ACE Working Group C. Sengul
Internet-Draft Nominet
Intended status: Standards Track A. Kirby
Expires: April 7, 2020 Oxbotica
P. Fremantle
University of Portsmouth
October 5, 2019
MQTT-TLS profile of ACE
draft-ietf-ace-mqtt-tls-profile-01
Abstract
This document specifies a profile for the ACE (Authentication and
Authorization for Constrained Environments) to enable authorization
in an MQTT-based publish-subscribe messaging system. Proof-of-
possession keys, bound to OAuth2.0 access tokens, are used to
authenticate and authorize MQTT Clients. The protocol relies on TLS
for confidentiality and server authentication.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 7, 2020.
Copyright Notice
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document authors. All rights reserved.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. ACE-Related Terminology . . . . . . . . . . . . . . . . . 4
1.3. MQTT-Related Terminology . . . . . . . . . . . . . . . . 4
2. Protocol Interactions . . . . . . . . . . . . . . . . . . . . 6
2.1. Authorizing Connection Establishment . . . . . . . . . . 7
2.1.1. Client Token Request to the Authorization Server (AS) 8
2.1.2. Client Connection Request to the Broker (C) . . . . . 8
2.1.2.1. Proof-of-Possession over Predefined Field . . . . 10
2.1.2.2. Proof-of-Possession via challenge/response . . . 11
2.1.2.3. Unauthorised Request: Authorisation Server
Discovery . . . . . . . . . . . . . . . . . . . . 12
2.1.3. Token Validation . . . . . . . . . . . . . . . . . . 12
2.1.4. The Broker's Response to Client Connection Request . 13
2.2. Authorizing PUBLISH Messages . . . . . . . . . . . . . . 13
2.2.1. PUBLISH Messages from the Publisher Client to the
Broker . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.2. PUBLISH Messages from the Broker to the Subscriber
Clients . . . . . . . . . . . . . . . . . . . . . . . 14
2.3. Authorizing SUBSCRIBE Messages . . . . . . . . . . . . . 14
2.4. Token Expiration and Reauthentication . . . . . . . . . . 15
2.5. Handling Disconnections and Retained Messages . . . . . . 15
3. Reduced Protocol Interactions for MQTT v3.1.1 . . . . . . . . 16
3.1. Token Transport . . . . . . . . . . . . . . . . . . . . . 16
3.2. Handling Authorization Errors . . . . . . . . . . . . . . 17
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1. Normative References . . . . . . . . . . . . . . . . . . 19
7.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Checklist for profile requirements . . . . . . . . . 21
Appendix B. The Authorization Information Endpoint . . . . . . . 21
Appendix C. Document Updates . . . . . . . . . . . . . . . . . . 22
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
This document specifies a profile for the ACE framework
[I-D.ietf-ace-oauth-authz]. In this profile, Clients and a Broker
use MQTT to exchange Application messages. The protocol relies on
TLS for communication security between entities. The MQTT protocol
interactions are described based on the MQTT v5.0 - the OASIS
Standard [MQTT-OASIS-Standard-v5]. It is expected that MQTT
deployments will retain backward compatibility for MQTT v3.1.1
clients, and therefore, this document describes a reduced set of
protocol interactions suited to MQTT v3.1.1 - the OASIS Standard
[MQTT-OASIS-Standard]. However, it is RECOMMENDED to use MQTT v5.0
as it works more naturally with ACE-style authentication and
authorization.
MQTT is a publish-subscribe protocol and supports two main types of
Client operation: publish and subscribe. Once connected, a Client
can publish to multiple topics, and subscribe to multiple topics.
The MQTT Broker is responsible for distributing messages published by
the publishers to the appropriate subscribers. Each publish message
contains a Topic Name, which is used by the Broker to filter the
subscribers for the message. Subscribers must subscribe to the
topics to receive the corresponding messages.
In this document, message topics are treated as resources. Clients
use an access token, bound to a key (the proof-of-possession key) to
authorize with the MQTT Broker their connection and publish/subscribe
permissions to topics. In the context of this ACE profile, the MQTT
Broker acts as the Resource Server (RS). In the rest of the document
RS and Broker are used interchangeably. To provide communication
confidentiality and Resource Server authentication, TLS is used.
This document makes the same assumptions as the Section 4 of the ACE
framework [I-D.ietf-ace-oauth-authz] regarding Client and RS
registration with the AS and establishing of keying material.
