ACE Working Group S. Gerdes
Internet-Draft O. Bergmann
Intended status: Standards Track C. Bormann
Expires: August 13, 2015 Universitaet Bremen TZI
February 09, 2015
Delegated CoAP Authentication and Authorization Framework (DCAF)
draft-gerdes-ace-dcaf-authorize-01
Abstract
This specification defines a protocol for delegating client
authentication and authorization in a constrained environment for
establishing a Datagram Transport Layer Security (DTLS) channel
between resource-constrained nodes. The protocol relies on DTLS to
transfer authorization information and shared secrets for symmetric
cryptography between entities in a constrained network. A resource-
constrained node can use this protocol to delegate authentication of
communication peers and management of authorization information to a
trusted host with less severe limitations regarding processing power
and memory.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 13, 2015.
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
(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Features . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1. Actors . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.2. Other Terms . . . . . . . . . . . . . . . . . . . . . 5
2. System Overview . . . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Unauthorized Resource Request Message . . . . . . . . . . 8
3.3. SAM Information Message . . . . . . . . . . . . . . . . . 9
3.4. Access Request . . . . . . . . . . . . . . . . . . . . . 10
3.5. Ticket Request Message . . . . . . . . . . . . . . . . . 11
3.6. Ticket Grant Message . . . . . . . . . . . . . . . . . . 12
3.7. Ticket Transfer Message . . . . . . . . . . . . . . . . . 14
3.8. DTLS Channel Setup Between C and RS . . . . . . . . . . . 15
3.9. Authorized Resource Request Message . . . . . . . . . . . 16
3.10. Dynamic Update of Authorization Information . . . . . . . 17
3.10.1. Handling of Ticket Transfer Messages . . . . . . . . 18
4. Ticket . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1. Face . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2. CI . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.3. Revocation . . . . . . . . . . . . . . . . . . . . . . . 20
4.3.1. Lifetime . . . . . . . . . . . . . . . . . . . . . . 20
4.3.2. Revocation Messages . . . . . . . . . . . . . . . . . 21
5. Payload Format and Encoding (application/dcaf+cbor) . . . . . 21
5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 24
6. DTLS PSK Generation Methods . . . . . . . . . . . . . . . . . 26
6.1. DTLS PSK Transfer . . . . . . . . . . . . . . . . . . . . 26
6.2. Distributed Key Derivation . . . . . . . . . . . . . . . 26
7. Authorization Configuration . . . . . . . . . . . . . . . . . 27
8. Trust Relationships . . . . . . . . . . . . . . . . . . . . . 27
9. Listing Authorization Manager Information in a Resource
Directory . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1. The "auth-request" Link Relation . . . . . . . . . . . . 28
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Access Granted . . . . . . . . . . . . . . . . . . . . . 29
10.2. Access Denied . . . . . . . . . . . . . . . . . . . . . 31
10.3. Access Restricted . . . . . . . . . . . . . . . . . . . 32
10.4. Implicit Authorization . . . . . . . . . . . . . . . . . 33
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11. Specific Usage Scenarios . . . . . . . . . . . . . . . . . . 34
11.1. Combined Authorization Manager and Client . . . . . . . 34
11.1.1. Creating the Ticket Request Message . . . . . . . . 34
11.1.2. Processing the Ticket Grant Message . . . . . . . . 35
11.2. Combined Client Authorization Manager and Server
Authorization Manager . . . . . . . . . . . . . . . . . 35
11.2.1. Processing the Access Request Message . . . . . . . 36
11.2.2. Creating the Ticket Transfer Message . . . . . . . . 36
11.3. Combined Server Authorization Manager and Resource
Server . . . . . . . . . . . . . . . . . . . . . . . . . 36
12. Security Considerations . . . . . . . . . . . . . . . . . . . 37
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
13.1. DTLS PSK Key Generation Methods . . . . . . . . . . . . 38
13.2. dcaf+cbor Media Type Registration . . . . . . . . . . . 38
13.3. CoAP Content Format Registration . . . . . . . . . . . . 39
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
14.1. Normative References . . . . . . . . . . . . . . . . . . 40
14.2. Informative References . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a transfer
protocol similar to HTTP which is designed for the special
requirements of constrained environments. A serious problem with
constrained devices is the realization of secure communication. The
devices only have limited system resources such as memory, stable
storage (such as disk space) and transmission capacity and often lack
input/output devices such as keyboards or displays. Therefore, they
are not readily capable of using common protocols. Especially
authentication mechanisms are difficult to realize, because the lack
of stable storage severely limits the number of keys the system can
store. Moreover, CoAP has no mechanism for authorization.
[I-D.gerdes-ace-actors] describes an architecture that is designed to
help constrained nodes with authorization-related tasks by
introducing less-constrained nodes. These Authorization Managers
perform complex security tasks for their nodes such as managing keys
for numerous devices, and enable the constrained nodes to enforce the
authorization policies of their owners.
DCAF uses access tokens to implement this architecture. A device
that wants to access an item of interest on a constrained node first
has to gain permission in the form of a token from the node's
Authorization Manager.
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As fine-grained authorization is not always needed on constrained
devices, DCAF supports an implicit authorization mode where no
authorization information is exchanged.
The main goals of DCAF are the setup of a Datagram Transport Layer
Security (DTLS) [RFC6347] channel with symmetric pre-shared keys
(PSK) [RFC4279] between two nodes and to securely transmit
authorization tickets.
1.1. Features
o Utilize DTLS communication with pre-shared keys.
o Authenticated exchange of authorization information.
o Simplified authentication on constrained nodes by handing the more
sophisticated authentication over to less-constrained devices.
o Support of secure constrained device to constrained device
communication.
o Authorization policies of the principals of both participating
parties are ensured.
o Simplified authorization mechanism for cases where implicit
authorization is sufficient.
o Using only symmetric encryption on constrained nodes.
1.2. 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 RFC 2119 [RFC2119].
Readers are expected to be familiar with the terms and concepts
defined in [I-D.gerdes-ace-actors].
1.2.1. Actors
Resource Server (RS): An endpoint that hosts and represents a CoAP
resource.
Client (C): An endpoint that attempts to access a CoAP resource on
the Resource Server.
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Server Authorization Manager (SAM): An entity that prepares and
endorses authentication and authorization data for a Resource
Server.
Client Authorization Manager (CAM): An entity that prepares and
endorses authentication and authorization data for a Client.
Authorization Manager (AM): An entity that is either a SAM or a CAM.
Client Owner: The principal that is responsible for the Client and
controls permissions concerning authorized representations of a
CoAP resource.
Resource Owner: The principal that is responsible for the CoAP
resource and controls its access permissions.
1.2.2. Other Terms
Resource: A CoAP resource.
Authorization information: Contains all information needed by RS to
decide if C is privileged to access a resource in a specific way.
Authentication information: Contains all information needed by RS
to decide if the entity in possession of a certain key is verified
by SAM.
Access information: Contains authentication information and, if
necessary, authorization information.
Access ticket: Contains the authentication and, if necessary, the
authorization information needed to access a resource. A Ticket
consists of the Ticket Face and the Client Information. The
access ticket is a representation of the access information.
