Pub-Sub Profile for Authentication and Authorization for Constrained Environments (ACE)
draft-ietf-ace-pubsub-profile-03
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| Authors | Francesca Palombini , Cigdem Sengul | ||
| Last updated | 2021-06-30 | ||
| Replaces | draft-palombini-ace-coap-pubsub-profile | ||
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draft-ietf-ace-pubsub-profile-03
ACE Working Group F. Palombini
Internet-Draft Ericsson
Intended status: Standards Track C. Sengul
Expires: 1 January 2022 Brunel University
30 June 2021
Pub-Sub Profile for Authentication and Authorization for Constrained
Environments (ACE)
draft-ietf-ace-pubsub-profile-03
Abstract
This specification defines an application profile for authentication
and authorization for publishers and subscribers in a constrained
pub-sub scenario, using the ACE framework. This profile relies on
transport layer or application layer security to authorize the pub-
sub clients to the broker. Moreover, it describes application layer
security for publisher-subscriber communication going through the
broker.
Note to Readers
Source for this draft and an issue tracker can be found at
https://github.com/ace-wg/pubsub-profile (https://github.com/ace-wg/
pubsub-profile).
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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This Internet-Draft will expire on 1 January 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Application Profile Overview . . . . . . . . . . . . . . . . 3
3. PubSub Authorisation . . . . . . . . . . . . . . . . . . . . 5
3.1. AS Discovery (Optional) . . . . . . . . . . . . . . . . . 6
3.2. Authorising to the Broker . . . . . . . . . . . . . . . . 6
4. Key Distribution for PubSub Content Protection . . . . . . . 7
4.1. Token POST . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Join Request . . . . . . . . . . . . . . . . . . . . . . 8
5. PubSub Protected Communication . . . . . . . . . . . . . . . 11
5.1. Using COSE Objects To Protect The Resource
Representation . . . . . . . . . . . . . . . . . . . . . 12
6. Profile-specific Considerations . . . . . . . . . . . . . . . 13
6.1. CoAP PubSub Application Profile . . . . . . . . . . . . . 13
6.2. MQTT PubSub Application Profile . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8.1. ACE Groupcomm Profile Registry . . . . . . . . . . . . . 16
8.1.1. CoAP Profile Registration . . . . . . . . . . . . . . 16
8.1.2. MQTT Profile Registration . . . . . . . . . . . . . . 17
8.2. ACE Groupcomm Key Registry . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Requirements on Application Profiles . . . . . . . . 19
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
In the publish-subscribe (pub-sub) scenario, devices with limited
reachability communicate via a broker, which enables store-and-
forward messaging between the devices. This document defines a way
to authorize pub-sub clients using the ACE framework
[I-D.ietf-ace-oauth-authz], and to provide the keys for protecting
the communication between them. The pub-sub communication using the
Constrained Application Protocol (CoAP) is specified in
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[I-D.ietf-core-coap-pubsub], while the one using MQTT is specified in
[MQTT-OASIS-Standard-v5]. This document gives detailed
specifications for MQTT and CoAP pub-sub, but can easily be adapted
for other transport protocols as well.
1.1. 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
described in [I-D.ietf-ace-oauth-authz],
[I-D.ietf-ace-key-groupcomm]. In particular, analogously to
[I-D.ietf-ace-oauth-authz], terminology for entities in the
architecture such as Client (C), Resource Server (RS), and
Authorization Server (AS) is defined in OAuth 2.0 [RFC6749] and
[I-D.ietf-ace-actors], and terminology for entities such as the Key
Distribution Center (KDC) and Dispatcher in
[I-D.ietf-ace-key-groupcomm].
Readers are expected to be familiar with terms and concepts of pub-
sub group communication, as described in [I-D.ietf-core-coap-pubsub],
or MQTT [MQTT-OASIS-Standard-v5].
