ACE Working Group F. Palombini
Internet-Draft Ericsson
Intended status: Standards Track March 13, 2017
Expires: September 14, 2017
CoAP Pub-Sub Profile for Authentication and Authorization for
Constrained Environments (ACE)
draft-palombini-ace-coap-pubsub-profile-00
Abstract
This specification defines a 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Publisher Profile . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Retrieval of COSE Key for protection of content . . . . . 5
3.2. AS1, AS2 Information . . . . . . . . . . . . . . . . . . 7
4. Subscriber Profile . . . . . . . . . . . . . . . . . . . . . 8
5. Pub-Sub Protected Communication . . . . . . . . . . . . . . . 9
5.1. Using COSE Objects to protect the resource representation 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
This specification defines a way to authorize nodes in a CoAP pub-sub
type of setting, using the ACE framework [I-D.ietf-ace-oauth-authz].
The pub-sub scenario is described in [I-D.ietf-core-coap-pubsub].
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] and
[I-D.ietf-core-coap-pubsub].
2. Overview
The objective of this specification is to specify how to protect a
CoAP pub-sub communication, as described in
[I-D.ietf-core-coap-pubsub], using Ace framework
([I-D.ietf-ace-oauth-authz]) and profiles
([I-D.gerdes-ace-dtls-authorize], [I-D.seitz-ace-oscoap-profile]).
The architecture of the scenario is shown in Figure 1.
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+----------------+ +----------------+
| | | |
| Authorization | | Authorization |
| Server 1 | | Server 2 |
| | | |
+----------------+ +----------------+
^ ^ ^
| | |
+---------(A)----+ | +-----(E)------+
| +--------------------(B)--------+ |
v v v
+------------+ +------------+ +------------+
| CoAP | ----(C)---> | CoAP | | CoAP |
| Client - | [<--(D)-->] | Server - | | Client - |
| | ----(F)---> | | | |
| Publisher | | Broker | <----(G)---- | Subscriber |
| | | | -----(H)---> | |
+------------+ +------------+ +------------+
Figure 1: Architecture CoAP pubsub with Authorization Servers
The RS is the broker, which contains the topic. The AS1 hosts the
policies about the Broker: what endpoints are allowed to Publish on
the Broker. The AS2 hosts the policies about the topic: what
endpoints are allowed to access what topic. There are four phases,
the first three can be done in parallel.
1. The Publisher requests publishing access to a broker at the AS1,
and communicates with the Broker to set up security.
2. The Publisher requests access to a specific topic at the AS2
3. The Subscriber requests access to a specific topic at the AS2.
4. The Publisher and the Subscriber securely post to and get
publications from the Broker.
This scenario requires the setup of 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, the Security Association 2 is set up using AS2.
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+------------+ +------------+ +------------+
| CoAP | | CoAP | | CoAP |
| Client - | | Server - | | Client - |
| | | | | |
| Publisher | | Broker | | Subscriber |
+------------+ +------------+ +------------+
: : : :
: '------ Security -------' :
: Association 1 :
'------------------------------- Security --------------'
Association 2
3. Publisher Profile
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 - | [<--(D)-->] | Server - |
| | | |
| Publisher | | Broker |
| | | |
+------------+ +------------+
Figure 2: Phase 1: Publisher side
This is a combination of two independent phases:
o one is the establishment of a secure connection between Publisher
and Broker, using an ACE profile such as DTLS
[I-D.gerdes-ace-dtls-authorize] or OSCOAP
[I-D.seitz-ace-oscoap-profile]. (A)(C)(D)
o the other is the Publisher's retrieval of keying material to
protect the publication. (B)
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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.
(D) corresponds to the exchange where the Publisher establishes a
secure connection with the Broker. Depending on the Information
received in (A), this can be for example DTLS handshake, or other
protocols such as EDHOC. Depending on the application, there may not
be the need for this set up phase: for example, if OSCOAP is used
directly and not without EDHOC first.