This document describes the authorization of the following exchanges
between Clients and the Broker.
o Connection establishment between the Clients and the Broker
o Publish messages from the Clients to the Broker, and from the
Broker to the Clients
o Subscribe messages from the Clients to the Broker
While the Client-Broker exchanges are over MQTT, the required Client-
AS and RS-AS interactions are described for HTTPS-based
communication, using 'application/ace+json' content type, and unless
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otherwise specified, using JSON encoding. The token may be a
reference, or JWT. For JWT tokens, this document follows RFC 7800
[RFC7800] for PoP semantics for JWTs. The Client-AS and RS-AS may
also be based on CoAP. It is also possible to use 'application/
ace+cbor' content type, and CBOR encoding, and CWT and associated PoP
semantics to reduce the protocol memory and bandwidth requirements.
For more information on Proof of Possession semantics for CWTs, see
Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)
[I-D.ietf-ace-cwt-proof-of-possession].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174], when, and only when, they appear in all
capitals, as shown here.
1.2. ACE-Related Terminology
The terminology for entities in the architecture is defined in OAuth
2.0 RFC 6749 [RFC6749] such as "Client" (C), "Resource Server" (RS)
and "Authorization Server" (AS).
The term "endpoint" is used following its OAuth definition, to denote
resources such as /token and /introspect at the AS.
The term "Resource" is used to refer to an MQTT Topic Name, which is
defined in Section 1.3. Hence, the "Resource Owner" is any entity
that can authoritatively speak for the topic.
Certain security-related terms such as "authentication",
"authorization", "confidentiality", "(data) integrity", "message
authentication code", and "verify" are taken from RFC 4949 [RFC4949].
1.3. MQTT-Related Terminology
The document describes message exchanges as MQTT protocol
interactions. The Clients are MQTT Clients, which connect to the
Broker to publish and subscribe to Application Messages. For
additional information, please refer to the MQTT v5.0 - the OASIS
Standard [MQTT-OASIS-Standard-v5] or the MQTT v3.1.1 - the OASIS
Standard [MQTT-OASIS-Standard].
MQTTS
Secured transport profile of MQTT. MQTTS runs over TLS.
Broker
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The Server in MQTT and acts as an intermediary between
Clients that publish Application Messages, and the Clients
that made Subscriptions. The Broker acts as the Resource
Server for the Clients.
Application Message
The data carried by the MQTT protocol. The data has an
associated QoS level and a Topic Name.
Topic Name
The label attached to an Application Message, which is
matched to a Subscription.
Subscription
A subscription comprises a Topic Filter and a maximum Quality
of Service (QoS).
Topic Filter
An expression that indicates interest in one or more Topic
Names. Topic Filters may include wildcards.
MQTT sends various control messages across a network connection. The
following is not an exhaustive list and the control packets that are
not relevant for authorization are not explained. These include, for
instance, the PUBREL and PUBCOMP packets used in the 4-step handshake
required for the QoS level 2.
CONNECT
Client request to connect to the Broker. After a network
connection is established, this is the first packet sent by a
Client.
CONNACK
The Broker connection acknowledgment. The first packet sent
from the Broker to a Client is a CONNACK packet. CONNACK
packets contain return codes indicating either a success or
an error state to a Client.
PUBLISH
Publish packet that can be sent from a Client to the Broker,
or from the Broker to a Client.
PUBACK
Response to PUBLISH packet with QoS level 1. PUBACK can be
sent from the Broker to a Client or a Client to the Broker.
PUBREC
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Response to PUBLISH packet with QoS level 2. PUBREC can be
sent from the Broker to a Client or a Client to the Broker.
SUBSCRIBE
The Client subscribe request.
SUBACK
Subscribe acknowledgment.
PINGREQ
A ping request sent from a Client to the Broker. It signals
to the Broker that the Client is alive, and is used to
confirm that the Broker is still alive.
Will
An application message published by the Server after the
network connection is closed in cases where the network
connection is not closed normally. If the Will Flag is set,
then the payload of the CONNECT message has information about
the Will. The Will consists of the Will Properties, Will
Topic, and Will Payload fields in the CONNECT message.
2. Protocol Interactions
This section describes the following exchanges between Clients, the
Broker, and the Authorization Server according to the MQTT v5.0.
o Authorizing connection establishment between the Clients and the
Broker
o Authorizing publish messages from the Clients to the Broker, and
from the Broker to the Clients
o Authorizing subscribe messages from Clients to the Broker
Section 3 describes how these exchanges can also be supported using
the MQTT v3.1.1. MQTT v5.0 brokers MAY also only support the basic
operation; however, this is NOT RECOMMENDED.
In this profile document, message topics are treated as resources.
The Clients are assumed to have identified the publish/subscribe
topics of interest out-of-band (topic discovery is not a feature of
the MQTT protocol). A resource owner can pre-configure policies at
the AS that give Clients publish or subscribe permissions to
different topics.