2. Application Profile Overview
The objective of this document is to specify how to authorize nodes,
provide keys, and protect a pub-sub communication, using
[I-D.ietf-ace-key-groupcomm], which expands from the ACE framework
([I-D.ietf-ace-oauth-authz]), and transport profiles
([I-D.ietf-ace-dtls-authorize], [I-D.ietf-ace-oscore-profile],
[I-D.ietf-ace-mqtt-tls-profile]). The pub-sub communication protocol
can be based on CoAP, as described in [I-D.ietf-core-coap-pubsub],
MQTT [MQTT-OASIS-Standard-v5] , or other transport. Note that both
publishers and subscribers use the same profiles.
The architecture of the scenario is shown in Figure 1.
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+----------------+ +----------------+
| | | Key |
| Authorization | | Distribution |
| Server | | Center |
| (AS) | | (KDC) |
+----------------+ +----------------+
^ ^
| |
+---------(A)----+ |
| +--------------------(B)--------+
v v
+------------+ +------------+
| | | |
| Pub-Sub | <-- (C)---> | Broker |
| Client | | |
| | | |
+------------+ +------------+
Figure 1: Architecture for Pub-Sub with Authorization Servers
Publisher or Subscriber Clients is referred to as Client in short.
This profile specifies:
1. The establishment of a secure connection between a Client and
Broker, using an ACE transport profile such as DTLS
[I-D.ietf-ace-dtls-authorize], OSCORE
[I-D.ietf-ace-oscore-profile], or MQTT-TLS
[I-D.ietf-ace-mqtt-tls-profile] (A and C).
2. The Clients retrieval of keying material for the Publisher Client
to publish protected publications to the Broker, and for the
Subscriber Client to read protected publications (B).
These exchanges aim at setting up two different security
associations. On the one hand, the Publisher and the Subscriber
clients have a security association with the Broker (i.e. RS), so
that RS can authorize the Clients (Security Association 1). On the
other hand, the Publisher has a security association with the
Subscriber, to protect the publication content (Security Association
2) while sending it through the broker (i.e. here, the broker
corresponds to the Dispatcher in [I-D.ietf-ace-key-groupcomm]). The
Security Association 1 is set up using AS and a transport profile of
[I-D.ietf-ace-oauth-authz], the Security Association 2 is set up
using AS, KDC and [I-D.ietf-ace-key-groupcomm]. Note that, given
that the publication content is protected, the Broker MAY accept
unauthorised Subscribers. In this case, the Subscriber client can
skip setting up Security Association 1 with the Broker.
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+------------+ +------------+ +------------+
| | | | | |
| Publisher | | Broker | | Subscriber |
| | | | | |
| | | | | |
+------------+ +------------+ +------------+
: : : : : :
: '------ Security -------' '-----------------------' :
: Association 1 :
'------------------------------- Security --------------'
Association 2
Figure 2: Security Associations between Publisher, Broker,
Subscriber pairs.
3. PubSub Authorisation
Since [I-D.ietf-ace-oauth-authz] recommends the use of CoAP and CBOR,
this document describes the exchanges assuming CoAP and CBOR are
used. However, using HTTP instead of CoAP is possible, using the
corresponding parameters and methods. Analogously, JSON [RFC8259]
can be used instead of CBOR, using the conversion method specified in
Sections 6.1 and 6.2 of [RFC8949]. In case JSON is used, the Content
Format or Media Type of the message has to be changed accordingly.
Exact definition of these exchanges are considered out of scope for
this document.
Figure 3 shows the message flow for authorisation purposes.
Client Broker AS KDC
| [--Resource Request (CoAP/MQTT/other)-->] | | |
| | | |
| [<----AS Information (CoAP/MQTT/other)--] | | |
| | |
| ----- Authorisation Request (CoAP/HTTP/other)---->| |
| | |
| <------Authorisation Response (CoAP/HTTP/other) --| |
| |
|----------------------Token Post (CoAP)------------------->|
| |
|------------------- Joining Request (CoAP) --------------->|
| |
|------------------ Joining Response (CoAP) --------------->|
Figure 3: Authorisation Flow
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3.1. AS Discovery (Optional)
Complementary to what is defined in [I-D.ietf-ace-oauth-authz]
(Section 5.1) for AS discovery, the Broker MAY send the address of
the AS to the Client in the 'AS' parameter in the AS Information as a
response to an Unauthorized Resource Request (Section 5.2). An
example using CBOR diagnostic notation and CoAP is given below:
4.01 Unauthorized
Content-Format: application/ace+cbor
{"AS": "coaps://as.example.com/token"}
Figure 4: AS Information example
Authorisation Server (AS) Discovery is also defined in
Section 2.2.6.1 of [I-D.ietf-ace-mqtt-tls-profile] for MQTT v5
clients (and not supported for MQTT v3 clients).