Ticket Face: The part of the ticket which is generated for the
Resource Server. It contains the authorization information and
all information needed by the Resource Server to verify that it
was granted by SAM.
Client Information (CI): The part of the ticket which is generated
for the Client. It contains the Verifier and optionally may
contain authorization information that represent CO's
authorization policies for C.
Verifier: It enables the client to verify that it is communicating
with an appropriate RS.
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Explicit authorization: SAM informs the RS in detail which
privileges are granted to the Client.
Implicit authorization: SAM authenticates the Client for the
Resource Server without specifying the privileges in detail. This
can be used for binary or unrestricted authorization (cf section 4
of [I-D.gerdes-ace-actors]).
2. System Overview
Within the DCAF Architecture each Resource Server (RS) has a Server
Authorization Manger (SAM) which conducts the authentication and
authorization for RS. RS and SAM share a symmetric key which has to
be exchanged initially to provide for a secure channel. The
mechanism used for this is not in the scope of this document.
To gain access to a specific resource on a RS, a Client (C) has to
request an access ticket from the SAM serving RS either directly or,
if it is a constrained device, using its Client Authorization Manager
(CAM). In the following, we always discuss the CAM role separately,
even if that is co-located within a (more powerful) C (see section
Section 11 for details about co-located actors).
CAM decides if RS is an authorized source for R according to the
policies set by CO and in this case transmits the request to SAM. If
SAM decides that C is allowed to access the resource according to the
policies set by RO, it generates a DTLS pre-shared key (PSK) for the
communication between C and RS and wraps it into an access ticket.
For explicit access control, SAM adds the detailed access permissions
to the ticket in a way that CAM and RS can interpret. CAM checks if
the permissions in the access ticket comply with CO's authorization
policies for C, and if this is the case sends it to C. After C
presented the ticket to RS, C and RS can communicate securely.
To be able to provide for the authentication and authorization
services, an Authorization Manager has to fulfill several
requirements:
o AM must have enough stable storage (such as disk space) to store
the necessary number of credentials (matching the number of
Clients and Resource Servers).
o AM must possess means for user interaction, for example directly
or indirectly connected input/output devices such as keyboard and
display, to allow for configuration of authorization information
by the respective Principal.
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o AM must have enough processing power to handle the authorization
requests for all constrained devices it is responsible for.
3. Protocol
The DCAF protocol comprises three parts:
1. transfer of authentication and, if necessary, authorization
information between C and RS;
2. transfer of access requests and the respective ticket grants
between C and CAM; and
3. transfer of access requests and the respective ticket grants
between SAM and CAM.
3.1. Overview
In Figure 1, a DCAF protocol flow is depicted (messages in square
brackets are optional):
CAM C RS SAM
| <== DTLS chan. ==> | | <== DTLS chan. ==> |
| | [Resource Req.-->] | |
| | | |
| | [<-- SAM Info.] | |
| | | |
| <-- Access Req. | | |
| | | |
| <==== TLS/DTLS channel (CAM/SAM Mutual Authentication) ====> |
| | | |
| Ticket Request ------------------------------------------> |
| | | |
| <------------------------------------------ Ticket Grant |
| | | |
| Ticket Transf. --> | | |
| | | |
| | <== DTLS chan. ==> | |
| | Auth. Res. Req. -> | |
Figure 1: Protocol Overview
To determine the SAM 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 SAM back to C.
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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]) that lists RS's resources as
discussed in Section 9.
Once C knows SAM's address, it can send a request for authorization
to SAM using its own CAM. CAM and SAM authenticate each other and
each determine if the request is to be authorized. If it is, SAM
generates an access ticket for C. The ticket contains keying
material for the establishment of a secure channel and, if necessary,
a representation of the permissions C has for the resource. C keeps
one part of the access ticket and presents the other part to RS to
prove its right to access. With their respective parts of the
ticket, C and RS are able to establish a secure channel.
The following sections specify how CoAP is used to interchange
access-related data between RS and SAM so that SAM can provide C and
RS with sufficient information to establish a secure channel, and
simultaneously convey authorization information specific for this
communication relationship to RS.
Note: Special implementation considerations apply when one single
entity takes the role of more than one actors. Section 11 gives
additional advice on some of these usage scenarios.
This document uses Concise Binary Object Representation (CBOR,
[RFC7049]) to express authorization information as set of attributes
passed in CoAP payloads. Notation and encoding options are discussed
in Section 5.
3.2. Unauthorized Resource Request Message
The optional Unauthorized Resource Request message is a request for a
resource hosted by RS for which no proper authorization is granted.
RS MUST treat any CoAP request as Unauthorized Resource Request
message when any of the following holds:
o The request has been received on an insecure channel.
o RS has no valid access ticket for the sender of the request
regarding the requested action on that resource.
o RS has a valid access ticket for the sender of the request, but
this does not allow the requested action on the requested
resource.
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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.
Unauthorized Resource Request messages MUST be denied with a client
error response. In this response, the Resource Server MUST provide
proper SAM Information to enable the Client to request an access
ticket from RS's SAM as described in Section 3.3.
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 ticket for C. If RS has an access ticket 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 ticket for C
but it does not cover the action C requested on the resource, RS MUST
reject the request with a 4.05 (Method Not Allowed).
Note: The use of the response codes 4.03 and 4.05 is intended to
prevent infinite loops where a dumb Client optimistically tries to
access a requested resource with any access token received from
the SAM. 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.
3.3. SAM Information Message
The SAM Information Message is sent by RS as a response to an
Unauthorized Resource Request message (see Section 3.2) to point the
sender of the Unauthorized Resource Request message to RS's SAM. The
SAM information is a set of attributes containing an absolute URI
(see Section 4.3 of [RFC3986]) that specifies the SAM in charge of
RS.
The message MAY also contain a timestamp generated by RS.
Figure 2 shows an example for an SAM Information message payload
using CBOR diagnostic notation. (Refer to Section 5 for a detailed
description of the available attributes and their semantics.)
4.01 Unauthorized
Content-Format: application/dcaf+cbor
{SAM: "coaps://sam.example.com/authorize", TS: 168537}
Figure 2: SAM Information Payload Example
In this example, the attribute SAM points the receiver of this
message to the URI "coaps://sam.example.com/authorize" to request
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access permissions. The originator of the SAM Information payload
(i.e. RS) uses a local clock that is loosely synchronized with a
time scale common between RS and SAM (e.g., wall clock time).
Therefore, it has included a time stamp on its own time scale that is
used as a nonce for replay attack prevention. Refer to Section 4.1
for more details concerning the usage of time stamps to ensure
freshness of access tickets.
The examples in this document are written in CBOR diagnostic notation
to improve readability. Figure 3 illustrates the binary encoding of
the message payload shown in Figure 2.
a2 # map(2)
00 # unsigned(0) (=SAM)
78 21 # text(33)
636f6170733a2f2f73616d2e6578
616d706c652e636f6d2f617574686f72
697a65 # "coaps://sam.example.com/authorize"
05 # unsigned(5) (=TS)
1a 00029259 # unsigned(168537)
Figure 3: SAM Information Payload Example encoded in CBOR
3.4. Access Request
To retrieve an access ticket for the resource that C wants to access,
C sends an Access Request to its CAM. The Access Request is
constructed as follows:
1. The request method is POST.
2. The request URI is set as described below.
3. The message payload contains a data structure that describes the
action and resource for which C requests an access ticket.