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2.1. Authorizing Connection Establishment
This section specifies how Clients establish an authorized connection
to an MQTT Broker. Figure 1 shows the basic protocol flow during
connection establishment.The token request and response use the
/token endpoint of the authorization server, specified in the
Section 5.6 of the ACE framework [I-D.ietf-ace-oauth-authz]. Steps
(D) and (E) are optional, and use the introspection endpoint,
specified in the Section 5.7 of the ACE framework. The Client and
Broker use HTTPS to communicate to AS via these endpoints. If the
Client is resource-constrained, a Client Authorisation Server may
carry out the token request on behalf of the Client, and later,
onboard the Client with the token. Also, these interfaces may be
implemented using other protocols, e.g., CoAP or MQTT. The
interactions between a Client and its Client Authorization Server for
token onboarding, and the MQTTS support for token requests are out of
scope of this document.
+---------------------+
| Client |
| |
+---(A) Token request--| Client - |
| | Authorization |
| +-(B) Access token-> Server Interface |
| | | (HTTPS) |
| | |_____________________|
| | | |
+--v-------------+ | Pub/Sub Interface |
| Authorization | | (MQTTS) |
| Server | +-----------^---------+
|________________| | |
| ^ (C)Connection (F)Connection
| | request + response
| | access token |
| | | |
| | +---v--------------+
| | | Broker (MQTTS) |
| | |__________________|
| +(D)Introspection-| |
| request (optional) | RS-AS interface |
| | (HTTPS) |
+-(E)Introspection---->|__________________|
response (optional)
Figure 1: Connection establishment
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2.1.1. Client Token Request to the Authorization Server (AS)
The first step in the protocol flow (Figure 1 (A)) is the token
acquisition by the Client from the AS. When requesting an access
token from the AS, the Client MAY include parameters in its request
as defined in Section 5.6.1 of the ACE framework
[I-D.ietf-ace-oauth-authz]. The media format is 'application/
ace+json'. The profile parameter MUST be set to 'mqtt_tls'. The
OAuth 2.0 AS uses a JSON structure in the payload of its responses
both to client and RS.
If the AS successfully verifies the access token request and
authorizes the Client for the indicated audience (e.g., RS) and
scopes (e.g., publish/subscribe permissions over topics), the AS
issues an access token (Figure 1 (B)). The response includes the
parameters described in Section 5.6.2 of the ACE framework
[I-D.ietf-ace-oauth-authz]. The included token is assumed to be
Proof-of-Possession (PoP) token by default. This document follows
RFC 7800 [RFC7800] for PoP semantics for JWTs. The PoP token
includes a 'cnf' parameter with a symmetric or asymmetric PoP key.
Note that the 'cnf' parameter in the web tokens are to be consumed by
the resource server and not the Client.
In the case of an error, the AS returns error responses for HTTP-
based interactions as ASCII codes in JSON content, as defined in
Section 5.2 of RFC 6749 [RFC6749].
2.1.2. Client Connection Request to the Broker (C)
This section describes how the Client transports the token to the
Broker (RS) via the CONNECT control message after the TLS handshake.
This is similar to an earlier proposal by Fremantle et al.
[fremantle14]. Alternatively, the token may be used for the TLS
session establishment as described in the DTLS profile for ACE
[I-D.gerdes-ace-dtls-authorize]. In this case, both the TLS PSK and
RPK handshakes MAY be supported. This may additionally require that
the Client transports the token to the Broker before the connection
establishment. To this end, the Broker MAY support /authz-info
endpoint via the "authz-info" topic. Then, to transport the token,
Clients publish to "authz-info" topic unauthorized. The topic
"authz-info" MUST be publish-only for Clients (i.e., the Clients are
not allowed to subscribe to it). This option is described in more
detail in Appendix B.
After the token acquisition, the Client connects to the RS (Broker)
using the CONNECT message of MQTT over TLS. For server
authentication, the client MAY either have the ability to receive and
validate a certificate or a raw public key from the Broker. The
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client needs to use this raw public key in the TLS handshake together
with an out-of-band validation technique (see RFC 7250 [RFC7250] for
details).
Figure 2 shows the structure of the MQTT CONNECT control message used
in MQTT v5.0. A CONNECT message is composed of a fixed header, a
variable header and a payload. The fixed header contains Control
Packet Type (CPT), Reserved, and Remaining Length. The Variable
Header contains the Protocol Name, Protocol Level, Connect Flags,
Keep Alive, and Properties. The Connect Flags in the variable header
specify the behavior of the MQTT connection. It also indicates the
presence or absence of fields in the Payload. The payload contains
one or more encoded fields, namely a unique Client identifier for the
Client, a Will Topic, Will Payload, User Name and Password. All but
the Client identifier can be omitted depending on flags in the
Variable Header.