3.2. Authorising to the Broker
After retrieving the AS address, the Client sends an Authorisation
Request to the AS for the KDC and the Broker. Note that the AS
authorises:
1. What endpoints are allowed to Publish or Subscribe to the Broker.
2. What endpoints are allowed to access to which topic(s).
The request includes the following fields from the Authorization
Request (Section 3.1 of [I-D.ietf-ace-key-groupcomm]):
* 'scope', containing the topic identifier, that the Client wishes
to access
* 'audience', an array with identifiers of the KDC and the Broker.
Other additional parameters can be included if necessary, as defined
in [I-D.ietf-ace-oauth-authz].
The 'scope' parameter is encoded as follows, where 'gname' is treated
as topic identifier or filter.
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gname = tstr
role = tstr
scope_entry = [ gname , ? ( role / [ 2*role ] ) ]
scope = << [ + scope_entry ] >>
Figure 5: CDLL definition of scope, using as example group name
encoded as tstr and role as tstr.
Other scope representations are also possible and are described in
(Section 3.1 of [I-D.ietf-ace-key-groupcomm]). Note that in the AIF-
MQTT data model is described in Section 3 of the
[I-D.ietf-ace-mqtt-tls-profile], the role values have been further
constrained to "pub" and "sub".
The AS responds with an Authorization Response as defined in
Section 5.8.2 of [I-D.ietf-ace-oauth-authz] and Section 3.2 of
[I-D.ietf-ace-key-groupcomm]. If a token is returned, then the
audience of this token are the KDC and the Broker, and the client
uses the same token for both. In case CoAP PubSub is used as
communication protocol, 'profile' is set to "coap_pubsub_app" as
defined in Section 8.1.1. In case MQTT PubSub is used as
communication protocol, 'profile' is set to "mqtt_pubsub_app" as
defined in Section 8.1.2.
4. Key Distribution for PubSub Content Protection
4.1. Token POST
After receiving a token from the AS, the Client posts the token to
the KDC (Section 3.3 [I-D.ietf-ace-key-groupcomm]). In addition to
the token post, a Subscriber Client MAY ask for the public keys in
the group, used for source authentication, as well as any other group
parameters. In this case, the message MUST have Content-Format set
to "application/ace+cbor" defined in Section 8.16 of
[I-D.ietf-ace-oauth-authz]. The message payload MUST be formatted as
a CBOR map, which MUST include the access token and the 'sign_info'
parameter. The details for the 'sign_info' parameter can be found in
Section 3.3 of [I-D.ietf-ace-key-groupcomm]. Alternatively, the
joining node may retrieve this information by other means as
described in [I-D.ietf-ace-key-groupcomm].
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The KDC verifies that the Client is authorized to access the topic
with the requested role. After successful verification, the Client
is authorized to receive the group keying material from the KDC and
join the group. The KDC replies to the Client with a 2.01 (Created)
response, using Content-Format "application/ace+cbor". The payload
of the 2.01 response is a CBOR map.
A Publisher Client MUST send its own public key to the KDC when
joining the group. Since the access token from a Publisher Client
will have "pub" role, the KDC MUST include 'kdcchallenge' in the CBOR
map, specifying a dedicated challenge N_S generated by the KDC. The
Client uses this challenge to prove possession of its own private key
(see [I-D.ietf-ace-key-groupcomm] for details).
4.2. Join Request
In the next step, a joining node MUST have a secure communication
association established with the KDC, before starting to join a group
under that KDC. Possible ways to provide a secure communication
association are described in the DTLS transport profile
[I-D.ietf-ace-dtls-authorize] and OSCORE transport profile
[I-D.ietf-ace-oscore-profile] of ACE.