The request URI identifies a resource at CAM for handling
authorization requests from C. The URI SHOULD be announced by CAM in
its resource directory as described in Section 9.
Note: Where capacity limitations of C do not allow for resource
directory lookups, the request URI in Access Requests could be
hard-coded during provisioning or set in a specific device
configuration profile.
The message payload is constructed from the SAM information that RS
has returned in its SAM Information message (see Section 3.3) and
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information that C provides to describe its intended request(s). The
Access Request MUST contain the following attributes:
1. Contact information for the SAM to use.
2. An absolute URI of the resource that C wants to access.
3. The actions that C wants to perform on the resource.
4. Any time stamp generated by RS.
An example Access Request from C to CAM is depicted in Figure 4.
(Refer to Section 5 for a detailed description of the available
attributes and their semantics.)
POST client-authorize
Content-Format: application/dcaf+cbor
{
SAM: "coaps://sam.example.com/authorize",
SAI: ["coaps://temp451.example.com/s/tempC", 5],
TS: 168537
}
Figure 4: Access Request Message Example
The example shows an Access Request message payload for the resource
"/s/tempC" on the Resource Server "temp451.example.com". Requested
operations in attribute SAI are GET and PUT.
The attributes SAM (that denotes the Server Authorization Manager to
use) and TS (a nonce generated by RS) are taken from the SAM
Information message from RS.
The response to an Authorization Request is delivered by CAM back to
C in a Ticket Transfer message.
3.5. Ticket Request Message
When CAM receives an Access Request message from C and CO specified
authorization policies for C, CAM MUST check if the requested actions
are allowed according to these policies. If this is not the case,
CAM MUST send a 4.03 response.
If no authorization policies were specified or the requested action
is allowed according to the authorization policies, CAM either
returns a cached response or attempts to create a Ticket Request
message.
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CAM MAY return a cached response if it is known to be fresh according
to Max-Age. CAM SHOULD NOT return a cached response if it expires in
less than a minute.
If CAM does not send a cached response, it checks whether the request
payload is of type "application/dcaf+cbor and contains at least the
fields SAM and SAI. CAM MUST respond with 4.00 (Bad Request) if the
type is "application/dcaf+cbor and any of these fields is missing or
does not conform to the format described in Section 5. Content
formats other than application/dcaf+cbor are out of scope of this
specification.
If the payload is correct, CAM creates a Ticket Request message from
the Access Request received from C as follows:
1. The destination of the Ticket Request message is derived from the
authority information in the URI contained in field "SAM" of the
Access Request message payload.
2. The request method is POST.
3. The request URI is constructed from the SAM field received in the
Access Request message payload.
4. The payload is copied from the Access Request sent by C.
5. A label that describes the Client is added to the payload
To send the Ticket Request message to SAM a secure channel between
CAM and SAM MUST be used. Depending on the URI scheme used in the
SAM field of the Access Request message payload (the less-constrained
devices CAM and SAM do not necessarily use CoAP to communicate with
each other), this could be, e.g., a DTLS channel (for "coaps") or a
TLS connection (for "https"). CAM and SAM MUST be able to mutually
authenticate each other, e.g. based on a public key infrastructure.
(Refer to Section 8 for a detailed discussion of the trust
relationship between Client Authorization Managers and Server
Authorization Managers.)
3.6. Ticket Grant Message
When SAM has received a Ticket Request message it has to evaluate the
access request information contained therein. First, it checks
whether the request payload is of type "application/dcaf+cbor" and
contains at least the fields SAM and SAI. SAM MUST respond with 4.00
(Bad Request) for CoAP (or 400 for HTTP) if the type is "application/
dcaf+cbor" and any of these fields is missing or does not conform to
the format described in Section 5.
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SAM decides whether or not access is granted to the requested
resource and then creates a Ticket Grant message that reflects the
result. To grant access to the requested resource, SAM creates an
access ticket comprised of a Face and the Client Information as
described in Section 4.1.
The Ticket Grant message then is constructed as a success response
indicating attached content, i.e. 2.05 for CoAP, or 200 for HTTP,
respectively. The payload of the Ticket Grant message is a data
structure that contains the result of the access request. When
access is granted, the data structure contains the Ticket Face, the
Client Information, which at this point only consists of the Verifier
and the Session Key Generation Method.
The Ticket Grant message MAY provide cache-control options to enable
intermediaries to cache the response. The message MAY be cached
according to the rules defined in [RFC7252] to facilitate ticket
retrieval when C has crashed and wants to recover the DTLS session
with RS.
SAM SHOULD set Max-Age according to the ticket lifetime in its
response (Ticket Grant Message).
Figure 5 shows an example Ticket Grant message using CoAP. The Face/
Verifier information is transferred as a CBOR data structure as
specified in Section 5. The Max-Age option tells the receiving CAM
how long this ticket will be valid.
2.05 Content
Content-Format: application/dcaf+cbor
Max-Age: 86400
{ F: {
SAI: [ "/s/tempC", 7 ],
TS: 0("2013-07-10T10:04:12.391"),
L: 86400,
G: hmac_sha256
},
V: h'f89947160c73601c7a65cb5e08812026
6d0f0565160e3ff7d3907441cdf44cc9'
}
Figure 5: Example Ticket Grant Message
A Ticket Grant message that declines any operation on the requested
resource is illustrated in Figure 6. As no ticket needs to be
issued, an empty payload is included with the response.
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2.05 Content
Content-Format: application/dcaf+cbor
Figure 6: Example Ticket Grant Message With Reject
3.7. Ticket Transfer Message
A Ticket Transfer message delivers the access information sent by SAM
in a Ticket Grant message to the requesting client C. The Ticket
Transfer message is the response to the Access Request message sent
from C to CAM and includes any access information from SAM contained
in the Ticket Grant message.
The Authorization Information provided by SAM in the Ticket Grant
Message may grant more permissions than C has requested. The
authorization policies of CO and RO may differ: CO might want
restrict the resources C is allowed to access, and the actions that C
is allowed to perform on the resource.
If CAM must ensure authorization policies CO configured , CAM MUST
add Authorization Information for C (CAI) to the CI. Since C and CAM
use a DTLS channel for communication, the autorization information
does not need to be encrypted.
CAM includes the Face and Verifier sent by SAM in the Ticket Transfer
message. CAM MUST NOT include any other information SAM provided.
In particular, CAM MUST NOT include any CAI information provided by
SAM.
Figure 7 shows an example Ticket Transfer message that conveys the
permissions for actions GET, POST, PUT (but not DELETE) on the
resource "/s/tempC" in field SAI. As CAM only wants to permit
outbound GET requests, it restricts C's permissions in the field CAI
accordingly.