0 8 16 24 32
+------------------------------------------------------+
|CPT=1 | Rsvd.|Remaining len.| Protocol name len. = 4 |
+------------------------------------------------------+
| 'M' 'Q' 'T' 'T' |
+------------------------------------------------------+
| Proto.level=5|Connect flags| Keep alive |
+------------------------------------------------------+
| Property length |
| Auth. Method (0x15) | 'ace' |
| Auth. Data (0x16) | empty or token or |
| token + PoP data |
+------------------------------------------------------+
| Payload: Client Identifier |
+------------------------------------------------------+
Figure 2: MQTT CONNECT control message. (CPT=Control Packet Type,
Rsvd=Reserved, len.=length, Proto.=Protocol)
Connect Flags include Clean Start, Will, Will QoS, Will Retain,
Password and Username flags. Figure 6 shows how the MQTT connect
flags MUST be set to initiate a connection with the Broker.
+-----------------------------------------------------------+
|User name|Pass.|Will retain|Will QoS|Will Flag|Clean| Rsvd.|
| flag |flag | | | | | |
+-----------------------------------------------------------+
| 0 | 0 | X | X X | X | 1 | 0 |
+-----------------------------------------------------------+
Figure 3: MQTT CONNECT flags. (Rsvd=Reserved)
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To achieve a clean session (i.e., the session starts without an
existing session), the Clean Start Flag MUST be set to 1. In
addition, if the Session Expiry Interval is present in the CONNECT
message, it MUST be set to 0.
The Will Flag indicates that a Will message needs to be sent if
network connection is not closed normally. The situations in which
the Will message is published include disconnections due to I/O or
network failures, and the server closing the network connection due
to a protocol error. The Client may set the Will Flag as desired
(marked as 'X' in Figure 3). If the Will Flag is set to 1 and the
Broker accepts the connection request, the Broker must store the Will
message, and publish it when the network connection is closed
according to Will QoS and Will retain parameters, and MQTT Will
management rules. To avoid publishing Will Messages in the case of
temporary network disconnections, the Client my specify a Will Delay
Interval in Will Properties. Section 2.5 explains how the Broker
deals with the retained messages in further detail.
For token transport, the RS SHOULD support AUTH (Authentication
Exchange) method. The RS MAY support token transport via username
and password, which is described in Section 3 for MQTT v3.1.1. The
rest of this section describes the AUTH method, for which the
username and password flags MUST be set to 0.
To implement the AUTH (Authentication Exchange) method, the Client
MUST set the Authentication Method as a property of a CONNECT packet
by using the property identifier 21 (0x15). This is followed by a
UTF-8 Encoded String containing the name of the authentication
method, which MUST be set to 'ace'. If the RS does not support this
profile, it sends a CONNACK with a Reason Code of '0x8C (Bad
authentication method)'.
The Authentication Method is followed by the Authentication Data,
which has a property identifier 22 (0x16) and is binary data. Based
on the Authentication Data, this profile allows:
o Proof-of-Possession over predefined field
o Proof-of-Possession via challenge/response
o Unauthorised request: Authorisation Server discovery
2.1.2.1. Proof-of-Possession over Predefined Field
For this option, the Authentication Data MUST contain the token and
the keyed message digest (MAC) or the Client signature. To calculate
the keyed message digest (MAC) or the Client signature, the Client
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SHOULD apply the PoP key to the CONNECT payload. The CONNECT payload
has at least a Client Identifier, and if the Will Flag is set to 1,
may contain Will-related information. The Client Identifier is a
MUST be a UTF-8 Encoded String (i.e., is prefixed with a two-byte
integer length field that gives the number of bytes in a UTF-8
encoded string itself). The Client Identifier may be 1-23 UTF-8
encoded bytes, and contain only the characters
"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".
However, according to MQTTv5 standard, the Broker may except longer
Client Identifiers, and characters not included in the list given
above. Clients MUST change their Client Identifier for each session,
if the Client Identifier is the only source of randomness in the
payload to defend against a replay attack. If the Client reuses its
Client Identifier across different sessions, the Authentication Data
MUST also contain a nonce, and the keyed message digest (MAC) or the
Client signature MUST be computed over this nonce. Finally, the
token is validated as described in Section 2.1.3 and the server
responds with a CONNACK.
2.1.2.2. Proof-of-Possession via challenge/response
For this option, the RS follows a challenge/response protocol. The
success case is illustrated in Figure 4. If the Authentication Data
only includes the token, the RS MUST respond with an AUTH packet,
with the Authenticate Reason Code set to '0x18 (Continue
Authentication)'. This packet includes the Authentication Method,
which MUST be set to 'ace' and Authentication Data. The
Authentication Data MUST NOT be empty and contains a challenge for
the Client. The Client responds to this with an AUTH packet with a
reason code '0x18 (Continue Authentication)'. Similarly, the Client
packet sets the Authentication Method to 'ace'. The Authentication
Data in the Client's response contains the signature or MAC computed
over the RS's challenge. Next, the token is validated as described
in Section 2.1.3.
