 bytes it should not be sent as a CoAP option.
For conveying authorization information to the RS we therefore
introduce a new resource to which the PoP tokens can be sent to
convey authorization information before the first resource request is
made by the client. This specification calls this resource "/authz-
info"; the URI may, however, vary in deployments.
5.3. Authorization Information Format
We introduce a new claim for describing access rights with a specific
format, the "aif" claim. In this memo we propose to use the compact
format provided by AIF [I-D.bormann-core-ace-aif]. Access rights may
be specified as a list of URIs of resources together with allowed
actions (GET, POST, PUT, PATCH, or DELETE).
5.4. CBOR Data Formats
The /token resource (called "endpoint" in OAuth 2.0), defined in
Section 3.2 of [RFC6749], is used by the client to obtain an access
token. Requests sent to the /token resource use the HTTP POST method
and the payload includes a query component, which is formatted as
application/x-www-form-urlencoded. CoAP payloads cannot be formatted
in the same way which requires the /token resource on the AS to be
profiled. Appendix C defines a CBOR-based format for sending
parameters to the /token resource.
5.5. CBOR Web Token
CBOR Web Tokens (CWT) are defined in Appendix D as compact analogs of
JSON Web Tokens (JWT) [RFC7519]. CWTs uses COSE [I-D.ietf-cose-msg]
to offer similar, but more compact security services. CWT supports
PoP token functionality.
6. Deployment Scenarios
There is a large variety of IoT deployments, as is indicated in
Appendix A, and this section highlights common variants. This
section is not normative but illustrates how the framework can be
applied.
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For each of the deployment variants there are a number of possible
security setups between clients, resource servers and authorization
servers. The main focus in the following subsections is on how
authorization of a client request for a resource hosted by a RS is
performed. This requires us to also consider how these requests and
responses between the clients and the resource servers are secured.
The security protocols between other pairs of nodes in the
architecture, namely client-to-AS and RS-to-AS, are not detailed in
these examples. Different security protocols may be used on
transport or application layer.
Note: We use the CBOR diagnostic notation for examples of requests
and responses.
6.1. Client and Resource Server are Offline
In this scenario we consider the case where both the resource server
and the client are offline, i.e., they are not connected to the AS at
the time of the resource request. This access procedure involves
steps A, B, C, and F of Figure 1, but assumes that step A and B have
been carried out during a phase when the client had connectivity to
AS.
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 2.
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 at AS. The request
contains the public key of the client and the Audience parameter
set to "tempSensorInLivingRoom", a value the 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 token and client information. The
PoP token contains the public key of the client, while the client
information contains the public key of the RS. For communication
security this example uses DTLS with raw public keys between the
client and the RS.
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Note: In this example we assume that the client knows what
resource it wants to access, and is therefore able to request
specific audience and scope claims for the access token.
Authorization
Client Server
| |
| |
A: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"token"
| | Payload: <Request-Payload>
| |
B: |<--------+ Header: 2.05 Content
| | Content-Type: application/cbor
| 2.05 | Payload: <Response-Payload>
| |
Figure 2: Token Request and Response Using Client Credentials.
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 3.
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",
"key" : b64'eyJhbGciOiJSU0ExXzUi ...'
}
Figure 3: Request and Response Payload Details.
The content of the "key" parameter and the access token are shown in
Figure 4 and Figure 5.
{
"kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4',
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"y" : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM'
}
Figure 4: Public Key of the RS.
{
"aud" : "tempSensorInLivingRoom",
"iat" : "1360189224",
"cnf" : {
"jwk" : {
"kid" : b64'1Bg8vub9tLe1gHMzV76e8',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
"y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
}
}
}
Figure 5: Access Token including Public Key of the Client.
Messages C and F are shown in Figure 6 - Figure 7.
C: The client then sends the PoP token to the /authz-info resource
at the RS. This is a plain CoAP request, i.e. no DTLS/OSCOAP
between client and RS, since the token is integrity protected
between AS and RS. The RS verifies that the PoP 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 token.
The client and resource server run 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.
F: The RS responds with a resource representation over DTLS.
Resource
Client Server
| |
C: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"authz-info"
| | Payload: SlAV32hkKG ...
| | (access token)
| |
|<--------+ Header: 2.04 Changed
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| 2.04 |
| |
Figure 6: Access Token provisioning to RS
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: {"t":"22.7"}
| |
Figure 7: Resource Request and Response protected by DTLS.
6.2. Resource Server Offline
In this deployment scenario we consider the case of an RS that may
not be able to access the AS at the time it receives an access
request from a client. We denote this case "RS offline", it involves
steps A, B, C and F of Figure 1.
If the RS is offline, then it must be possible for the RS to locally
validate the access token. This requires self-contained tokens to be
used.
The validity time for the token should always be chosen as short as
possible to reduce the possibility that a token contains out-of-date
authorization information. Therefore the value for the Expiration
Time claim ("exp") should be set only slightly larger than the value
for the Issuing Time claim ("iss"). A constrained RS with means to
reliably measure time must validate the expiration time of the access
token.