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2.05 Content
Content-Format: application/dcaf+cbor
Max-Age: 86400
{ F: {
SAI: [ "/s/tempC", 7 ],
TS: 0("2013-07-10T10:04:12.391"),
L: 86400,
G: hmac_sha256
},
V: h'f89947160c73601c7a65cb5e08812026
6d0f0565160e3ff7d3907441cdf44cc9'
CAI: [ "/s/tempC", 1 ],
TS: 0("2013-07-10T10:04:12.855"),
L: 86400
}
Figure 7: Example Ticket Transfer Message
3.8. DTLS Channel Setup Between C and RS
Using the information contained in a positive response to its Access
Request (i.e. a Ticket Transfer message that contains a Face and a
Client Information), C can initiate establishment of a new DTLS
channel with RS. To use DTLS with pre-shared keys, C follows the PSK
key exchange algorithm specified in Section 2 of [RFC4279], with the
following additional requirements:
1. C sets the psk_identity field of the ClientKeyExchange message to
the ticket Face received in the Ticket Transfer message.
2. C uses the ticket Verifier as PSK when constructing the premaster
secret.
Note1: As RS cannot provide C with a meaningful PSK identity hint in
response to C's ClientHello message, RS SHOULD NOT send a
ServerKeyExchange message.
Note2: According to [RFC7252], CoAP implementations MUST support the
ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655]. C is therefore
expected to offer at least this ciphersuite to RS.
Note3: The ticket is constructed by SAM such that RS can derive the
authorization information as well as the PSK (refer to Section 6 for
details).
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3.9. Authorized Resource Request Message
If the Client Information in the Ticket Transfer message contains
CAI, C MUST ensure that it only sends requests that according to them
are allowed. C therefore MUST check CAI, L and T before every
request. If CAI is no longer valid according to L, C MUST terminate
the DTLS connection with RS and re-request the CAI from CAM using an
Access Request Message.
On the Resource Server side, successful establishment of the DTLS
channel between C and RS ties the SAM authorization information
contained in the psk_identity field to this channel. Any request
that RS receives on this channel is checked against these
authorization rules. Incoming CoAP requests that are not Authorized
Resource Requests MUST be rejected by RS with 4.01 response as
described in Section 3.2.
RS SHOULD treat an incoming CoAP request as Authorized Resource
Request if the following holds:
1. The message was received on a secure channel that has been
established using the procedure defined in Section 3.8.
2. The authorization information tied to the secure channel is
valid.
3. The request is destined for RS.
4. The resource URI specified in the request is covered by the
authorization information.
5. The request method is an authorized action on the resource with
respect to the authorization information.
Note that the authorization information is not restricted to a single
resource URI. For example, role-based authorization can be used to
authorize a collection of semantically connected resources
simultaneously. Implicit authorization also provides access rights
to authenticated clients for all actions on all resources that RS
offers. As a result, C can use the same DTLS channel not only for
subsequent requests for the same resource (e.g. for block-wise
transfer as defined in [I-D.ietf-core-block] or refreshing observe-
relationships [I-D.ietf-core-observe]) but also for requests to
distinct resources.
Incoming CoAP requests received on a secure channel according to the
procedure defined in Section 3.8 MUST be rejected
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1. with response code 4.03 (Forbidden) when the resource URI
specified in the request is not covered by the authorization
information, and
2. with response code 4.05 (Method Not Allowed) when the resource
URI specified in the request covered by the authorization
information but not the requested action.
Since SAM may limit the set of requested actions in its Ticket Grant
message, C cannot know a priori if an Authorized Resource Request
will succeed.
3.10. Dynamic Update of Authorization Information
Once a security association exists between a Client and a Resource
Server, the Client can update the Authorization Information stored at
the Resource Server at any time. To do so, the Client creates a new
Access Request for the intended action on the respective resource and
sends this request to its CAM which checks and relays this request to
the Resource Server's SAM as described in Section 3.4.
Note: Requesting a new Access Ticket also can be a Client's reaction
on a 4.03 or 4.05 error that it has received in response to an
Authorized Resource Request.
Figure 8 depicts the message flow where C requests a new Access
Tickets after a security association between C and RS has been
established using this protocol.
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CAM C RS SAM
| <== DTLS chan. ==> | <== DTLS chan. ==> | <== DTLS chan. ==> |
| | | |
| | [Unauth. R. Req->] | |
| |[<- 4.0x+SAM Info.] | |
| | | |
| <-- Access Req. | | |
| | | |
| <==== TLS/DTLS channel (CAM/SAM Mutual Authentication) ====> |
| | | |
| Ticket Request ------------------------------------------> |
| | | |
| <------------------------------------------ Ticket Grant |
| | | |
| Ticket Transf. --> | | |
| | | |
| | <== Update SAI ==> | |
Figure 8: Overview of Dynamic Update Operation
Processing the Ticket Request is done at the SAM as specified in
Section 3.6, i.e. the SAM checks whether or not the requested
operation is permitted by the Resource Owner's policy, and then
return a Ticket Grant message with the result of this check. If
access is granted, the Ticket Grant message contains an Access Ticket
comprised of a public Ticket Face and a private Ticket Verifier.
This authorization payload is relayed by CAM to the Client in a
Ticket Transfer Message as defined in Section 3.7.
The major difference between dynamic update of Authorization
Information and the initial handshake is the handling of a Ticket
Transfer message by the Client that is described in Section 3.10.1.
3.10.1. Handling of Ticket Transfer Messages
If the security association with RS still exists and RS has indicated
support for session renegotiation according to [RFC5746], the ticket
Face SHOULD be used to renegotiate the existing DTLS session. In
this case, the ticket Face is used as psk_identity as defined in
Section 3.8. Otherwise, the Client MUST perform a new DTLS handshake
according to Section 3.8 that replaces the existing DTLS session.
After successful completion of the DTLS handshake RS updates the
existing SAM Authorization Information for C according to the
contents of the ticket Face.
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Note: No mutual authentication between C and RS is required for
dynamic updates when a DTLS channel exists that has been
established as defined in Section 3.8. RS only needs to verify
the authenticity and integrity of the ticket Face issued by SAM
which is achieved by having performed a successful DTLS handshake
with the ticket Face as psk_identity. This could even be done
within the existing DTLS session by tunneling a CoDTLS
[I-D.schmertmann-dice-codtls] handshake.
4. Ticket
Access tokens in DCAF are tickets that consist of two parts, namely
the Face and the Client Information. The Face goes to RS, the CI
goes to the Client. The Face and the CI are parts of the same
ticket.
RS only needs the information contained in the Ticket Face to
authorize the client and make sure that SAM generated the Ticket Face
(RS cannot make authorization decisions by itself and hence needs SAM
to do it). No additional information about the Client is needed. RS
keeps the Ticket Face as long as it is valid.
4.1. Face
Face is the part of the ticket generated for RS. Face MUST contain
all information needed for authorized access to a resource:
o SAM Authorization Information
o A timestamp generated by SAM
Optionally, Face MAY also contain:
o A lifetime (optional)
o A DTLS pre-shared key (optional)
RS MUST verify the integrity of Face, i.e. the information contained
in Face stems from SAM and was not manipulated by anyone else.
Face MUST contain a timestamp to verify that the contained
information is fresh. As constrained devices may not have a clock,
timestamps MAY be generated using the clock ticks since the last
reboot. To circumvent synchronization problems the timestamp MAY be
generated by RS and included in the first SAM Information message.