After establishing a secure communication, the Client sends a Joining
Request to the KDC as described in Section 4.3 of
[I-D.ietf-ace-key-groupcomm]. More specifically, the Client sends a
POST request to the /ace-group/GROUPNAME endpoint on KDC, with
Content-Format = "application/ace+cbor" that MUST contain in the
payload (formatted as a CBOR map, Section 4.1.2.1 of
[I-D.ietf-ace-key-groupcomm]):
* 'scope' parameter as defined earlier
* 'get_pub_keys' parameter set to the empty array if the Client
needs to retrieve the public keys of the other pubsub members,
* 'client_cred' parameter containing the Client's public key
formatted as a COSE_Key (as defined in Section 8.2), if the Client
is a Publisher,
* 'cnonce', encoded as a CBOR byte string, and including a dedicated
nonce N_C generated by the Client, if 'client_cred' is present,
* 'client_cred_verify', set to a singature computed over the
'rsnonce' concatenated with cnonce, if 'client_cred' is present,
* OPTIONALLY, if needed, the 'pub_keys_repos' parameter
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TODO: Check 'cnonce'
Note that for a Subscriber-only Client, the Joining Request MUST NOT
contain the 'client_cred parameter', the role element in the 'scope'
parameter MUST be set to "sub". The Subscriber MUST have access to
the public keys of all the Publishers; this MAY be achieved in the
Joining Request by using the parameter 'get_pub_keys' set to receive
the public key of all Publishers using "pub" as the 'role_filter' (as
described in Section 4.1.2.1 of [I-D.ietf-ace-key-groupcomm]).
If the 'client_cred' parameter is present, KDC stores the public key
of the Client. Note that the alg parameter in the 'client_cred'
COSE_Key MUST be a signing algorithm, as defined in section 8 of
[RFC8152], and that it is the same algorithm used to compute the
signature sent in 'client_cred_verify'.
The KDC response to Joining Response has the Content-Format =
"application/ace+cbor". The payload (formatted as a CBOR map) MUST
contain the following fields from the Joining Response (Section 4.2
of [I-D.ietf-ace-key-groupcomm]):
* 'kty' identifies a key type "COSE_Key".
* 'key', which contains a "COSE_Key" object (defined in [RFC8152],
containing:
- 'kty' with value 4 (symmetric)
- 'alg' with value defined by the AS2 (Content Encryption
Algorithm)
- 'Base IV' with value defined by the AS2
- 'k' with value the symmetric key value
- OPTIONALLY, 'kid' with an identifier for the key value
* OPTIONALLY, 'exp' with the expiration time of the key
* 'pub_keys', containing the public keys of all authorized signing
members formatted as COSE_Keys, if the 'get_pub_keys' parameter
was present and set to the empty array in the Key Distribution
Request. For Subscriber Clients, the Joining Response MUST
contain the 'pub_keys' parameter.
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An example of the Joining Request and corresponding Response for a
CoAP Publisher using CoAP and CBOR is specified in Figure 6 and
Figure 7, where SIG is a signature computed using the private key
associated to the public key and the algorithm in 'client_cred'.
{
"scope" : ["Broker1/Temp", "pub"],
"client_cred" :
{ / COSE_Key /
/ type / 1 : 2, / EC2 /
/ kid / 2 : h'11',
/ alg / 3 : -7, / ECDSA with SHA-256 /
/ crv / -1 : 1 , / P-256 /
/ x / -2 : h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de1
08de439c08551d',
/ y /-3 : h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e
9eecd0084d19c',
"cnonce" : h'd36b581d1eef9c7c,
"client_cred_verify" : SIG
}
}
Figure 6: Joining Request payload for a Publisher
{
"kty" : "COSE_Key",
"key" : {1: 4, 2: h'1234', 3: 12, 5: h'1f389d14d17dc7',
-1: h'02e2cc3a9b92855220f255fff1c615bc'}
}
Figure 7: Joining Response payload for a Publisher
An example of the payload of a Joining Request and corresponding
Response for a Subscriber using CoAP and CBOR is specified in
Figure 8 and Figure 9.