The following example shows interactions between a client (AC control
unit), an offline resource server (temperature sensor) and an
authorization server. The message exchanges A and B are shown in
Figure 8.
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A: The client sends the request POST to /token at AS. The request
contains the Audience parameter set to "tempSensor109797", a value
that the temperature sensor identifies itself with. The scope the
client want's the AS to authorize the access token for is "owner",
which means that the token can be used to both read temperature
data and upgrade the firmware on the RS. The AS evaluates the
request and authorizes the client to access the resource.
B: The AS responds with a PoP token and client information. The
PoP token is wrapped in a COSE message, object secured content
from AS to RS. The client information contains a symmetric key.
In this case communication security between C and RS is OSCOAP
with an authenticated encryption algorithm. The client derives
two unidirectional security contexts to use with the resource
request and response messages. The access token includes the
claim "aif" with the authorized access that an owner of the
temperature device can enjoy. The "aif" claim, issued by the AS,
informs the RS that the owner of the access token, that can prove
the possession of a key is authorized to make a GET request
against the /tempC resource and a POST request on the /firmware
resource.
Authorization
Client Server
| |
| |
A: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path: "token"
| | Payload: <Request-Payload>
| |
B: |<--------+ Header: 2.05 Content
| | Content-Type: application/cbor
| 2.05 | Payload: <Response-Payload>
| |
| |
Figure 8: Token Request and Response
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 9.
Request-Payload:
{
"grant_type" : "client_credentials",
"client_id" : "myclient",
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"client_secret" : "qwerty",
"aud" : "tempSensor109797",
"scope" : "owner"
}
Response-Payload:
{
"access_token": b64'SlAV32hkKG ...',
"token_type" : "pop",
"csp" : "OSCOAP",
"key" : b64'eyJhbGciOiJSU0ExXzUi ...'
}
Figure 9: Request and Response Payload for RS offline
Figure 10 shows examples of the key and the access_token parameters
of the Response-Payload, decoded to CBOR.
access_token:
{
"aud" : "tempSensor109797",
"exp" : 1311281970,
"iat" : 1311280970,
"aif" : [["/tempC", 0], ["/firmware", 2]],
"cnf" : {
"ck":b64'JDLUhTMjU2IiwiY3R5Ijoi ...'
}
}
key:
{
"alg" : "AES_128_CCM_8",
"kid" : b64'U29tZSBLZXkgSWQ',
"k" : b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE'
}
Figure 10: Access Token and symmetric key from the Response-Payload
Message exchanges C and F are shown in Figure 11 and Figure 12.
C: The client then sends the PoP token to the /authz-info resource
in the RS. This is a plain CoAP request, i.e. no DTLS/OSCOAP
between client and RS, since the token is integrity protected
between AS and RS. The RS verifies that the PoP token was created
by a known and trusted AS, is valid, and responds to the client.
The RS derives and caches the security contexts together with
authorization information about this client contained in the PoP
token.
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The client sends the CoAP requests GET to /tempC on the RS using
OSCOAP. The RS verifies the request and that it is authorized.
F: The RS responds with a protected status code using OSCOAP. The
client verifies the response.
Resource
Client Server
| |
C: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"authz-info"
| | Payload: <Access Token>
| |
| |
|<--------+ Header: 2.04 Changed
| 2.04 |
| |
| |
Figure 11: Access Token provisioning to RS
Resource
Client Server
| |
+-------->| Header: GET (Code=0.01)
| GET | Object-Security:
| | (<seq>,<cid>,[Uri-Path:"tempC"],<tag>)
| |
F: |<--------+ Header: 2.05 Content
| 2.05 | Object-Security:
| | (<seq>,<cid>,[22.7 C],<tag>)
| |
Figure 12: Resource request and response protected by OSCOAP
In Figure 12 the GET request contains an Object-Security option and
an indication of the content of the COSE object: a sequence number
("seq", starting from 0), a context identifier ("cid") indicating the
security context, the ciphertext containing the encrypted CoAP option
identifying the resource, and the Message Authentication Code ("tag")
which also covers the Code in the CoAP header.
The Object-Security ciphertext in the response [22.7 C] represents an
encrypted temperature reading. (The COSE object is actually carried
in the CoAP payload when possible but that is omitted to simplify
notation.)
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6.3. Token Introspection with an Offline Client
In this deployment scenario we assume that a client is not be able to
access the AS at the time of the access request. Since the RS is,
however, connected to the back-end infrastructure it 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.
The resource server may use its online connectivity to validate the
access token with the authorization server, which is shown in the
example below.
In the example we show the interactions between an offline client
(key fob), a resource server (online lock), and an authorization
server. We assume 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 13.
A: The client sends the request using POST to /token at AS. The
request contains the Audience parameter set to "lockOfDoor4711", a
value the that the online door in question identifies itself with.
The AS generates an access token as on opaque string, which it can
match to the specific client, a targeted audience and a symmetric
key security context.
B: The AS responds with the an access token and client
information, the latter containing a symmetric key. Communication
security between C and RS will be OSCOAP with authenticated
encryption.