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Resource
Client Server
| |
|<===========>| TLS connection establishment
| |
| |
+------------>| CONNECT with Authentication Data
| | contains only token
| |
<-------------+ AUTH '0x18 (Continue Authentication)'
| | challenge
| |
|------------>| AUTH '0x18 (Continue Authentication)'
| | signature
| |
| |-----+ Token validation (may involve introspection)
| | |
| |<----+
| |
|<------------+ CONNACK '0x00 (Success)'
Figure 4: PoP Challenge/Response Protocol Flow - Success
2.1.2.3. Unauthorised Request: Authorisation Server Discovery
Finally, this document allows the CONNECT message to have an empty
Authentication Data field. This is the AS discovery option and the
RS responds with the CONNACK reason code '0x87 (Not Authorized)' and
includes a User Property (identified by 38 (0x26)) for the AS
creation hints as dedined in the Section 5.1.2 of the ACE framework
[I-D.ietf-ace-oauth-authz].
2.1.3. Token Validation
The RS MUST verify the validity of the token. This validation MAY be
done locally (e.g., in the case of a self-contained token) or the RS
MAY send an introspection request to the AS. If introspection is
used, this section follows similar steps to those described in
Sections 5.7 of the ACE framework [I-D.ietf-ace-oauth-authz]. The
communication between AS and RS MUST be confidential, mutually
authenticated and integrity protected.
The Broker MUST check if the token is active either using 'exp' claim
of the token or 'active' parameter of the introspection response.
Also, if present in the access token, RS must check that the 'iss'
corresponds to AS, the 'aud' field corresponds to RS. It also has to
check whether the 'nbf' and the 'iat' claims are present and valid.
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To authenticate the Client, the RS validates the signature or the
MAC, depending on how the PoP protocol is implemented. To authorize
the Client, the Broker uses the scope field in the token (or in the
introspection result). The scope field contains the publish and
subscribe permissions for the Client. If the Will Flag is set,then
the Broker MUST check that the token allows the publication of the
Will message.
Scope strings SHOULD be encoded as a permission, followed by an
underscore, followed by a topic filter. Two permissions apply to
topics: 'publish' and 'subscribe'. An example scope field may
contain multiple such strings, space delimited, e.g., 'publish_topic1
subscribe_topic2/#'. Hence, this access token would give 'publish'
permission to the 'topic1', 'subscribe' permission to all the
subtopics of 'topic2'.
2.1.4. The Broker's Response to Client Connection Request
Based on the validation result (obtained either via local inspection
or using the /introspection interface of the AS), the Broker MUST
send a CONNACK message to the Client. The reason code of the CONNACK
is '0x00 (Success)' if the authentication is successful. In case of
an invalid PoP token, the CONNACK reason code is '0x87 (Not
Authorized)'.
If the RS accepts the connection, it MUST store the token until the
end of connection. On Client or RS disconnection, the token is
discarded, and the Client MUST provide a token inside each CONNECT
message.
If the token is not self-contained and the Broker uses token
introspection, it MAY cache the validation result to authorize the
subsequent PUBLISH and SUBSCRIBE messages. PUBLISH and SUBSCRIBE
messages, which are sent after a connection set-up, do not contain
access tokens. If the introspection result is not cached, then the
RS needs to introspect the saved token for each request. The Broker
SHOULD use a cache time out to introspect tokens regularly.
2.2. Authorizing PUBLISH Messages
2.2.1. PUBLISH Messages from the Publisher Client to the Broker
On receiving the PUBLISH message, the Broker MUST use the type of
message (i.e., PUBLISH) and the Topic name in the message header to
compare against the cached token or its introspection result.
If the Client is allowed to publish to the topic, the RS must publish
the message to all valid subscribers of the topic. The Broker may
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also return an acknowledgment message if the QoS level is greater
than or equal to 1.
In case of an authorization failure, an error MAY be returned to the
Client. For this the QoS level of the PUBLISH message, should be set
to greater than or equal to 1. This guarantees that RS responds with
either a PUBACK or PUBREC packet with reason code '0x87 (Not
authorized)'.
On receiving a PUBACK with '0x87 (Not authorized)', the Client MAY
reauthenticate as described in Section 2.4, and pass a new token
following the same PoP methods as described in Figure 2.
2.2.2. PUBLISH Messages from the Broker to the Subscriber Clients
To forward PUBLISH messages to the subscribing Clients, the Broker
identifies all the subscribers that have valid matching topic
subscriptions (i.e., the tokens are valid, and token scopes allow a
subscription to the particular topic). The Broker sends a PUBLISH
message with the Topic name to all the valid subscribers.