{
"scope" : ["Broker1/Temp", "sub"],
"get_pub_keys" : [true, ["pub"], []]
}
Figure 8: Joining Request payload for a Subscriber
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{
"scope" : ["Broker1/Temp", "sub"],
"kty" : "COSE_Key"
"key" : {1: 4, 2: h'1234', 3: 12, 5: h'1f389d14d17dc7',
-1: h'02e2cc3a9b92855220f255fff1c615bc'},
"pub_keys" : [
{
1 : 2, / type EC2 /
2 : h'11', / kid /
3 : -7, / alg ECDSA with SHA-256 /
-1 : 1 , / crv P-256 /
-2 : h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de43
9c08551d', / x /
-3 : h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd
0084d19c' / y /
}
]
}
Figure 9: Joining Response payload for a Subscriber
5. PubSub Protected Communication
+------------+ +------------+ +------------+
| | | | | |
| Publisher | ----(D)---> | Broker | | Subscriber |
| | | | <----(E)---- | |
| | | | -----(F)---> | |
+------------+ +------------+ +------------+
Figure 10: Secure communication between Publisher and Subscriber
(D) corresponds to the publication of a topic on the Broker. The
publication (the resource representation) is protected with COSE
([RFC8152]). The (E) message is the subscription of the Subscriber.
The subscription MAY be unprotected. The (F) message is the response
from the Broker, where the publication is protected with COSE.
Publisher Broker Subscriber
| --- PUT /topic ----> | |
| protected with COSE | |
| | <--- GET /topic ----- |
| | |
| | ---- response ------> |
| | protected with COSE |
Figure 11: (E), (F), (G): Example of protected communication for CoAP
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The flow graph is presented below for CoAP. The message flow is
similar for MQTT, where PUT corresponds to a PUBLISH message, and GET
corresponds to a SUBSCRIBE message. Whenever a Client publishes a
new message, the Broker sends this message to all valid subscribers.
5.1. Using COSE Objects To Protect The Resource Representation
The Publisher uses the symmetric COSE Key received from the KDC
(Section 4) to protect the payload of the PUBLISH operation
(Section 4.3 of [I-D.ietf-core-coap-pubsub] and
[MQTT-OASIS-Standard-v5]). Specifically, the COSE Key is used to
create a COSE_Encrypt0 with algorithm specified by KDC. The
Publisher uses the private key corresponding to the public key sent
to the KDC in exchange B (Section 4) to countersign the COSE Object
as specified in Section 4.5 of [RFC8152]. The payload is replaced by
the COSE object before the publication is sent to the Broker.
The Subscriber uses the 'kid' in the 'countersignature' field in the
COSE object to retrieve the right public key to verify the
countersignature. It then uses the symmetric key received from KDC
to verify and decrypt the publication received in the payload from
the Broker (in the case of CoAP the publication is received by the
CoAP Notification and for MQTT, it is received as a PUBLISH message
from the Broker to the subscribing client).
The COSE object is constructed in the following way:
* The protected Headers (as described in Section 3 of [RFC8152]) MAY
contain the kid parameter, with value the kid of the symmetric
COSE Key received in Section 4 and MUST contain the content
encryption algorithm.
* The unprotected Headers MUST contain the Partial IV, with value a
sequence number that is incremented for every message sent, and
the counter signature that includes:
- the algorithm (same value as in the asymmetric COSE Key
received in (B)) in the protected header;
- the kid (same value as the kid of the asymmetric COSE Key
received in (B)) in the unprotected header;
- the signature computed as specified in Section 4.5 of
[RFC8152].
* The ciphertext, computed over the plaintext that MUST contain the
message payload.
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The 'external_aad' is an empty string.