Authorization
Client Server
| |
| |
A: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"token"
| | Payload: <Request-Payload>
| |
B: |<--------+ Header: 2.05 Content
| | Content-Type: application/cbor
| 2.05 | Payload: <Response-Payload>
| |
Figure 13: Token Request and Response using Client Credentials.
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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 is used to authorize the key for his car at
the the car manufacturers AS.
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 14.
Request-Payload:
{
"grant_type" : "token",
"aud" : "lockOfDoor4711",
"client_id" : "myclient",
}
Response-Payload:
{
"access_token" : b64'SlAV32hkKG ...'
"token_type" : "pop",
"csp" : "OSCOAP",
"key" : b64'eyJhbGciOiJSU0ExXzUi ...'
}
Figure 14: 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
resource in the RS. This is a plain CoAP request, i.e. no DTLS/
OSCOAP 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 step E and only
acknowledges on the CoAP message layer (indicated with a dashed
line).
Resource
Client Server
| |
C: +-------->| Header: POST (T=CON, Code=0.02
| POST | Token 0x2a12)
| | Uri-Path:"authz-info"
| | Payload: SlAV32hkKG ...
| | (access token)
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| |
|<- - - - + Header: T=ACK
| |
Figure 15: Access Token provisioning to RS
D: The RS forwards the token to the /introspect resource on the
AS. Introspection assumes a secure connection between the AS and
the RS, e.g. using DTLS or OSCOAP, which is not detailed in this
example.
E: The AS provides the introspection response containing claims
about the token. This includes the confirmation key (cnf) claim
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 Code 2.04 (Changed), using CoAP Token 0x2a12. This
step is not shown in the figures.
Resource Authorization
Server Server
| |
D: +--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "introspect"
| | Payload: <Request-Payload>
| |
E: |<---------+ Header: 2.05 Content
| 2.05 | Content-Type: application/cbor)
| | Payload: <Response-Payload>
| |
Figure 16: Token Introspection for C offline
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 17.
Request-Payload:
{
"token" : b64'SlAV32hkKG...',
"client_id" : "myRS",
"client_secret" : "ytrewq"
}
Response-Payload:
{
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"active" : true,
"aud" : "lockOfDoor4711",
"scope" : "open, close",
"iat" : 1311280970,
"cnf" : {
"ck" : b64'JDLUhTMjU2IiwiY3R5Ijoi ...'
}
}
Figure 17: Request and Response Payload for Introspection
The client sends the CoAP requests PUT 1 (= "close the lock") to /
lock on RS using OSCOAP with a security context derived from the
key supplied in step B. The RS verifies the request with the key
supplied in step E and that it is authorized by the token supplied
in step C.
F: The RS responds with a protected status code using OSCOAP. The
client verifies the response.
Resource
Client Server
| |
+-------->| Header: PUT (Code=0.03)
| PUT | Object-Security:
| | (<seq>,<cid>,[Uri-Path:"lock", 1],<tag>)
| |
F: |<--------+ Header: 2.04 Changed
| 2.04 | Object-Security:
| | (<seq>,<cid>,,<tag>)
| |
Figure 18: Resource request and response protected by OSCOAP
The Object-Security ciphertext [...] of the PUT request contains CoAP
options that are encrypted, as well as the payload value '1' which is
the value of PUT to the door lock.
In this example there is no ciphertext of the PUT response, but "tag"
contains a MAC which covers the request sequence number and context
identifier as well as the Code which allows the Client to verify that
this actuator command was well received (door is locked).
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6.4. Always-On Connectivity
A popular deployment scenario for IoT devices is to have them always
be connected to the Internet so that they can be reachable to receive
commands. As a continuation from the previous scenarios we assume
that both the client and the RS are online at the time of the access
request.
If the client and the resource server are online then the AS should
be configured to issue short-lived access tokens for the resource to
the client. The resource server must then validate self-contained
access tokens or otherwise must use token introspection to obtain the
up-to-date claim information. If transmission costs are high or the
channel is lossy, the CWT token format may be used instead of a JWT
to reduce the volume of network traffic. In terms of messaging this
deployment scenario uses the patterns described in the previous sub-
sections.
Note that despite the lack of connectivity constraints there may
still be other restrictions a deployment may face.
6.5. Token-less Authorization
In this deployment scenario we consider the case of an RS which is
severely energy constrained, sleeps most of the time and need to have
a tight messaging budget. It is not only infeasible to access the AS
at the time of the access request, as in the "RS offline" case
Section 6.2, it must be offloaded as much message communication as
possible.
OAuth 2.0 is already an efficient protocol in terms of message
exchanges and can be further optimized by compact encodings of
tokens. The scenario illustrated in this section goes beyond that
and removes the access tokens from the protocol. This may be
considered a degenerate case of OAuth 2.0 but it allows us to do two
things:
1. The common case where authorization is performed by means of
authentication fits into the same protocol framework.
Authentication protocol and key is specified by client
information, and access token is omitted.
2. Authentication, and thereby authorization, may even be implicit,
i.e. anyone with access to the right key is authorized to access
the protected resource.
In case 2., the RS does not need to receive any message from the
client, and therefore enables offloading recurring resource request
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and response processing to a third party, such as a Message Broker
(MB) in a publish-subscribe setting.