Alternatively, SAM MAY generate the timestamp. In this case, SAM and
RS MUST use a time synchronization mechanism to make sure that RS
interprets the timestamp correctly.
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Face MAY be encrypted. If Face contains a DTLS PSK, the whole
content of Face MUST be encrypted.
The integrity of Face can be ensured by various means. Face may be
encrypted by SAM with a key it shares with RS. Alternatively, RS can
use a mechanism to generate the DTLS PSK which includes Face. RS
generates the key from the Face it received. The correct key can
only be calculated with the correct Face (refer to Section 6 for
details).
4.2. CI
The CI part of the ticket is generated for C. It contains the
Verifier, i.e. the DTLS PSK for C and MAY contain Client
Authorization Information generated by CAM to provide the CO's
authorization policies to C. The Verifier MUST NOT be transmitted
over insecure channels.
4.3. Revocation
The existence of access tickets SHOULD be limited in time to avoid
stale tickets that waste resources on RS and C. This can be achieved
either by explicit Revocation Messages to invalidate a ticket or
implicitly by attaching a lifetime to the ticket.
4.3.1. Lifetime
Tickets MAY have a lifetime. SAM is responsible for defining the
ticket lifetime. If SAM sets a lifetime for a ticket, SAM and RS
MUST use a time synchronization method to ensure that RS is able to
interpret the lifetime correctly. RS SHOULD end the DTLS connection
to C if the lifetime of a ticket has run out and it MUST NOT accept
new requests. RS MUST NOT accept tickets with an invalid lifetime.
If CAM provides CAI in the CI part of the ticket, CAM MAY add a
lifetime for this CAI. If CI contains a lifetime, CAM and C MUST use
a time synchronization method to ensure that C is able to interpret
the lifetime correctly. C SHOULD end the DTLS connection to RS and
MUST NOT send new requests if the CAI in the ticket is no longer
valid. C MUST NOT accept tickets with an invalid lifetime.
Note: Defining reasonable ticket lifetimes is difficult to
accomplish. How long a client needs to access a resource depends
heavily on the application scenario and may be difficult to decide
for SAM.
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4.3.2. Revocation Messages
SAM MAY revoke tickets by sending a ticket revocation message to RS.
If RS receives a ticket revocation message, it MUST end the DTLS
connection to C and MUST NOT accept any further requests from C.
If ticket revocation messages are used, RS MUST check regularly if
SAM is still available. If RS cannot contact SAM, it MUST end all
DTLS connections and reject any further requests from C.
Note: The loss of the connection between RS and SAM prevents all
access to RS. This might especially be a severe problem if SAM is
responsible for several Resource Servers or even a whole network.
5. Payload Format and Encoding (application/dcaf+cbor)
Various messages types of the DCAF protocol carry payloads to express
authorization information and parameters for generating the DTLS PSK
to be used by C and RS. In this section, a representation in Concise
Binary Object Representation (CBOR, [RFC7049]) is defined.
DCAF data structures are defined as CBOR maps that contain key value
pairs. For efficient encoding, the keys defined in this document are
represented as unsigned integers in CBOR, i. e. major type 0. For
improved reading, we use symbolic identifiers to represent the
corresponding encoded values as defined in Table 1.
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+---------------+-----+
| Encoded Value | Key |
+---------------+-----+
| 0b000_00000 | SAM |
| | |
| 0b000_00001 | SAI |
| | |
| 0b000_00010 | CAI |
| | |
| 0b000_00011 | E |
| | |
| 0b000_00100 | K |
| | |
| 0b000_00101 | TS |
| | |
| 0b000_00110 | L |
| | |
| 0b000_00111 | G |
| | |
| 0b000_01000 | F |
| | |
| 0b000_01001 | V |
+---------------+-----+
Table 1: DCAF field identifiers encoded in CBOR
The following list describes the semantics of the keys defined in
DCAF.
SAM: Server Authorization Manager. This attribute denotes the
Server Authorization Manager that is in charge of the resource
specified in attribute R. The attribute's value is a string that
contains an absolute URI according to Section 4.3 of [RFC3986].
SAI: SAM Authorization Information. A data structure used to convey
authorization information from SAM to RS. It describes C's
permissions for RS according to SAM, e.g., which actions C is
allowed to perform on an R of RS. The SAI attribute contains an
AIF object as defined in [I-D.bormann-core-ace-aif]. C uses SAI
for its Access Request messages.
CAI: CAM Authorization Information. A data structure used to convey
authorization information from CAM to C. It describes the C's
permissions for RS according to CAM, e.g., which actions C is
allowed to perform on an R of RS. The CAI attribute contains an
AIF object as defined in [I-D.bormann-core-ace-aif].
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E: Encrypted Ticket Face. A binary string containing an encrypted
ticket Face.
K: Key. A string that identifies the shared key between RS and SAM
that can be used to decrypt the contents of E. If the attribute E
is present and no attribute K has been specified, the default is
to use the current session key for the secured channel between RS
and SAM.
TS: Time Stamp. An optional time stamp that indicates the instant
when the access ticket request was formed. This attribute can be
used by the Resource Server in an SAM Information message to
convey a time stamp in its local time scale (e.g. when it does not
have a real time clock with synchronized global time). When the
attribute's value is encoded as a string, it MUST contain a valid
UTC timestamp without time zone information. When encoded as
integer, TS contains a system timestamp relative to the local time
scale of its generator, usually RS.
L: Lifetime. A lifetime of the ticket. When encoded as a string, L
MUST denote the ticket's expiry time as a valid UTC timestamp
without time zone information. When encoded as an integer, L MUST
denote the ticket's validity period in seconds relative to TS.
G: DTLS PSK Generation Method. A numeric identifier for the method
that RS MUST use to derive the DTLS PSK from the ticket Face.
This attribute MUST NOT be used when attribute V is present within
the contents of F. This specification uses symbolic identifiers
for improved readability. The corresponding numeric values
encoded in CBOR are defined in Table 2. A registry for these
codes is defined in Section 13.1.
F: Ticket Face. An object containing the fields SAI, TS, and
optionally G, L and V.
V: Ticket Verifier. A binary string containing the shared secret
between C and RS.
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+---------------+-------------+-----------+
| Encoded Value | Mnemonic | Support |
+---------------+-------------+-----------+
| 0b000_00000 | hmac_sha256 | mandatory |
| | | |
| 0b000_00001 | hmac_sha384 | optional |
| | | |
| 0b000_00010 | hmac_sha512 | optional |
+---------------+-------------+-----------+
Table 2: CBOR encoding for DTLS PSK Key Generation Methods
5.1. Examples
The following example specifies a SAM that will be accessed using
HTTP over TLS. The request URI is set to
"/a?ep=%5B2001:DB8::dcaf:1234%5D" (hence denoting the endpoint
address to authorize). TS denotes a local timestamp in UTC.
POST /a?ep=%5B2001:DB8::dcaf:1234%5D HTTP/1.1
Host: sam.example.com
Content-Type: application/dcaf+cbor
{SAM: "https://sam.example.com/a?ep=%5B2001:DB8::dcaf:1234%5D",
SAI: ["coaps://temp451.example.com/s/tempC", 1],
TS: 0("2013-07-14T11:58:22.923")}
The following example shows a ticket for the distributed key
generation method (cf. Section 6.2), comprised of a Face (F) and a
Verifier (V). The Face data structure contains authorization
information SAI, a client descriptor, a timestamp using the local
time scale of RS, and a lifetime relative to RS's time scale.