An example is given in Figure 12:
16(
[
/ protected / h'a2010c04421234' / {
\ alg \ 1:12, \ AES-CCM-64-64-128 \
\ kid \ 4: h'1234'
} / ,
/ unprotected / {
/ iv / 5:h'89f52f65a1c580',
/ countersign / 7:[
/ protected / h'a10126' / {
\ alg \ 1:-7
} / ,
/ unprotected / {
/ kid / 4:h'11'
},
/ signature / SIG / 64 bytes signature /
]
},
/ ciphertext / h'8df0a3b62fccff37aa313c8020e971f8aC8d'
]
)
Figure 12: Example of COSE Object sent in the payload of a
PUBLISH operation
The encryption and decryption operations are described in sections
5.3 and 5.4 of [RFC8152].
6. Profile-specific Considerations
This section summarises the CoAP and MQTT specific pub-sub
communications, and considerations respectively.
6.1. CoAP PubSub Application Profile
A CoAP Pub-Sub Client and Broker use an ACE transport profile such as
DTLS [I-D.ietf-ace-dtls-authorize], OSCORE
[I-D.ietf-ace-oscore-profile].
As shown in Figure 1, (A) is an Access Token Request and Response
exchange between Publisher and Authorization Server to retrieve the
Access Token and RS (Broker) Information. As specified, the Client
has the role of a CoAP client, the Broker has the role of the CoAP
server.
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(B) corresponds to the retrieval of the keying material to protect
the publication end-to-end (see Section 5.1), and uses
[I-D.ietf-ace-key-groupcomm]. The details are defined in Section 4.
(C) corresponds to the exchange between the Client and the Broker,
where the Client sends its access token to the Broker and establishes
a secure connection with the Broker. Depending on the Information
received in (A), this can be for example DTLS handshake, or other
protocols. Depending on the application, there may not be the need
for this set up phase: for example, if OSCORE is used directly. Note
that, in line with what defined in the ACE transport profile used,
the access token includes the scope (i.e. pubsub topics on the
Broker) the Publisher is allowed to publish to. For implementation
simplicity, it is RECOMMENDED that the ACE transport profile used and
this specification use the same format of "scope".
After the previous phases have taken place, the pub-sub communication
can commence. The operations of publishing and subscribing are
defined in [I-D.ietf-core-coap-pubsub].
6.2. MQTT PubSub Application Profile
The steps MQTT clients go through are similar to the CoAP clients as
described in Section 6.1. The payload that is carried in MQTT
messages will be protected using COSE.
In MQTT, topics are organised as a tree, and in the
[I-D.ietf-ace-mqtt-tls-profile] 'scope' captures permissions for not
a single topic but a topic filter. Therefore, topic names (i.e.,
group names) may include wildcards spanning several levels of the
topic tree. Hence, it is important to distinguish application groups
and security groups defined in [I-D.ietf-core-groupcomm-bis]. An
application group has relevance at the application level - for
example, in MQTT an application group could denote all topics stored
under ""home/lights/". On the other hand, a security group is a
group of endpoints that each store group security material to
exchange secure communication within the group. The group
communication in [I-D.ietf-ace-key-groupcomm] refers to security
groups.
To be able join the right security group associated with requested
topics (application groups), the client needs to discover the
(application group, security group) association. In MQTT, $SYS/ has
been widely adopted as a prefix to topics that contain broker-
specific information, and hence, can be used by the broker for this
purpose. In typical implementations, Clients that subscribe to one
or more SYS-Topics receive the current value on the SYS topics as
soon as they subscribe, and then after periodically.
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For an MQTT client we envision the following steps to take place:
1. Client learns the (application group, security group)
associations from the $SYS topic (this topic is RECOMMENDED to be
a protected topic). These associations MAY be published under
another topic.
2. Client computes the corresponding security groups for its
application groups, and sends token requests for the security
groups to AS.
3. Client sends join requests to KDC to gets the keys for these
security groups.
4. Client authorises to the Broker with the token (described in
[I-D.ietf-ace-mqtt-tls-profile]).
5. A Publisher Client sends PUBLISH messages for a given topic and
protects the payload with the corresponding key for the
associated security group. RS validates the PUBLISH message by
checking the topic's security group association and the stored
token.
