| ACE Working Group | F. Palombini |
| Internet-Draft | Ericsson |
| Intended status: Standards Track | November 04, 2019 |
| Expires: May 7, 2020 |
CoAP Pub-Sub Profile for Authentication and Authorization for Constrained Environments (ACE)
draft-palombini-ace-coap-pubsub-profile-06
This specification defines an application profile for authentication and authorization for publishers and subscribers in a pub-sub setting scenario in a constrained environment, using the ACE framework. This profile relies on transport layer or application layer security to authorize the publisher to the broker. Moreover, it relies on application layer security for publisher-broker and subscriber-broker communication.
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The publisher-subscriber setting allows for devices with limited reachability to communicate via a broker that enables store-and-forward messaging between the devices. The pub-sub scenario using the Constrained Application Protocol (CoAP) is specified in [I-D.ietf-core-coap-pubsub]. This document defines a way to authorize nodes in a CoAP pub-sub type of setting, using the ACE framework [I-D.ietf-ace-oauth-authz], and to provide the keys for protecting the communication between these nodes.
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] and [I-D.ietf-core-coap-pubsub]. 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].
The objective of this document is to specify how to authorize nodes, provide keys, and protect a CoAP pub-sub communication, as described in [I-D.ietf-core-coap-pubsub], using [I-D.ietf-ace-key-groupcomm], which itself expands the Ace framework ([I-D.ietf-ace-oauth-authz]), and transport profiles ([I-D.ietf-ace-dtls-authorize], [I-D.ietf-ace-oscore-profile]).
The architecture of the scenario is shown in Figure 1.
+----------------+ +----------------+
| | | |
| Authorization | | Authorization |
| Server 1 | | Server 2 |
| | | |
+----------------+ +----------------+
^ ^ ^
| | |
+---------(A)----+ | +-----(D)------+
| +--------------------(B)--------+ |
v v v
+------------+ +------------+ +------------+
| CoAP | ----(C)---> | CoAP | | CoAP |
| Client - | ----(E)---> | Server - | | Client - |
| | | | <----(F)---- | |
| Publisher | | Broker | -----(G)---> | Subscriber |
+------------+ +------------+ +------------+
Figure 1: Architecture CoAP pubsub with Authorization Servers
The RS is the broker, which contains the topic. This node corresponds to the Dispatcher, in [I-D.ietf-ace-key-groupcomm]. The AS1 hosts the policies about the Broker: what endpoints are allowed to Publish on the Broker. The Clients access this node to get write access to the Broker. The AS2 hosts the policies about the topic: what endpoints are allowed to access what topic. This node represents both the AS and Key Distribution Center roles from [I-D.ietf-ace-key-groupcomm].
There are four phases, the first three can be done in parallel.
This exchange aims at setting up 2 different security associations: on the one hand, the Publisher has a security association with the Broker, to protect the communication and securely authorize the Publisher to publish on a topic (Security Association 1). On the other hand, the Publisher has a security association with the Subscriber, to protect the publication content itself (Security Association 2). The Security Association 1 is set up using AS1 and a transport profile of [I-D.ietf-ace-oauth-authz], the Security Association 2 is set up using AS2 and [I-D.ietf-ace-key-groupcomm].
Note that, analogously to the Publisher, the Subscriber can also set up an additional security association with the Broker, using an AS, in the same way the Publisher does with AS1. In this case, only authorized Subscribers would be able to get notifications from the Broker. The overhead would be that each Subscriber should access the AS and get all the information to start a secure exchange with the Broker.
+------------+ +------------+ +------------+
| CoAP | | CoAP | | CoAP |
| Client - | | Server - | | Client - |
| | | | | |
| Publisher | | Broker | | Subscriber |
+------------+ +------------+ +------------+
: : : :
: '------ Security -------' :
: Association 1 :
'------------------------------- Security --------------'
Association 2
Note that AS1 and AS2 might either be co-resident or be 2 separate physical entities, in which case access control policies must be exchanged between AS1 and AS2, so that they agree on rights for joining nodes about specific topics. How the policies are exchanged is out of scope for this specification.
This profile uses [I-D.ietf-ace-key-groupcomm], which expands the ACE framework. This document specifies which exact parameters from [I-D.ietf-ace-key-groupcomm] have to be used, and the values for each parameter.
The Publisher and the Subscriber map to the Client in [I-D.ietf-ace-key-groupcomm], the AS2 maps to the AS and to the KDC, the Broker maps to the Dispatcher.