This scenario involves steps A, B, C and F of Figure 1 and four
parties: a client (subscriber), an offline RS (publisher), a trusted
AS, and a MB, not necessarily trusted with access to the plain text
publications. Message exchange A, B is shown in Figure 19.
A: The client sends the request POST to /token at AS. The request
contains the Audience parameter set to "birchPollenSensor301", a
value that characterizes a certain pollen sensor resource. The AS
evaluates the request and authorizes the client to access the
resource.
B: The AS responds with an empty token and client information with
a security context to be used by the client. The empty token
signifies that authorization is performed by means of
authentication using the communication security protocol indicated
with "csp". In this case it is object security of content (OSCON)
i.e. protection of CoAP payload only. The security context
contains the symmetric decryption key and a public signature
verification key of the RS.
Authorization
Client Server
| |
| |
A: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"token"
| | Payload: <Request-Payload>
| |
B: |<--------+ Header: 2.05 Content
| | Content-Type: application/cbor
| 2.05 | Payload: <Response-Payload>
| |
| |
Figure 19: Token Request and Response
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 20.
Request-Payload :
{
"grant_type" : "client_credentials",
"aud" : "birchPollenSensor301",
"client_id" : "myclient",
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"client_secret" : "qwerty"
}
Response-Payload :
{
"access_token" : NULL,
"token_type" : "none",
"csp" : "OSCON",
"key" : b64'eyJhbGciOiJSU0ExXzUi ...'
}
Figure 20: Request and Response Payload for RS severely constrained
The content of the "key" parameter is shown in Figure 21.
key :
{
"alg" : "AES_128_CTR_ECDSA",
"kid" : b64'c29tZSBvdGhlciBrZXkgaWQ';
"k" : b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE',
"crv" : "P-256",
"x" : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4',
"y" : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM'
}
Figure 21: The 'key' Parameter
The RS, which sleeps most of the time, occasionally wakes up,
measures the number birch pollens per cubic meters, publishes the
measurements to the MB, and then returns to sleep. See Figure 22.
In this case the birch pollen count stopped at 270, which is
encrypted with the symmetric key and signed with the private key of
the RS. The MB verifies that the message originates from RS using
the public key of RS, that it is not a replay of an old measurement
using the sequence number of the OSCON COSE profile, and caches the
object secured content. The MB does not have the secret key so is
unable to read the plain text measurement.
Message exchanges C and F are shown in Figure 22.
C: Since there is no access token, the client does not address the
/authz-info resource in the RS. The client sends the CoAP request
GET to /birchPollen on MB which is a plain CoAP request.
F: The MB responds with the cached object secured content.
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Message Resource
Client Broker Server
| | |
| |<--------| Header: PUT (Code=0.02)
| | PUT | Uri-Path: "birchPollen"
| | | Payload: (<seq>,<cid>,["270"],<tag>)
| | |
| |-------->| Header: 2.04 Changed
| | 2.04 |
| |
| |
C: +-------->| Header: GET (Code=0.01)
| GET | Uri-Path: "birchPollen"
| |
| |
F: |<--------+ Header: 2.05 Content
| 2.05 | Payload: (<seq>,<cid>,["270"],<tag>)
| |
Figure 22: Sensor measurement protected by COSE
The payload is a COSE message consisting of sequence number 'seq'
stepped by the RS for each publication, the context identifier 'cid'
in this case coinciding with the key identifier 'kid' of Figure 21,
the encrypted measurement and the signature by the RS.
Note that the same COSE message format may be used as in OSCOAP but
that only CoAP payload is protected in this case.
The authorization step is implicit, so while any client could request
access the COSE object, only authorized clients have access to the
symmetric key needed to decrypt the content.
Note that in this case the order of the message exchanges A,B and C,F
could in principle be interchanged, i.e. the client could first
request and obtain the protected resource in steps C,F; and after
that request client information containing the keys decrypt and
verify the message.
6.6. Securing Group Communication
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There are use cases that require securing communication between a
(group of) senders and a group of receivers. One prominent example
is lighting. Often, a set of lighting nodes (e.g., luminaires, wall-
switches, sensors) are grouped together and only authorized members
of the group must be able read and process messages. Additionally,
receivers of group messages must be able to verify the integrity of
received messages as being generated within the group.
The requirements for securely communicating in such group use cases
efficiently is outlined in [I-D.somaraju-ace-multicast] along with an
architectural description that aligns with the content of this
document. The requirements for conveying the necessary identifiers
to reference groups and also the process of commissioning devices can
be accomplished using the protocol described in this document. For
details about the lighting-unique use case aspects, the architecture,
as well as other multicast-specific considerations we refer the
reader to [I-D.somaraju-ace-multicast].
7. Security Considerations
The entire document is about security. 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. Finally
[I-D.ietf-oauth-pop-architecture] discusses security and privacy
threats as well as mitigation measures for Proof-of-Possession
tokens.
8. IANA Considerations
TBD
9. Acknowledgments
We would like 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 ACRE proposal
FIXME:REF which was one source of inspiration for this work.