6. A Subscriber Client may send SUBSCRIBE messages with one or
multiple topic filters. A topic filter may correspond to
multiple topics but MUST belong to a single security group. If
requested topics are in multiple security groups, then these
topics SHOULD be separated into the corresponding topic filters
in the SUBSCRIBE message.
7. Security Considerations
In the profile described above, the Publisher and Subscriber use
asymmetric crypto, which would make the message exchange quite heavy
for small constrained devices. Moreover, all Subscribers must be
able to access the public keys of all the Publishers to a specific
topic to be able to verify the publications. Such a database could
be set up and managed by the same entity having control of the topic,
i.e. KDC.
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An application where it is not critical that only authorized
Publishers can publish on a topic may decide not to make use of the
asymmetric crypto and only use symmetric encryption/MAC to
confidentiality and integrity protection of the publication.
However, this is not recommended since, as a result, any authorized
Subscribers with access to the Broker may forge unauthorized
publications without being detected. In this symmetric case the
Subscribers would only need one symmetric key per topic, and would
not need to know any information about the Publishers, that can be
anonymous to it and the Broker.
Subscribers can be excluded from future publications through re-
keying for a certain topic. This could be set up to happen on a
regular basis, for certain applications. How this could be done is
out of scope for this work.
The Broker is only trusted with verifying that the Publisher is
authorized to publish, but is not trusted with the publications
itself, which it cannot read nor modify. In this setting, caching of
publications on the Broker is still allowed.
TODO: expand on security and privacy considerations
8. IANA Considerations
8.1. ACE Groupcomm Profile Registry
The following registrations are done for the "ACE Groupcomm Profile"
Registry following the procedure specified in
[I-D.ietf-ace-key-groupcomm].
Note to RFC Editor: Please replace all occurrences of "[[This
document]]" with the RFC number of this specification and delete this
paragraph.
8.1.1. CoAP Profile Registration
Name: coap_pubsub_app
Description: Profile for delegating client authentication and
authorization for publishers and subscribers in a CoAP pub-sub
setting scenario in a constrained environment.
CBOR Key: TBD
Reference: [[This document]]
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8.1.2. MQTT Profile Registration
Name: mqtt_pubsub_app
Description: Profile for delegating client authentication and
authorization for publishers and subscribers in a MQTT pub-sub
setting scenario in a constrained environment.
CBOR Key: TBD
Reference: [[This document]]
8.2. ACE Groupcomm Key Registry
The following registrations are done for the ACE Groupcomm Key
Registry following the procedure specified in
[I-D.ietf-ace-key-groupcomm].
Note to RFC Editor: Please replace all occurrences of "[[This
document]]" with the RFC number of this specification and delete this
paragraph.
Name: COSE_Key
Key Type Value: TBD
Profile: coap_pubsub_app
Description: COSE_Key object
References: [RFC8152], [[This document]]
9. References
9.1. Normative References
[I-D.ietf-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", Work in Progress, Internet-
Draft, draft-ietf-ace-key-groupcomm-11, 22 February 2021,
<https://www.ietf.org/archive/id/draft-ietf-ace-key-
groupcomm-11.txt>.
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
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draft-ietf-ace-oauth-authz-40, 26 April 2021,
<https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
authz-40.txt>.
[I-D.ietf-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", Work in Progress, Internet-Draft, draft-ietf-
core-coap-pubsub-09, 30 September 2019,
<https://www.ietf.org/archive/id/draft-ietf-core-coap-
pubsub-09.txt>.
[I-D.ietf-core-groupcomm-bis]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", Work in
Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
03, 22 February 2021, <https://www.ietf.org/archive/id/
draft-ietf-core-groupcomm-bis-03.txt>.
[MQTT-OASIS-Standard-v5]
Banks, A., Briggs, E., Borgendale, K., and R. Gupta,
"OASIS Standard MQTT Version 5.0", 2017,
<http://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-
v5.0-os.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
9.2. Informative References
[I-D.ietf-ace-actors]
Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
architecture for authorization in constrained
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environments", Work in Progress, Internet-Draft, draft-
ietf-ace-actors-07, 22 October 2018,
<https://www.ietf.org/archive/id/draft-ietf-ace-actors-
07.txt>.