Note that both publishers and subscribers use the same profile, called “coap_pubsub_app”.
This phase is common to both Publisher and Subscriber. To maintain the generality, the Publisher or Subscriber is referred as Client in this section.
Client Broker AS2
| [----- Resource Request ---->] | |
| | |
| [<-- AS1, AS2 Information ---] | |
| |
| [------ Pub Key Format Negociation Request --->] |
| |
| [<---- Pub Key Format Negociation Response ----] |
| |
| -- Authorization + Key Distribution Request ---> |
| |
| <-- Authorization + Key Distribution Response -- |
| |
Figure 2: B: Access request - response
Complementary to what is defined in [I-D.ietf-ace-oauth-authz] (Section 5.1.1), to determine the AS2 in charge of a topic hosted at the Broker, the Broker MAY send the address of both the AS in charge of the topic back to the Client in the ‘AS’ parameter in the AS Information, as a response to an Unauthorized Resource Request (Section 5.1.2). The uri of AS2 is concatenated to the uri of AS1, and separated by a comma. An example using CBOR diagnostic notation is given below:
4.01 Unauthorized
Content-Format: application/ace+cbor
{"AS": "coaps://as1.example.com/token,
coaps://as2.example.com/pubsubkey"}
Figure 3: AS1, AS2 Information example
After retrieving the AS2 address, the Client MAY send a request to the AS, in order to retrieve necessary information concerning the public keys in the group, as well as concerning the algorithm and related parameters for computing signatures in the group. This request is a subset of the Token POST request defined in Section 3.3 of [I-D.ietf-ace-key-groupcomm], specifically a CoAP POST request to a specific resource at the AS, including only the parameters ‘sign_info’ and ‘pub_key_enc’ in the CBOR map in the payload. The default url-path for this resource is /ace-group/gid/cs-info, where “gid” is the topic identifier, but implementations are not required to use this name, and can use their own instead. The AS MUST respond with the response defined in Section 3.3 of [I-D.ietf-ace-key-groupcomm], specifically including the parameters ‘sign_info’, ‘pub_key_enc’, and ‘rsnonce’ (8 bytes pseudo-random nonce generated by the AS).
After that, the Client sends an Authorization + Joining Request, which is an Authorization Request merged with a Joining Request, as described in [I-D.ietf-ace-key-groupcomm], Sections 3.1 and 4.2. The reason for merging these two messages is that the AS2 is both the AS and the KDC, in this setting, so the Authorization Response and the Post Token message are not necessary.
More specifically, the Client sends a POST request to the /ace-group/gid endpoint on AS2, with Content-Format = “application/ace+cbor” that MUST contain in the payload (formatted as a CBOR map):
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’.
Examples of the payload of a Authorization + Joining Request are specified in Figure 5 and Figure 8.
The AS2 verifies that the Client is authorized to access the topic and, if the ‘client_cred’ parameter is present, stores the public key of the Client.
The AS2 response is an Authorization + Joining Response, with Content-Format = “application/ace+cbor”. The payload (formatted as a CBOR map) MUST contain:
Examples for the response payload are detailed in Figure 6 and Figure 9.
In this section, it is specified how the Publisher requests, obtains and communicates to the Broker the access token, as well as the retrieval of the keying material to protect the publication.
+----------------+ +----------------+
| | | |
| Authorization | | Authorization |
| Server 1 | | Server 2 |
| | | |
+----------------+ +----------------+
^ ^
| |
+---------(A)----+ |
| +--------------------(B)--------+
v v
+------------+ +------------+
| CoAP | ----(C)---> | CoAP |
| Client - | | Server - |
| | | |
| Publisher | | Broker |
+------------+ +------------+
Figure 4: Phase 1: Publisher side
This is a combination of two independent phases:
In detail:
(A) corresponds to the Access Token Request and Response between Publisher and Authorization Server to retrieve the Access Token and RS (Broker) Information. As specified, the Publisher has the role of a CoAP client, the Broker has the role of the CoAP server.
(C) corresponds to the exchange between Publisher and Broker, where the Publisher 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.
(A) and (C) details are specified in the profile used.
(B) corresponds to the retrieval of the keying material to protect the publication end-to-end with the subscribers (see Section 6.1), and uses [I-D.ietf-ace-key-groupcomm]. The details are defined in Section 3.1.
An example of the payload of an Authorization + Joining Request and corresponding Response for a Publisher is specified in Figure 5 and Figure 6, where SIG is a signature computed using the private key associated to the public key and the algorithm in “client_cred”.