(A), (C) and (D) details are specified in the profile used.
(B) corresponds to the retrieval of the keying material to protect
the publication. The detailed message flow is defined below.
3.1. Retrieval of COSE Key for protection of content
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 ---] | |
| |
| ------- Topic Keying Material Request ---------> |
| |
| <------------ Keying Material ------------------ |
| |
Figure 3: B: Access request - response
Complementary to what is defined in the DTLS profile (section 2.), 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, as a response to a Resource Request
(Section 2.1). Analogously to the DTLS profile, instead of the
initial Unauthorized Resource Request message, the Client MAY look up
the desired topic in a resource directory (see
[I-D.ietf-core-resource-directory]).
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After retrieving the AS2 address, the Client sends a Topic Keying
Material Request, which is a token-less authorization as described in
[I-D.seitz-ace-oauth-authz], section 6.5. More specifically, the
Client sends a POST request to the /token endpoint on AS2, that MUST
contain in the payload:
o the grant type set to "client_credentials",
o the audience parameter set to the Broker,
o the scope parameter set to the topic,
o the cnf parameter containing the Client's COSE key, if the Client
is a publisher, and
o OPTIONALLY, other additional parameters such as the client id or
the algorithm.
Note that, if present, the algorithm MUST be a Content Encryption
Algorithm, as defined in Section 10 of [I-D.ietf-cose-msg]. An
example of the payload of a Topic Keying Material Request for a
Publisher is specified in Figure 4.
{
"grant_type" : "client_credentials",
"aud" : "Broker1",
"scope" : "Temp",
"client_id" : "publisher1",
"cnf" :
{ / 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'
}
}
Figure 4: Example of Topic Keying Material Request payload for a
Publisher
The AS2 verifies that the Client is authorized to access the topic
and, if the "cnf" parameter is present, stores the public key of the
Client.
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The AS2 response contains an empty token and the keying material to
protect the publication ("key" field in the payload). Moreover, the
payload MUST contain the "profile" parameter, set to value "OSCON",
and the "token_type" set to "none".
TODO: define "key" parameter following ACE framework
The "key" parameter value MUST be a serialized COSE Key (see
Section 7 of [I-D.ietf-cose-msg]), with the following values:
o kty with value 4 (symmetric)
o alg with value defined by the AS2 (Content Encryption Algorithm)
o k with value the symmetric key value
o OPTIONALLY, kid with an identifier for the key value
An example for the response is detailed in Figure 5.
{
"access_token" : NULL,
"token_type" : "none",
"profile" : "OSCON",
"key" : h'a4010402421234030c205002e2cc3a9b92855220f255fff1c615bc'
/{1: 4, 2: h'1234', 3: 12, -1: h'02e2cc3a9b92855220f255fff1c615bc'}/
}
Figure 5: Example of Topic Keying Material response payload for a
Publisher
3.2. AS1, AS2 Information
The Client MUST be able to process the following response message
from the Broker, in order to retrieve the correct AS1 and AS2
addresses.
This CoAP message MUST have the following characteristics: the CoAP
Code MUST be 4.01 "Unauthorized", the payload MUST be present and
MUST include the full URI of both AS. An example using CBOR
diagnostic notation is given below:
4.01 Unauthorized
Content-Format: application/ace+cbor
{"AS1": "coaps://as1.example.com/token",
"AS2": "coaps://as2.example.com/pubsubkey"}
Figure 6: AS1, AS2 Information example
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4. Subscriber Profile
In this section, it is specified how the Subscriber retrieves the
keying material to protect the publication.