[I-D.ietf-ace-dtls-authorize]
Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
L. Seitz, "Datagram Transport Layer Security (DTLS)
Profile for Authentication and Authorization for
Constrained Environments (ACE)", Work in Progress,
Internet-Draft, draft-ietf-ace-dtls-authorize-16, 8 March
2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
dtls-authorize-16.txt>.
[I-D.ietf-ace-mqtt-tls-profile]
Sengul, C. and A. Kirby, "Message Queuing Telemetry
Transport (MQTT)-TLS profile of Authentication and
Authorization for Constrained Environments (ACE)
Framework", Work in Progress, Internet-Draft, draft-ietf-
ace-mqtt-tls-profile-11, 14 April 2021,
<https://www.ietf.org/archive/id/draft-ietf-ace-mqtt-tls-
profile-11.txt>.
[I-D.ietf-ace-oscore-profile]
Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
"OSCORE Profile of the Authentication and Authorization
for Constrained Environments Framework", Work in Progress,
Internet-Draft, draft-ietf-ace-oscore-profile-18, 14 April
2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
oscore-profile-18.txt>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
Appendix A. Requirements on Application Profiles
This section lists the specifications on this profile based on the
requirements defined in Appendix A of [I-D.ietf-ace-key-groupcomm]
* REQ1: Specify the encoding and value of the identifier of group or
topic of 'scope': see Section 4).
* REQ2: Specify the encoding and value of roles of 'scope': see
Section 4).
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* REQ3: Optionally, specify the acceptable values for 'sign_alg':
TODO
* REQ4: Optionally, specify the acceptable values for
'sign_parameters': TODO
* REQ5: Optionally, specify the acceptable values for
'sign_key_parameters': TODO
* REQ6: Optionally, specify the acceptable values for 'pub_key_enc':
TODO
* REQ7: Specify the exact format of the 'key' value: COSE_Key, see
Section 4.
* REQ8: Specify the acceptable values of 'kty' : "COSE_Key", see
Section 4.
* REQ9: Specity the format of the identifiers of group members: TODO
* REQ10: Optionally, specify the format and content of
'group_policies' entries: not defined
* REQ11: Specify the communication protocol the members of the group
must use: CoAP pub/sub.
* REQ12: Specify the security protocol the group members must use to
protect their communication. This must provide encryption,
integrity and replay protection: Object Security of Content using
COSE, see Section 5.1.
* REQ13: Specify and register the application profile identifier :
"coap_pubsub_app", see Section 8.1.
* REQ14: Optionally, specify the encoding of public keys, of
'client_cred', and of 'pub_keys' if COSE_Keys are not used: NA.
* REQ15: Specify policies at the KDC to handle id that are not
included in get_pub_keys: TODO
* REQ16: Specify the format and content of 'group_policies': TODO
* REQ17: Specify the format of newly-generated individual keying
material for group members, or of the information to derive it,
and corresponding CBOR label : not defined
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* REQ18: Specify how the communication is secured between Client and
KDC. Optionally, specify tranport profile of ACE
[I-D.ietf-ace-oauth-authz] to use between Client and KDC: pre-set,
as KDC is AS.
* OPT1: Optionally, specify the encoding of public keys, of
'client_cred', and of 'pub_keys' if COSE_Keys are not used: NA
* OPT2: Optionally, specify the negotiation of parameter values for
signature algorithm and signature keys, if 'sign_info' and
'pub_key_enc' are not used: NA
* OPT3: Optionally, specify the format and content of
'mgt_key_material': not defined
* OPT4: Optionally, specify policies that instruct clients to retain
unsuccessfully decrypted messages and for how long, so that they
can be decrypted after getting updated keying material: not
defined
Acknowledgments
The author wishes to thank Ari Keraenen, John Mattsson, Ludwig Seitz,
Goeran Selander, Jim Schaad and Marco Tiloca for the useful
discussion and reviews that helped shape this document.
Authors' Addresses
Francesca Palombini
Ericsson
Email: francesca.palombini@ericsson.com
Cigdem Sengul
Brunel University
Email: csengul@acm.org
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