RS MUST stop forwarding messages to the unauthorized subscribers.
For Clients with invalid tokens, there is no way to inform the Client
that an authorization error has occurred other than sending a
DISCONNECT message. The RS SHOULD send a DISCONNECT message with the
reason code '0x87 (Not authorized)'. Note that the server-side
DISCONNECT is a new feature of MQTT v5.0 (in MQTT v3.1.1, the server
needs to drop the connection).
2.3. Authorizing SUBSCRIBE Messages
In MQTT, a SUBSCRIBE message is sent from a Client to the Broker to
create one or more subscriptions to one or more topics. The
SUBSCRIBE message may contain multiple Topic Filters. The Topic
Filters may include wildcard characters.
On receiving the SUBSCRIBE message, the Broker MUST use the type of
message (i.e., SUBSCRIBE) and the Topic Filter in the message header
to compare against the stored token or introspection result.
As a response to the SUBSCRIBE message, the Broker issues a SUBACK
message. For each Topic Filter, the SUBACK packet includes a return
code matching the QoS level for the corresponding Topic Filter. In
the case of failure, the return code is 0x87, indicating that the
Client is 'Not authorized'. A reason code is returned for each Topic
Filter. Therefore, the Client may receive success codes for a subset
of its Topic Filters while being unauthorized for the rest.
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2.4. Token Expiration and Reauthentication
The Broker MUST check for token expiration whenever a CONNECT,
PUBLISH or SUBSCRIBE message is received or sent. The Broker SHOULD
check for token expiration on receiving a PINGREQUEST message. This
may allow for early detection of a token expiry.
The token expiration is checked by checking the 'exp' claim of a JWT
or introspection response, or via performing an introspection request
with the Authorization server as described in Section 5.7 of the ACE
framework [I-D.ietf-ace-oauth-authz]. Token expirations may trigger
the RS to send PUBACK, SUBACK and DISCONNECT messages with return
code set to 'Not authorised'. As a response, the Client MAY re-
authenticate by sending an AUTH packet with a Reason Code of 0x19
(Re-authentication)
To re-authenticate, the Client sends an AUTH packet with reason code
'0x19 (Re-authentication)'. The Client MUST set the authentication
method as 'ace' and transport the new token in the Authentication
Data. The Client and the RS go through the same steps for proof of
possession validation as described in Section 2.1.2. If the re-
authentication fails, the server MUST send a DISCONNECT with the
reason code '0x87 (Not Authorized)'. The Clients can also
proactively update their tokens before they receive a message with
'Not authorized' return code.
2.5. Handling Disconnections and Retained Messages
In the case of a Client DISCONNECT, due to the Clean Session flag,
the Broker deletes all session state but MUST keep the retained
messages. By setting a RETAIN flag in a PUBLISH message, the
publisher indicates to the Broker that it should store the most
recent message for the associated topic. Hence, the new subscribers
can receive the last sent message from the publisher of that
particular topic without waiting for the next PUBLISH message. The
Broker MUST continue publishing the retained messages as long as the
associated tokens are valid.
In case of disconnections due to network errors or server
disconnection due to a protocol error (which includes authorization
errors), the Will message must be sent if the Client supplied a Will
in the CONNECT message. The Client's token scopes MUST include the
Will Topic. The Will message MUST be published to the Will Topic
when the network connection is closed regardless of whether the
corresponding token has expired. In the case of a server-side
DISCONNECT, the server returns the '0x87 Not Authorized' return code
to the Client.
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3. Reduced Protocol Interactions for MQTT v3.1.1
This section describes a reduced set of protocol interactions for the
MQTT v3.1.1 Client.
3.1. Token Transport
To transport the token to the Broker, the Clients use the username
and password fields of the CONNECT control message after the TLS
handshake. Figure 5 shows the structure of the MQTT CONNECT message.
0 8 16 24 32
+------------------------------------------------------+
|CPT=1 | Rsvd.|Remaining len.| Protocol name len. = 4 |
+------------------------------------------------------+
| 'M' 'Q' 'T' 'T' |
+------------------------------------------------------+
| Proto.level=4|Connect flags| Keep alive |
+------------------------------------------------------+
| Payload |
| Client Identifier |
| Username as access token (UTF-8) |
| Password length (2 Bytes) |
| Password data as signature/MAC (binary) |
+------------------------------------------------------+
Figure 5: MQTT CONNECT control message. (CPT=Control Packet Type,
Rsvd=Reserved, len.=length, Proto.=Protocol)
Figure 6 shows how the MQTT connect flags MUST be set to initiate a
connection with the Broker.