Finally, we would like to thank the ACE working group in general for
their feedback.
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10. References
10.1. Normative References
[I-D.bormann-core-ace-aif]
Bormann, C., "An Authorization Information Format (AIF)
for ACE", draft-bormann-core-ace-aif-02 (work in
progress), March 2015.
[I-D.ietf-cose-msg]
Schaad, J. and B. Campbell, "CBOR Encoded Message Syntax",
draft-ietf-cose-msg-05 (work in progress), September 2015.
[I-D.ietf-oauth-introspection]
Richer, J., "OAuth 2.0 Token Introspection", draft-ietf-
oauth-introspection-09 (work in progress), May 2015.
[I-D.ietf-oauth-pop-architecture]
Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
Architecture", draft-ietf-oauth-pop-architecture-02 (work
in progress), July 2015.
[I-D.ietf-oauth-pop-key-distribution]
Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
"OAuth 2.0 Proof-of-Possession: Authorization Server to
Client Key Distribution", draft-ietf-oauth-pop-key-
distribution-01 (work in progress), March 2015.
[I-D.selander-ace-object-security]
Selander, G., Mattsson, J., and L. Seitz, "March 9, 2015",
draft-selander-ace-object-security-01 (work in progress),
March 2015.
[I-D.wahlstroem-ace-oauth-introspection]
Wahlstroem, E., "OAuth 2.0 Introspection over the
Constrained Application Protocol (CoAP)", draft-
wahlstroem-ace-oauth-introspection-01 (work in progress),
March 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, June 2014.
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10.2. Informative References
[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-00 (work in
progress), August 2015.
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP",
draft-ietf-core-block-18 (work in progress), September
2015.
[I-D.somaraju-ace-multicast]
Somaraju, A., Kumar, S., and H. Tschofenig, "Multicast
Security for the Lighting Domain", draft-somaraju-ace-
multicast-00 (work in progress), July 2015.
[RFC4680] Santesson, S., "TLS Handshake Message for Supplemental
Data", RFC 4680, October 2006.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
[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>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
6749, October 2012.
[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>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, October 2013.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
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[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, May 2014.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content", RFC 7231, June 2014.
[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>.
Appendix A. Design Justification
This section provides further insight into the design decisions of
the solution documented in this document. Section 3 lists several
building blocks and briefly summarizes their importance. The
justification for offering some of those building blocks, as opposed
to using OAuth 2.0 as is, is given below.
Common IoT constraints are:
Low Power Radio:
Many IoT devices are equipped with a small battery which needs to
last for a long time. For many constrained wireless devices the
highest energy cost is associated to transmitting or receiving
messages. 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
of message format and protocol. By using CoAP over UDP, and CBOR
encoded messages some of these aspects are addressed. Security
protocols contribute to the communication overhead and can in some
cases can be optimized. For example authentication and key
establishment may in certain cases where security requirements so
allows be replaced by provisioning of security context by a
trusted third party, using transport or application layer
security.
Low CPU Speed:
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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, which in
turn impacts e.g. protocol latency. Symmetric key cryptography
may be used instead of the computationally more expensive public
key cryptography where the security requirements so 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. 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
not want to give access to the data collected by their IoT device
or to functions it may offer to third parties. Since the
classical approach of requesting permissions from end users via a
rich user interface does not work in many IoT deployment scenarios
these functions need to be delegated to user controlled devices
that are better suitable for such tasks, such as smart phones and
tablets.
Communication Constraints:
In certain constrained settings an IoT device may not be able to
communicate with a given device at all times. Devices may be
sleeping, or just disconnected from the Internet because of
general lack of connectivity in the area, for cost reasons, or for
security reasons, e.g. to avoid an entry point for Denial-of-
Service attacks.
The communication interactions this framework builds upon (as
shown graphically in Figure 1) may be accomplished using a variety
of different protocols, and not all parts of the message flow are
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used in all applications due to the communication constraints.
While we envision deployments to make use of CoAP we explicitly
want to support HTTP, HTTP/2 or specific protocols, such as
Bluetooth Smart communication, which does not necessarily use IP.
The latter raises the need for application layer security over the
various interfaces.
Appendix B. Optimizations
This section sketches some potential optimizations to the presented
solution.
Access token in DTLS handshake
In the case of CSP=DTLS/TLS, the access token provisoning exchange
in step C of the protocol may be embedded in the security
handshake. Different solutions are possible, where one
standardized method would be the use of the TLS supplemental data
extension [RFC4680] for transferring the access token.
Reference token and introspection
In case of introspection it may be useful with access tokens which
are not self-contained (also known as "reference tokens") that are
used to lookup detailed information about the authorization. The
RS uses the introspection message exchange not only for validating
token claims, but also for obtaining claims that potentially were
not known at the time when the access token was issued.
A reference token can be made much more compact than a CWT, since
it does not need to contain any of claims that it represents.
This could be very useful in particular if the client is
constrained and offline most of the time.
Reference token in CoAP option
While large access tokens must be sent in CoAP payload, if the
access token is known to be of a certain limited size, for example
in the case of a reference token, then it would be favorable to
combine the access token provisioning request with the resource
request to the RS.