The DTLS PSK Generation Method is set to hmac_sha256 denoting that
the distributed key derivation is used as defined in Section 6.2 with
SHA-256 as HMAC function.
The Verifier V contains a shared secret to be used as DTLS PSK
between C and RS.
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HTTP/1.1 200 OK
Content-Type: application/dcaf+cbor
{
F: {
SAI: [ "/s/tempC", 1 ],
TS: 2938749,
L: 3600,
G: hmac_sha256
},
V: h'48ae5a81b87241d81618f56cab0b65ec
441202f81faabbe10075b20cb57fa939'
}
The Face may be encrypted as illustrated in the following example.
Here, the field E carries an encrypted Face data structure that
contains the same information as the previous example, and an
additional Verifier. Encryption was done with a secret shared by SAM
and RS. (This example uses AES128_CCM with the secret { 0x00, 0x01,
0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c,
0x0d, 0x0e, 0x0f } and RS's timestamp { 0x00, 0x2C, 0xD7, 0x7D } as
nonce.) Line breaks have been inserted to improve readability.
The attribute K describes the identity of the key to be used by RS to
decrypt the contents of attribute E. Here, The value "key0" in this
example is used to indicate that the shared session key between RS
and SAM was used for encrypting E.
{
E: h'2e75eeae01b831e0b65c2976e06d90f4
82135bec5efef3be3d31520b2fa8c6fb
f572f817203bf7a0940bb6183697567c
e291b03e9fca5e9cbdfa7e560322d4ed
3a659f44a542e55331a1a9f43d7f',
K: "key0",
V: h'48ae5a81b87241d81618f56cab0b65ec
441202f81faabbe10075b20cb57fa939'
}
The decrypted contents of E are depicted below (whitespace has been
added to improve readability). The presence of the attribute V
indicates that the DTLS PSK Transfer is used to convey the session
key (cf. Section 6.1).
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{
F: {
SAI: [ "/s/tempC", 1 ],
TS: 2938749,
L: 3600,
G: hmac_sha256
},
V: h'48ae5a81b87241d81618f56cab0b65ec
441202f81faabbe10075b20cb57fa939'
}
6. DTLS PSK Generation Methods
One goal of the DCAF protocol is to provide for a DTLS PSK shared
between C and RS. SAM and RS MUST negotiate the method for the DTLS
PSK generation.
6.1. DTLS PSK Transfer
The DTLS PSK is generated by AS and transmitted to C and RS using a
secure channel.
The DTLS PSK transfer method is defined as follows:
o SAM generates the DTLS PSK using an algorithm of its choice
o SAM MUST include a representation of the DTLS PSK in Face and
encrypt it together with all other information in Face with a key
K(SAM,RS) it shares with RS. How SAM and RS exchange K(SAM,RS) is
not in the scope of this document. SAM and RS MAY use their
preshared key as K(SAM,RS).
o SAM MUST include a representation of the DTLS PSK in the Verifier.
o As SAM and C do not have a shared secret, the Verifier MUST be
transmitted to C using encrypted channels.
o RS MUST decrypt Face using K(SAM,RS)
6.2. Distributed Key Derivation
SAM generates a DTLS PSK for C which is transmitted using a secure
channel. RS generates its own version of the DTLS PSK using the
information contained in Face (see also Section 4.1).
The distributed key derivation method is defined as follows:
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o SAM and RS both generate the DTLS PSK using the information
included in Face. They use an HMAC algorithm on Face with a
shared key K(SAM,RS). The result serves as the DTLS PSK. How SAM
and RS exchange K(SAM,RS) is not in the scope of this document.
They MAY use their preshared key as K(SAM,RS). How SAM and RS
negotiate the used HMAC algorithm is also not in the scope of this
document. They MAY however use the HMAC algorithm they use for
their DTLS connection.
o SAM MUST include a representation of the DTLS PSK in the Verifier.
o As SAM and C do not have a shared secret, the Verifier MUST be
transmitted to C using encrypted channels.
o SAM MUST NOT include a representation of the DTLS PSK in Face.
o SAM MUST NOT encrypt Face.
7. Authorization Configuration
For the protocol defined in this document, proper configuration of
CAM and SAM is crucial. The principals that are in charge of the
resource, RS and SAM, and the principals that are in charge of C and
CAM need to define the respective permissions. The data
representation of these permissions are not in the scope of this
document.
8. Trust Relationships
C has a trust relationship with CAM: C trusts CAM to act in behalf of
C's owner. RS has a trust relationship with SAM: RS trusts SAM to
act in behalf of RS's owner.
Obviously, CAM trusts C with the specific permissions it hands over
to it. How this trust is established, is not in the scope of this
document. It may be achieved by using a bootstrapping mechanism
similar to [bergmann12].
Additionally, SAM and CAM need to have a trust relationship
established. Its establishment is also not in the scope of this
document. It fulfills the following conditions:
1. SAM has means to authenticate CAM (e.g. it has a certificate of
CAM or a PKI in which CAM is included) and vice versa
2. As far as SAM needs to rely on the different clients of CAM to
receive different permissions, it can be sure that CAM correctly
identifies these clients towards SAM and does not leak tickets
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that have been generated for a specific client C to another
client.
SAM trusts C indirectly because it trusts CAM and CAM vouches for C.
The DCAF Protocol does not provide any means for SAM to validate that
a resource request stems from C.
C indirectly trusts SAM with some potentially confidential
information, and that SAM correctly represents RS, because CAM trusts
SAM.
CAM trusts RS indirectly because it trusts SAM and SAM vouches for
RS.
C implicitly trusts RS with some potentially confidential information
because it trusts CAM and because RS can prove that it shares a key
with SAM.
CAM <------------------> SAM
/|\ /|\
| |
\|/ \|/
C ..................... RS
9. Listing Authorization Manager Information in a Resource Directory
CoAP utilizes the Web Linking format [RFC5988] to facilitate
discovery of services in an M2M environment. [RFC6690] defines
specific link parameters that can be used to describe resources to be
listed in a resource directory [I-D.ietf-core-resource-directory].
9.1. The "auth-request" Link Relation
This section defines a resource type "auth-request" that can be used
by clients to retrieve the request URI for a server's authorization
service. When used with the parameter rt in a web link, "auth-
request" indicates that the corresponding target URI can be used in a
POST message to request authorization for the resource and action
that are described in the request payload.
The Content-Format "application/dcaf+cbor with numeric identifier
TBD1 defined in this specification MAY be used to express access
requests and their responses.
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The following example shows the web link used by CAM in this document
to relay incoming Authorization Request messages to SAM. (Whitespace
is included only for readability.)
<client-authorize>;rt="auth-request";ct=TBD1
;title="Contact Remote Authorization Manager"
The resource directory that hosts the resource descriptions of RS
could list the following description. In this example, the URI
"ep/node138/a/switch2941" is relative to the resource context
"coaps://sam.example.com/", i.e. the Server Authorization Manager
SAM.