+-----------------------------------------------------------+
|User name|Pass.|Will retain|Will QoS|Will Flag|Clean| Rsvd.|
| flag |flag | | | | | |
+-----------------------------------------------------------+
| 1 | 1 | X | X X | X | 1 | 0 |
+-----------------------------------------------------------+
Figure 6: MQTT CONNECT flags. (Rsvd=Reserved)
The Clean Session Flag MUST be set to 1. The Client may set the Will
Flag as desired (marked as 'X' in Figure 6). Username and Password
flags MUST be set to 1 to ensure that the Payload of the CONNECT
message includes both Username and Password fields.
The CONNECT message defaults to ACE for authentication and
authorization. The Username field MUST be set to the access token.
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The Password field MUST be set to the keyed message digest (MAC) or
signature associated with the access token for proof-of-possession.
The Client MUST apply the PoP key to the payload as described in
Section 2.1.2.1.
In MQTT v3.1.1, the MQTT Username as a UTF-8 encoded string (i.e., is
prefixed by a 2-byte length field followed by UTF-8 encoded character
data) and may be up to 65535 bytes. Therefore, an access token that
is not a valid UTF-8 MUST be Base64 [RFC4648] encoded. (The MQTT
Password allows binary data up to 65535 bytes.)
3.2. Handling Authorization Errors
Handling errors are more primitive in MQTT v3.1.1 due to not having
appropriate error fields, error codes, and server-side DISCONNECTS.
In the following, we list how errors are handled without such
protocol support.
o CONNECT without a token: It is not possible to support AS
discovery via sending a tokenless CONNECT message to the Broker.
This is because a CONNACK packet in MQTT v3.1.1 does not include a
means to provide additional information to the Client. Therefore,
AS discovery needs to take place out-of-band. CONNECT attempt
MUSY fail.
o Client-RS PUBLISH authorization failure: In case of a failure, it
is not possible to return an error in MQTT v3.1.1.
Acknowledgement messages only indicate success. In the case of an
authorization error, the Broker SHOULD disconnect the Client.
Otherwise, it MUST ignore the PUBLISH message. Also, DISCONNECT
messages are only sent from a Client to the Broker. So, server
disconnection needs to take place below the application layer.
o SUBSCRIBE authorization failure: In the SUBACK packet, the return
code must be 0x80 indicating 'Failure' for the unauthorized
topic(s). Note that, in both MQTT versions, a reason code is
returned for each Topic Filter.
o RS-Client PUBLISH authorization failure: When RS is forwarding
PUBLISH messages to the subscribed Clients, it may discover that
some of the subscribers are no more authorized due to expired
tokens. These token expirations SHOULD lead to disconnecting the
Client, rather than silently dropping messages.
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4. IANA Considerations
The following registrations are done for the ACE OAuth Profile
Registry following the procedure specified in
[I-D.ietf-ace-oauth-authz].
Note to the RFC editor: Please replace all occurrences of "[RFC-
XXXX]" with the RFC number of this specification and delete this
paragraph.
Profile name: mqtt_tls
Profile description: Profile for delegating Client authentication and
authorization using MQTT as the application protocol and TLS For
transport layer security.
Profile ID:
Change controller: IESG
Reference: [RFC-XXXX]
5. Security Considerations
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework
[I-D.ietf-ace-oauth-authz]. Therefore, the security considerations
outlined in [I-D.ietf-ace-oauth-authz] apply to this work.
In addition, the security considerations outlined in MQTT v5.0 - the
OASIS Standard [MQTT-OASIS-Standard-v5] and MQTT v3.1.1 - the OASIS
Standard [MQTT-OASIS-Standard] apply. Mainly, this document provides
an authorization solution for MQTT, the responsibility of which is
left to the specific implementation in MQTT v5.0 standard. In the
following, we comment on a few relevant issues based on the current
MQTT specifications.
In this document, RS uses the PoP access token to authenticate the
Client. If the Client is able, TLS certificates sent from the Client
can be used by the RS to authenticate the Client. The Client may
authenticate the RS either using a server cerficate or the RPK
method. In the case of RPK, client needs to use this raw public key
in the TLS handshake together with an out-of-band validation
technique (see [RFC7250] for details).
To authorize a Client's publish and subscribe requests in an ongoing
session, the RS caches the access token after accepting the
connection from the Client. However, if some permissions are revoked
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in the meantime, the RS may still grant publish/subscribe to revoked
topics. If the RS caches the token introspection responses, then the
RS should use a reasonable cache timeout to introspect tokens
regularly. When permissions change dynamically, it is expected that
AS also follows a reasonable expiration strategy for the access
tokens.