One way to achieve this is to define a new CoAP option for
carrying reference tokens, called "Ref-Token" as shown in the
example in Figure 23.
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Resource
Client Server
| |
C: +-------->| Header: PUT (Code=0.02)
| PUT | Ref-Token:SlAV32hkKG
| | Object-Security:
| | <seq>,<cid>,[Uri-Path:"lock", 1],<tag>)
| |
. .
. .
. .
| |
F: |<--------+ Header: 2.04 Changed
| 2.04 | Object-Security:
| | (<seq>,<cid>,,<tag>)
| |
Figure 23: Reference Token in CoAP Option
Appendix C. CoAP and CBOR profiles for OAuth 2.0
Many IoT devices can support OAuth 2.0 without any additional
extensions, but for certain constrained settings additional profiling
is needed. In this appendix we define CoAP resources for the HTTP
based token and introspection endpoints used in vanilla OAuth 2.0.
We also define a CBOR alternative to the JSON and form based POST
structures used in HTTP.
C.1. Profile for Token resource
The token resource is used by the client to obtain an access token by
presenting its authorization grant or client credentials to the /
token resource the AS.
C.1.1. Token Request
The client makes a request to the token resource by sending a CBOR
structure with the following attributes.
grant_type:
REQUIRED. The grant type, "code", "client_credentials",
"password" or others.
client_id:
OPTIONAL. The client identifier issued to the holder of the token
(client or RS) during the registration process.
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client_secret:
OPTIONAL. The client secret.
scope:
OPTIONAL. The scope of the access request as described by
Section 3.1.
aud:
OPTIONAL. Service-specific string identifier or list of string
identifiers representing the intended audience for this token, as
defined in CWT Appendix D.
alg:
OPTIONAL. The value in the 'alg' parameter together with value
from the 'token_type' parameter allow the client to indicate the
supported algorithms for a given token type.
key:
OPTIONAL. This field contains information about the public key
the client would like to bind to the access token in the COSE Key
Structure format.
The parameters defined above use the following CBOR major types.
/-----------+--------------+-----------------------\
| Value | Major Type | Key |
|-----------+--------------+-----------------------|
| 0 | 0 | grant_type |
| 1 | 0 | client_id |
| 2 | 0 | client_secret |
| 3 | 0 | scope |
| 4 | 0 | aud |
| 5 | 0 | alg |
| 6 | 0 | key |
\-----------+--------------+-----------------------/
Figure 24: CBOR mappings used in token requests
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C.1.2. Token Response
The AS responds by sending a CBOR structure with the following
attributes.
access_token:
REQUIRED. The access token issued by the authorization server.
token_type:
REQUIRED. The type of the token issued. "pop" is recommended.
key:
REQUIRED, if symmetric key cryptography is used. A COSE Key
Structure containing the symmetric proof of possession key. The
members of the structure can be found in section 7.1 of
[I-D.ietf-cose-msg].
csp:
REQUIRED. Information on what communication protocol to use in
the communication between the client and the RS. Details on
possible values can be found in Section 5.1.
scope:
OPTIONAL, if identical to the scope requested by the client;
otherwise, REQUIRED.
alg:
OPTIONAL. The 'alg' parameter provides further information about
the algorithm, such as whether a symmetric or an asymmetric
crypto-system is used.
The parameters defined above use the following CBOR major types.
/-----------+--------------+-----------------------\
| Value | Major Type | Key |
|-----------+--------------+-----------------------|
| 0 | 0 | access_token |
| 1 | 0 | token_type |
| 2 | 0 | key |
| 3 | 0 | csp |
| 4 | 0 | scope |
| 5 | 0 | alg |
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\-----------+--------------+-----------------------/
Figure 25: CBOR mappings used in token responses
C.2. CoAP Profile for OAuth Introspection
This section defines a way for a holder of access tokens, mainly
clients and RS's, to get metadata like validity status, claims and
scopes found in access token. The OAuth Token Introspection
specification [I-D.ietf-oauth-introspection] defines a way to
validate the token using HTTP POST or HTTP GET. This document reuses
the work done in the OAuth Token Introspection and defines a mapping
of the request and response to CoAP [RFC7252] to be used by
constrained devices.
C.2.1. Introspection Request
The token holder makes a request to the Introspection CoAP resource
by sending a CBOR structure with the following attributes.
token:
REQUIRED. The string value of the token.
resource_id:
OPTIONAL. A service-specific string identifying the resource that
the client doing the introspection is asking about.
client_id:
OPTIONAL. The client identifier issued to the holder of the token
(client or RS) during the registration process.
client_secret:
OPTIONAL. The client secret.
The parameters defined above use the following CBOR major types:
/-----------+--------------+-----------------------\
| Value | Major Type | Key |
|-----------+--------------+-----------------------|
| 0 | 0 | token |
| 1 | 0 | resource_id |
| 2 | 0 | client_id |
| 3 | 0 | client_secret |
\-----------+--------------+-----------------------/
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Figure 26: CBOR Mappings to Token Introspection Request Parameters.