+----------------+
| |
| Authorization |
| Server 2 |
| |
+----------------+
^
|
+-----(E)------+
|
v
+------------+
| CoAP |
| Client - |
| |
| Subscriber |
| |
+------------+
Figure 7: Phase 2: Subscriber side
Step (E) between Subscriber and AS2 corresponds to the retrieval of
the keying material to verify the publication, and is the same as (B)
between Publisher and AS2 (Section 3.1), with the following
differences:
o The POST request to the /token endpoint on AS2, does not contain
the cnf parameter containing the Client's COSE key.
o The AS2 response contains a "cnf" parameter whose value is set to
a COSE Key Set, (Section 7 of [I-D.ietf-cose-msg]) i.e. an array
of COSE Keys, which contains the public keys of all authorized
Publishers
An example of the payload of a Topic Keying Material Request and
corresponding response for a Subscriber is specified in Figure 8 and
Figure 9.
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{
"grant_type" : "client_credentials",
"aud" : "Broker1",
"scope" : "Temp",
"client_id" : "subscriber1"
}
Figure 8: Example of Topic Keying Material Request payload for a
Subscriber
{
"access_token" : NULL,
"token_type" : "none",
"profile" : "OSCON",
"key" : h'a4010402421234030c205002e2cc3a9b92855220f255fff1c615bc',
/{1: 4, 2: h'1234', 3: 12, -1: h'02e2cc3a9b92855220f255fff1c615bc'}/
"cnf" : [
{
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: Example of Topic Keying Material response payload for a
Subscriber
5. Pub-Sub Protected Communication
This section specifies the communication Publisher-Broker and
Subscriber-Broker, after the previous phases have taken place.
+------------+ +------------+ +------------+
| CoAP | | CoAP | | CoAP |
| Client - | | Server - | | Client - |
| | ----(F)---> | | | |
| Publisher | | Broker | <----(G)---- | Subscriber |
| | | | -----(H)---> | |
+------------+ +------------+ +------------+
Figure 10: Phase 3: Secure communication between Publisher and
Subscriber
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The (F) message corresponds to the publication of a topic on the
Broker. The publication (the resource representation) is protected
with COSE ([I-D.ietf-cose-msg]). The (G) message is the subscription
of the Subscriber, which is unprotected. The (H) 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: (F), (G), (H): Example of protected communication
5.1. Using COSE Objects to protect the resource representation
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 [I-D.ietf-cose-msg]. 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:
o The protected Headers (as described in Section 3 of
[I-D.ietf-cose-msg]) MAY contain the kid parameter, with value the
kid of the symmetric COSE Key received in Section 3.1 and MUST
contain the content encryption algorithm
o The unprotected Headers MUST contain the IV and the counter
signature that includes:
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* 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
[I-D.ietf-cose-msg]
o The ciphertext, computed over the plaintext that MUST contain the
CoAP payload.
The external_aad, when using AEAD, 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 [I-D.ietf-cose-msg].
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6. 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. 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
7. IANA Considerations
TODO: "key" parameter, OSCON profile identifier
8. Acknowledgments
The author wishes to thank John Mattsson, Ludwig Seitz and Goeran
Selander for the useful discussion that helped shape this document.
9. References
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9.1. Normative References
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE)", draft-ietf-ace-oauth-
authz-05 (work in progress), February 2017.
[I-D.ietf-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-ietf-core-coap-pubsub-00 (work in
progress), October 2016.
[I-D.ietf-cose-msg]
Schaad, J., "CBOR Object Signing and Encryption (COSE)",
draft-ietf-cose-msg-24 (work in progress), November 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
9.2. Informative References
[I-D.gerdes-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)", draft-gerdes-ace-dtls-
authorize-01 (work in progress), March 2017.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-09
(work in progress), October 2016.
[I-D.seitz-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authorization for the Internet of Things
using OAuth 2.0", draft-seitz-ace-oauth-authz-00 (work in
progress), October 2015.
[I-D.seitz-ace-oscoap-profile]
Seitz, L. and F. Palombini, "OSCOAP profile of ACE",
draft-seitz-ace-oscoap-profile-01 (work in progress),
October 2016.
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Author's Address
Francesca Palombini
Ericsson
Email: francesca.palombini@ericsson.com
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