The RS may monitor Client behaviour to detect potential security
problems, especially those affecting availability. These include
repeated token transfer attempts to the public "authz-info" topic,
repeated connection attempts, abnormal terminations, and Clients that
connect but do not send any data. If the RS supports the public
"authz-info" topic, described in Appendix B, then this may be
vulnerable to a DDoS attack, where many Clients use the "authz-info"
public topic to transport fictitious tokens, which RS may need to
store indefinitely.
6. Privacy Considerations
The privacy considerations outlined in [I-D.ietf-ace-oauth-authz]
apply to this work.
In MQTT, the RS is a central trusted party and may forward
potentially sensitive information between Clients. Clients may
choose to encrypt the payload of their messages. However, this would
not provide privacy for other properties of the message such as Topic
Name.
7. References
7.1. Normative 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-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)", draft-ietf-ace-oauth-authz-24
(work in progress), March 2019.
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[MQTT-OASIS-Standard]
Banks, A., Ed. and R. Gupta, Ed., "OASIS Standard MQTT
Version 3.1.1 Plus Errata 01", 2015, <http://docs.oasis-
open.org/mqtt/mqtt/v3.1.1/mqtt-v3.1.1.html>.
[MQTT-OASIS-Standard-v5]
Banks, A., Ed., Briggs, E., Ed., Borgendale, K., Ed., and
R. Gupta, Ed., "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>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[fremantle14]
Fremantle, P., Aziz, B., Kopecky, J., and P. Scott,
"Federated Identity and Access Management for the Internet
of Things", research International Workshop on Secure
Internet of Things, September 2014,
<http://dx.doi.org/10.1109/SIoT.2014.8>.
[I-D.ietf-ace-cwt-proof-of-possession]
Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
possession-08 (work in progress), October 2019.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
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[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>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>.
Appendix A. Checklist for profile requirements
o AS discovery: AS discovery is possible with the MQTT v5.0
described in Section 2.1.2.
o The communication protocol between the Client and RS: MQTT
o The security protocol between the Client and RS: TLS
o Client and RS mutual authentication: RS provides a server
certificate or RPK during TLS handshake. Client transports token
and MAC via the MQTT CONNECT message.
o Content format: For the HTTPS interactions with AS, "application/
ace+json". The MQTT payloads may be formatted in JSON.
o PoP protocols: Either symmetric or asymmetric keys can be
supported.
o Unique profile identifier: mqtt_tls
o Token introspection: RS uses HTTPS /introspect interface of AS.
o Token request: CAS uses HTTPS /token interface of AS.
o /authz-info endpoint: It MAY be supported using the method
described in Appendix B, but is not protected.
o Token transport: In MQTT CONNECT message for both versions of
MQTT. AUTH extensions also used for authentication and re-
authentication for MQTT v5.0 as described in Section 2.1.2.
Appendix B. The Authorization Information Endpoint
The main document described a method for transporting tokens inside
MQTT CONNECT messages. In this section, we describe an alternative
method to transport an access token.
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The method consists of the MQTT Broker accepting PUBLISH messages to
a public "authz-info" topic. A Client using this method MUST first
connect to the Broker, and publish the access token using the "authz-
info" topic. The Broker must verify the validity of the token (i.e.,
through local validation or introspection). After publishing the
token, the Client disconnects from the Broker and is expected to try
reconnecting over TLS.
In MQTT v5.0, the Broker can return 'Not authorized' error to a
PUBLISH request for QoS greater or equal to 1. In MQTT v3.1.1, after
the Client published to the "authz-info" topic, it is not possible
for the Broker to communicate the result of the token verification.
In any case, any token authorization failure affect the subsequent
TLS handshake, which can prompt the Client to obtain a valid token.
Appendix C. Document Updates
Version 00 to 01:
o Present the MQTTv5 as the RECOMMENDED version, and MQTT v3.1.1 for
backward compatibility.
o Clarified Will message.
o Improved consistency in the use of terminology, and upper/lower
case.
o Defined Broker and MQTTS.
o Clarified HTTPS use for C-AS and RS-AS communication. Removed
reference to actors document, and clarified the use of client
authorization server.
o Clarified the Connect message payload and Client Identifier.
o Presented different methods for passing the token, and PoP.
o Added new figures for AUTH methods, updated CONNECT message
figure.
Acknowledgements
The authors would like to thank Ludwig Seitz for his review and his
input on the authorization information endpoint, presented in the
appendix.
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Authors' Addresses
Cigdem Sengul
Nominet
4 Kingdom Street
London W2 6BD
UK
Email: Cigdem.Sengul@nominet.uk
Anthony Kirby
Oxbotica
1a Milford House, Mayfield Road, Summertown
Oxford OX2 7EL
UK
Email: anthony@anthony.org
Paul Fremantle
University of Portsmouth
School of Computing, Buckingham House
Portsmouth PO1 3HE
UK
Email: paul.fremantle@port.ac.uk
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