<ep/node138/a/switch2941>;rt="auth-request";ct=TBD1;ep="node138"
;title="Request Client Authorization"
;anchor="coaps://sam.example.com/"
10. Examples
This section gives a number of short examples with message flows for
the initial Unauthorized Resource Request and the subsequent
retrieval of a ticket from SAM. The notation here follows the actors
conventions defined in Section 1.2.1. The payload format is encoded
as proposed in Section 5. The IP address of SAM is 2001:DB8::1, the
IP address of RS is 2001:DB8::dcaf:1234, and C's IP address is
2001:DB8::c.
10.1. Access Granted
This example shows an Unauthorized PUT request from C to RS that is
answered with a SAM Information message. C then sends a POST request
to CAM with a description of its intended request. CAM forwards this
request to SAM using CoAP over a DTLS-secured channel. The response
from SAM contains an access ticket that is relayed back to CAM.
C --> RS
PUT a/switch2941 [Mid=1234]
Content-Format: application/senml+json
{"e": [{"bv": "1"}]}
C <-- RS
4.01 Unauthorized [Mid=1234]
Content-Format: application/dcaf+cbor
{SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941"}
C --> CAM
POST client-authorize [Mid=1235,Token="tok"]
Content-Format: application/dcaf+cbor
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{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 4]
}
CAM --> SAM [Mid=23146]
POST ep/node138/a/switch2941
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 4]
}
CAM <-- SAM
2.05 Content [Mid=23146]
Content-Format: application/dcaf+cbor
{ F: {
SAI: ["a/switch2941", 5],
TS: 0("2013-07-04T20:17:38.002"),
G: hmac_sha256
},
V: h'7ba4d9e287c8b69dd52fd3498fb8d26d
9503611917b014ee6ec2a570d857987a'
}
C <-- CAM
2.05 Content [Mid=1235,Token="tok"]
Content-Format: application/dcaf+cbor
{ F: {
SAI: ["a/switch2941", 5],
TS: 0("2013-07-04T20:17:38.002"),
G: hmac_sha256
},
V: h'7ba4d9e287c8b69dd52fd3498fb8d26d
9503611917b014ee6ec2a570d857987a'
}
C --> RS
ClientHello (TLS_PSK_WITH_AES_128_CCM_8)
C <-- RS
ServerHello (TLS_PSK_WITH_AES_128_CCM_8)
ServerHelloDone
C --> RS
ClientKeyExchange
psk_identity=0xa301826c612f73776974636832393431
0x0505c077323031332d30372d30345432
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0x303a31373a33382e3030320700
(C decodes the contents of V and uses the result as PSK)
ChangeCipherSpec
Finished
(RS calculates PSK from SAI, TS and its session key
HMAC_sha256(0xa301826c612f73776974636832393431
0x0505c077323031332d30372d30345432
0x303a31373a33382e3030320700,
0x736563726574)
= 0x7ba4d9e287c8...
)
C <-- RS
ChangeCipherSpec
Finished
10.2. Access Denied
This example shows a denied Authorization request for the DELETE
operation.
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C --> RS
DELETE a/switch2941
C <-- RS
4.01 Unauthorized
Content-Format: application/dcaf+cbor
{SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941"}
C --> CAM
POST client-authorize
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 8]
}
CAM --> SAM
POST ep/node138/a/switch2941
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 8]
}
CAM <-- SAM
2.05 Content
Content-Format: application/dcaf+cbor
C <-- CAM
2.05 Content
Content-Format: application/dcaf+cbor
10.3. Access Restricted
This example shows a denied Authorization request for the operations
GET, PUT, and DELETE. SAM grants access for PUT only.
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CAM --> SAM
POST ep/node138/a/switch2941
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 13]
}
CAM <-- SAM
2.05 Content
Content-Format: application/dcaf+cbor
{ F: {
SAI: ["a/switch2941", 5],
TS: 0("2013-07-04T21:33:11.930"),
G: hmac_sha256
},
V: h'c7b5774f2ddcbd548f4ad74b30a1b2e5
b6b04e66a9995edd2545e5a06216c53d'
}
10.4. Implicit Authorization
This example shows an Authorization request using implicit
authorization. CAM initially requests the actions GET and POST on
the resource "coaps://[2001:DB8::dcaf:1234]/a/switch2941". SAM
returns a ticket that has no SAI field in its ticket Face, hence
implicitly authorizing C.
CAM --> SAM
POST ep/node138/a/switch2941
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 3]
}
CAM <-- SAM
2.05 Content
Content-Format: application/dcaf+cbor
{ F: {
TS: 0("2013-07-16T10:15:43.663"),
G: hmac_sha256
},
V: h'4f7b0e7fdcc498fb2ece648bf6bdf736
61a6067e51278a0078e5b8217147ea06'
}
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11. Specific Usage Scenarios
The general DCAF architure outlined in Section 3.1 illustrates the
various actors who participate in the message exchange for
authenticated authorization. The message types defined in this
document cover the most general case where all four actors are
separate entities that may or may not reside on the same device.
Special implementation considerations apply when one single entity
takes the role of more than one actor. This section gives advice on
the most common usage scenarios where the Client Authorization
Manager and Client, the Server Authorization Manager and Resource
Server or both Authorization Managers reside on the same (less-
constrained) device and have a means of secure communication outside
the scope of this document.
11.1. Combined Authorization Manager and Client
When CAM and C reside on the same (less-constrained) device, the
Access Request and Ticket Transfer messages can be substituted by
other means of secure communication. Figure 9 shows a simplified
message exchange for a combined CAM+C device.
CAM+C RS SAM
| | <== DTLS chan. ==> |
| [Resource Req.-->] | |
| | |
| [<-- SAM Info.] | |
| | |
| <==== TLS/DTLS chan. (Mutual Auth) ===> |
| | |
| Ticket Request ---------------------> |
| | |
| <--------------------- Ticket Grant |
| | |
| <== DTLS chan. ==> | |
| Auth. Res. Req. -> | |
Figure 9: Combined Client Authorization Manager and Client
11.1.1. Creating the Ticket Request Message
When CAM+C receives an SAM Information message as a reaction to an
Unauthorized Request message, it creates a Ticket Request message as
follows:
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1. The destination of the Ticket Request message is derived from the
authority information in the URI contained in field "SAM" of the
SAM Information message payload.
2. The request method is POST.
3. The request URI is constructed from the SAM field received in the
SAM Information message payload.
4. The payload contains the SAM field from the SAM Information
message, an absolute URI of the resource that CAM+C wants to
access, the actions that CAM+C wants to perform on the resource,
and any time stamp generated by RS that was transferred with the
SAM Information message.
5. A label that describes CAM+C is added to the payload.
11.1.2. Processing the Ticket Grant Message
Based on the Ticket Grant message, CAM+C is able to establish a DTLS
channel with RS. To do so, CAM+C sets the psk_identity field of the
DTLS ClientKeyExchange message to the ticket Face received in the
Ticket Grant message and uses the ticket Verifier as PSK when
constructing the premaster secret.