{
"scope" : ["Broker1/Temp", "publisher"],
"client_id" : "publisher1",
"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 5: Authorization + Joining Request payload for a Publisher
{
"profile" : "coap_pubsub_app",
"kty" : "COSE_Key",
"key" : {1: 4, 2: h'1234', 3: 12, 5: h'1f389d14d17dc7',
-1: h'02e2cc3a9b92855220f255fff1c615bc'}
}
Figure 6: Authorization + Joining Response payload for a Publisher
In this section, it is specified how the Subscriber retrieves the keying material to protect the publication.
+----------------+
| |
| Authorization |
| Server 2 |
| |
+----------------+
^
|
+-----(D)------+
|
v
+------------+
| CoAP |
| Client - |
| |
| Subscriber |
| |
+------------+
Figure 7: Phase 2: Subscriber side
Step (D) between Subscriber and AS2 corresponds to the retrieval of the keying material to verify the publication end-to-end with the publishers (see Section 6.1). The details are defined in Section 3.1
This step is the same as (B) between Publisher and AS2 (Section 3.1), with the following differences:
An example of the payload of an Authorization + Joining Request and corresponding Response for a Subscriber is specified in Figure 8 and Figure 9.
{
"scope" : ["Broker1/Temp", "subscriber"],
"get_pub_keys" : [ ]
}
Figure 8: Authorization + Joining Request payload for a Subscriber
{
"profile" : "coap_pubsub_app",
"scope" : ["Broker1/Temp", "subscriber"],
"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: Authorization + Joining Response payload for a Subscriber
This section specifies the communication Publisher-Broker and Subscriber-Broker, after the previous phases have taken place. The operations of publishing and subscribing are defined in [I-D.ietf-core-coap-pubsub].
+------------+ +------------+ +------------+ | CoAP | | CoAP | | CoAP | | Client - | | Server - | | Client - | | | ----(E)---> | | | | | Publisher | | Broker | <----(F)---- | Subscriber | | | | | -----(G)---> | | +------------+ +------------+ +------------+
Figure 10: Phase 3: Secure communication between Publisher and Subscriber
The (E) message corresponds to the publication of a topic on the Broker. The publication (the resource representation) is protected with COSE ([RFC8152]). The (F) message is the subscription of the Subscriber, which is unprotected, unless a profile of ACE [I-D.ietf-ace-oauth-authz] is used between Subscriber and Broker. The (G) message is the response from the Broker, where the publication is protected with COSE.
The flow graph is presented below.
Publisher Broker Subscriber
| --- PUT /topic ----> | |
| protected with COSE | |
| | <--- GET /topic ----- |
| | |
| | ---- response ------> |
| | protected with COSE |
Figure 11: (E), (F), (G): Example of protected communication
The Publisher uses the symmetric COSE Key received from AS2 in exchange B (Section 3.1) to protect the payload of the PUBLISH operation (Section 4.3 of [I-D.ietf-core-coap-pubsub]). Specifically, the COSE Key is used to create a COSE_Encrypt0 with algorithm specified by AS2. The Publisher uses the private key corresponding to the public key sent to the AS2 in exchange B (Section 3.1) to countersign the COSE Object as specified in Section 4.5 of [RFC8152]. The CoAP 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 AS2 to verify and decrypt the publication received in the payload of the CoAP Notification from the Broker.
The COSE object is constructed in the following way:
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].
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. AS2.
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 protect the publication, but 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
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.
Name: coap_pubsub_app
Description: Profile for delegating client authentication and authorization for publishers and subscribers in a pub-sub setting scenario in a constrained environment.
CBOR Key: TBD
Reference: [[This document]]
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]]
| [I-D.ietf-ace-actors] | Gerdes, S., Seitz, L., Selander, G. and C. Bormann, "An architecture for authorization in constrained environments", Internet-Draft draft-ietf-ace-actors-07, October 2018. |
| [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)", Internet-Draft draft-ietf-ace-dtls-authorize-08, April 2019. |
| [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", Internet-Draft draft-ietf-ace-oscore-profile-08, July 2019. |
This section lists the specifications on this profile based on the requirements defined in Appendix A of [I-D.ietf-ace-key-groupcomm]
The author wishes to thank Ari Keränen, John Mattsson, Ludwig Seitz, Göran Selander, Jim Schaad and Marco Tiloca for the useful discussion and reviews that helped shape this document.