C.2.2. Introspection Response
If the introspection request is valid and authorized, the
authorization server returns a CoAP message with the response encoded
as a CBOR structure in the payload of the message. If the request
failed client authentication or is invalid, the authorization server
returns an error response using the CoAP 4.00 'Bad Request' response
code.
The JSON structure in the payload response includes the top-level
members defined in Section 2.2 in the OAuth Token Introspection
specification [I-D.ietf-oauth-introspection]. It is RECOMMENDED to
only return the 'active' attribute considering constrained nature of
CoAP client and server networks.
Introspection responses in CBOR use the following mappings:
active:
REQUIRED. The active key is an indicator of whether or not the
presented token is currently active. The specifics of a token's
"active" state will vary depending on the implementation of the
authorization server, and the information it keeps about its
tokens, but a "true" value return for the "active" property will
generally indicate that a given token has been issued by this
authorization server, has not been revoked by the resource owner,
and is within its given time window of validity (e.g., after its
issuance time and before its expiration time).
scope:
OPTIONAL. A string containing a space-separated list of scopes
associated with this token, in the format described in Section 3.3
of OAuth 2.0 [RFC6749].
client_id:
OPTIONAL. Client identifier for the client that requested this
token.
username:
OPTIONAL. Human-readable identifier for the resource owner who
authorized this token.
token_type:
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OPTIONAL. Type of the token as defined in Section 5.1 of OAuth
2.0 [RFC6749] or PoP token.
exp:
OPTIONAL. Integer timestamp, measured in the number of seconds
since January 1 1970 UTC, indicating when this token will expire,
as defined in CWT Appendix D.
iat:
OPTIONAL. Integer timestamp, measured in the number of seconds
since January 1 1970 UTC, indicating when this token will expire,
as defined in CWT Appendix D.
nbf:
OPTIONAL. Integer timestamp, measured in the number of seconds
since January 1 1970 UTC, indicating when this token will expire,
as defined in CWT Appendix D.
sub:
OPTIONAL. Subject of the token, as defined in CWT Appendix D.
Usually a machine-readable identifier of the resource owner who
authorized this token.
aud:
OPTIONAL. Service-specific string identifier or list of string
identifiers representing the intended audience for this token, as
defined in CWT Appendix D.
iss:
OPTIONAL. String representing the issuer of this token, as
defined in CWT Appendix D.
cti:
OPTIONAL. String identifier for the token, as defined in CWT
Appendix D
The parameters defined above use the following CBOR major types:
/-----------+--------------+-----------------------\
| Value | Major Type | Key |
|-----------+--------------+-----------------------|
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| 0 | 0 | active |
| 1 | 0 | scopes |
| 2 | 0 | client_id |
| 3 | 0 | username |
| 4 | 0 | token_type |
| 5 | 0 | exp |
| 6 | 0 | iat |
| 7 | 0 | nbf |
| 8 | 0 | sub |
| 9 | 0 | aud |
| 10 | 0 | iss |
| 11 | 0 | cti |
\-----------+--------------+-----------------------/
Figure 27: CBOR Mappings to Token Introspection Response Parameters.
Appendix D. CBOR Web Token (CWT)
CBOR Web Token (CWT) is a compact means of representing claims to be
transferred between two parties. CWT is a profile of JSON Web Tokens
that is optimized for constrained devices. The claims in a CWT are
encoded in CBOR and COSE is used for signature and encryption. A
claim is a piece of information asserted about a subject. A claim is
represented as a name/value pair consisting of a Claim Name and a
Claim Value.
The suggested pronunciation of CWT is the same as the English word
"cot".
The set of claims that a CWT must contain to be considered valid is
context dependent and is outside the scope of this specification.
Specific applications of CWTs will require implementations to
understand and process some claims in particular ways. However, in
the absence of such requirements, all claims that are not understood
by implementations MUST be ignored.
D.1. Claim Names
The following Claim Names are asserted by the AS and interpreted by
the RS. None of the claims defined below are intended to be
mandatory to use or implement in all cases, but rather they provide a
starting point for a set of useful, interoperable claims.
Applications using CWTs should define which specific claims they use
and when they are required or optional.
D.1.1. iss (Issuer) Claim
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The "iss" (issuer) claim identifies the principal that issued the
CWT. The processing of this claim is generally application specific.
The "iss" value is a case-sensitive string containing a StringOrURI
value. Use of this claim is OPTIONAL.
D.1.2. sub (Subject) Claim
The "sub" (subject) claim identifies the principal that is the
subject of the CWT. The claims in a CWT are normally statements
about the subject. The subject value MUST either be scoped to be
locally unique in the context of the issuer or be globally unique.
The processing of this claim is generally application specific. The
"sub" value is a case-sensitive string containing a StringOrURI
value. Use of this claim is OPTIONAL.
D.1.3. aud (Audience) Claim
The "aud" (audience) claim identifies the recipients that the CWT is
intended for. Each principal intended to process the CWT MUST
identify itself with a value in the audience claim. If the principal
processing the claim does not identify itself with a value in the
"aud" claim when this claim is present, then the CWT MUST be
rejected. In the general case, the "aud" value is an array of case-
sensitive strings, each containing a StringOrURI value. In the
special case when the CWT has one audience, the "aud" value MAY be a
single case-sensitive string containing a StringOrURI value. The
interpretation of audience values is generally application specific.