11.2. Combined Client Authorization Manager and Server Authorization
Manager
In certain scenarios, CAM and SAM may be combined to a single entity
that knows both, C and RS, and decides if their actions are
authorized. Therefore, no explicit communication between CAM and SAM
is necessary, resulting in omission of the Ticket Request and Ticket
Grant messages. Figure 10 depicts the resulting message sequence in
this simplified architecture.
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C CAM+SAM RS
| <== DTLS chan. ==> | <== DTLS chan. ==> |
| | |
| [Resource Req.----------------------->] |
| | |
| [<-------------------- SAM Information] |
| | |
| Access Request --> | |
| | |
| <-- Ticket Transf. | |
| | |
| <=========== DTLS channel ===========> |
| | |
| Authorized Resource Request ----------> |
Figure 10: Combined Client Authorization Manager and Server
Authorization Manager
11.2.1. Processing the Access Request Message
When receiving an Access Request message, CAM+SAM performs the checks
specified in Section 3.5 and returns a 4.00 (Bad Request) response in
case of failure. Otherwise, if the checks have succeeded, CAM+SAM
evaluates the contents of Access Request message as described in
Section 3.6.
The decision on the access request is performed by CAM+SAM with
respect to the stored policies. When the requested action is
permitted on the respective resource, CAM+SAM generates an access
ticket as outlined in Section 4.1 and creates a Ticket Transfer
message to convey the access ticket to the Client.
11.2.2. Creating the Ticket Transfer Message
A Ticket Transfer message is constructed as a 2.05 response with the
access ticket contained in its payload. The response MAY contain a
Max-Age option to indicate the ticket's lifetime to the receiving
Client.
This specification defines a CBOR data representation for the access
ticket as illustrated in Section 3.6.
11.3. Combined Server Authorization Manager and Resource Server
If SAM and RS are colocated in one entity (SAM+RS), the main
objective is to allow CAM to delegate access to C. Accordingly, the
authorization information could be replaced by a nonce internal to
SAM+RS. (TBD.)
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CAM C SAM+RS
| <== DTLS chan. ==> | |
| | [Resource Req.-->] |
| | |
| | [<-- SAM Info.] |
| | |
| <-- Access Req. | |
| | |
| <========= TLS/DTLS channel =========> |
| | |
| Ticket Request ---------------------> |
| | |
| <--------------------- Ticket Grant |
| | |
| Ticket Transf. --> | |
| | |
| | <== DTLS chan. ==> |
| | Auth. Res. Req. -> |
Figure 11: Combined Server Authorization Manager and Resource Server
12. Security Considerations
As this protocol builds on transitive trust between Authorization
Managers as mentioned in Section 8, SAM has no direct means to
validate that a resource request originates from C. It has to trust
CAM that it correctly vouches for C and that it does not give
authorization tickets meant for C to another client nor disclose the
contained session key.
The Authorization Managers also could constitute a single point of
failure. If the Server Authorization Manager fails, the resources on
all Resource Servers it is responsible for cannot be accessed any
more. If a Client Authorization Manager fails, all clients it is
responsible are not able to access resources on a Resource Server.
Thus, it is crucial for large networks to use Authorization Managers
in a redundant setup.
13. IANA Considerations
The following registrations are done following the procedure
specified in [RFC6838].
Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
with the RFC number of this specification.
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13.1. DTLS PSK Key Generation Methods
A sub-registry for the values indicating the PSK key generation
method as contents of the field G in a payload of type application/
dcaf+cbor is defined. Values in this sub-registry are numeric
integers encoded in Concise Binary Object Notation (CBOR, [RFC7049]).
This document follows the notation of [RFC7049] for binary values,
i.e. a number starts with the prefix "0b". The major type is
separated from the actual numeric value by an underscore to emphasize
the value's internal structure.
Initial entries in this sub-registry are as follows:
+---------------+-------------+------------+
| Encoded Value | Name | Reference |
+---------------+-------------+------------+
| 0b000_00000 | hmac_sha256 | [RFC-XXXX] |
| | | |
| 0b000_00001 | hmac_sha384 | [RFC-XXXX] |
| | | |
| 0b000_00010 | hmac_sha512 | [RFC-XXXX] |
+---------------+-------------+------------+
Table 3: DTLS PSK Key Generation Methods
New methods can be added to this registry based on designated expert
review according to [RFC5226].
(TBD: criteria for expert review.)
13.2. dcaf+cbor Media Type Registration
Type name: application
Subtype name: dcaf+cbor
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using a subset of the
encoding allowed in [RFC7049]. Specifically, only the primitive data
types String and Number are allowed. The type Number is restricted
to unsigned integers (i.e., no negative numbers, fractions or
exponents are allowed). Encoding MUST be UTF-8. These restrictions
simplify implementations on devices that have very limited memory
capacity.
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Security considerations: TBD
Interoperability considerations: TBD
Published specification: [RFC-XXXX]
Applications that use this media type: TBD
Additional information:
Magic number(s): none
File extension(s): dcaf
Macintosh file type code(s): none
Person & email address to contact for further information: TBD
Intended usage: COMMON
Restrictions on usage: None
Author: TBD
Change controller: IESG
13.3. CoAP Content Format Registration
This document specifies a new media type application/dcaf+cbor (cf.
Section 13.2). For use with CoAP, a numeric Content-Format
identifier is to be registered in the "CoAP Content-Formats" sub-
registry within the "CoRE Parameters" registry.
Note to RFC Editor: Please replace all occurrences of "RFC-XXXX" with
the RFC number of this specification.
+-----------------------+----------+------+------------+
| Media type | Encoding | Id. | Reference |
+-----------------------+----------+------+------------+
| application/dcaf+cbor | - | TBD1 | [RFC-XXXX] |
+-----------------------+----------+------+------------+
14. References
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14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279, December
2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC
6838, January 2013.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, October 2013.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, June 2014.
14.2. Informative References
[I-D.bormann-core-ace-aif]
Bormann, C., "An Authorization Information Format (AIF)
for ACE", draft-bormann-core-ace-aif-01 (work in
progress), July 2014.
[I-D.gerdes-ace-actors]
Gerdes, S., "Actors in the ACE Architecture", draft-
gerdes-ace-actors-02 (work in progress), October 2014.
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-16 (work in progress), October 2014.
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[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP", draft-ietf-
core-observe-16 (work in progress), December 2014.
[I-D.ietf-core-resource-directory]
Shelby, Z. and C. Bormann, "CoRE Resource Directory",
draft-ietf-core-resource-directory-02 (work in progress),
November 2014.
[I-D.schmertmann-dice-codtls]
Schmertmann, L., Hartke, K., and C. Bormann, "CoDTLS: DTLS
handshakes over CoAP", draft-schmertmann-dice-codtls-01
(work in progress), August 2014.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655, July 2012.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, August 2012.
[bergmann12]
Bergmann, O., Gerdes, S., Schaefer, S., Junge, F., and C.
Bormann, "Secure Bootstrapping of Nodes in a CoAP
Network", IEEE Wireless Communications and Networking
Conference Workshops (WCNCW), April 2012.
Authors' Addresses
Stefanie Gerdes
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63906
Email: gerdes@tzi.org
Olaf Bergmann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63904
Email: bergmann@tzi.org
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Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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