Use of this claim is OPTIONAL.
D.1.4. exp (Expiration Time) Claim
The "exp" (expiration time) claim identifies the expiration time on
or after which the CWT MUST NOT be accepted for processing. The
processing of the "exp" claim requires that the current date/time
MUST be before the expiration date/time listed in the "exp" claim.
Implementers MAY provide for some small leeway, usually no more than
a few minutes, to account for clock skew. Its value MUST be a number
containing a NumericDate value. Use of this claim is OPTIONAL.
D.1.5. nbf (Not Before) Claim
The "nbf" (not before) claim identifies the time before which the CWT
MUST NOT be accepted for processing. The processing of the "nbf"
claim requires that the current date/time MUST be after or equal to
the not-before date/time listed in the "nbf" claim. Implementers MAY
provide for some small leeway, usually no more than a few minutes, to
account for clock skew. Its value MUST be a number containing a
NumericDate value. Use of this claim is OPTIONAL.
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D.1.6. iat (Issued At) Claim
The "iat" (issued at) claim identifies the time at which the CWT was
issued. This claim can be used to determine the age of the CWT. Its
value MUST be a number containing a NumericDate value. Use of this
claim is OPTIONAL.
D.1.7. cti (CWT ID) Claim
The "cti" (CWT ID) claim provides a unique identifier for the CWT.
The identifier value MUST be assigned in a manner that ensures that
there is a negligible probability that the same value will be
accidentally assigned to a different data object; if the application
uses multiple issuers, collisions MUST be prevented among values
produced by different issuers as well. The "cti" claim can be used
to prevent the CWT from being replayed. The "cti" value is a case-
sensitive string. Use of this claim is OPTIONAL.
D.1.8. cnf (Confirmation) Claim
The "cnf" (confirmation) claim is used in the CWT to contain members
used to identify a proof-of-possession key. The "cnf" claim is used
to express a declaration in a CWT that a Client of the CWT possesses
a particular key and that the recipient can cryptographically confirm
proof-of-possession of the key by the client.
D.1.9. cks (COSE Key Structure) Claim
The "cks" (COSE Key Structure) claim holds members representing a
COSE Key Structure. The members of the structure can be found in
Section 7.1 of [I-D.ietf-cose-msg].
D.1.10. aif (Authorization Information Format) Claim
The "aif" (Authorization Information Format) claim uses the AIF
format defined in [I-D.bormann-core-ace-aif] to transfer information
about the authorization from the AS to the RS.
D.2. CBOR major types for Claims
/-----------+--------------+-----------------------\
| Value | Major Type | Key |
|-----------+--------------+-----------------------|
| 0 | 0 | iss |
| 1 | 0 | sub |
| 2 | 0 | aud |
| 3 | 0 | nonce |
| 4 | 0 | exp |
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| 5 | 0 | iat |
| 6 | 4 | cnf |
| 7 | 0 | ck |
| 8 | 4 | aif |
\-----------+--------------+-----------------------/
Figure 28: CBOR Mappings used in CWT Access Tokens.
Note: Claims defined by the OpenID Foundation have not yet been
included in the table above.
D.3. CBOR Web Token Example
This section illustrates a CWT in the CBOR diagnostic notation. This
example CWT was issued by the AS identified as "coap://
as.example.com" in the "iss" (issuer) claim. The CWT is only valid
at a resource server at "coap://light.example.com". It's validity is
2 minutes and it includes a symmetric key that will be used to secure
the communication, either using object security, or transport
security, between the client and the resource server. The "aif"
claim includes AIF objects that assert that subject is authorized to
make a PUT request against the "/s/light" resource, a PUT and a GET
against the "/a/led" resource and a POST against the "/dlts"
resource.
{
"iss" : "coap://as.example.com",
"aud" : "coap://light.example.com",
"exp" : 1444064944,
"iat" : 1443944944,
"aif" : [["/s/light", 1], ["/a/led", 5], ["/dtls", 2]],
"cnf" : {
"jwk" : b64'JDLUhTMjU2IiwiY3R5Ijoi ...'
}
}
Figure 29: CWT Example in the CBOR Diagnostic Notation.
Authors' Addresses
Ludwig Seitz
SICS
Scheelevaegen 17
Lund 223 70
SWEDEN
Email: ludwig@sics.se
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Goeran Selander
Ericsson
Faroegatan 6
Kista 164 80
SWEDEN
Email: goran.selander@ericsson.com
Erik Wahlstroem
Nexus Technology
Telefonvagen 26
Hagersten 126 26
Sweden
Email: erik.wahlstrom@nexusgroup.com
Samuel Erdtman
Nexus Technology
Telefonvagen 26
Hagersten 126 26
Sweden
Email: samuel.erdtman@nexusgroup.com
Hannes Tschofenig
ARM Ltd.
Hall in Tirol 6060
Austria
Email: Hannes.Tschofenig@arm.com
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