Internet Engineering Task Force (IETF) C. Sengul

Internet Engineering Task Force (IETF) C. Sengul
Request for Comments: 9431 Brunel University
Category: Standards Track A. Kirby
ISSN: 2070-1721 Oxbotica
July 2023

Message Queuing Telemetry Transport (MQTT) and Transport Layer Security
(TLS) Profile of Authentication and Authorization for Constrained
Environments (ACE) Framework

Abstract

This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework to enable
authorization in a publish-subscribe messaging system based on
Message Queuing Telemetry Transport (MQTT). Proof-of-Possession
keys, bound to OAuth 2.0 access tokens, are used to authenticate and
authorize MQTT Clients. The protocol relies on TLS for
confidentiality and MQTT server (Broker) authentication.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9431.

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.

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in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Requirements Language
1.2. ACE-Related Terminology
1.3. MQTT-Related Terminology
2. Authorizing Connection Requests
2.1. Client Token Request to the Authorization Server (AS)
2.2. Client Connection Request to the Broker (C)
2.2.1. Overview of Client-RS Authentication Methods over TLS
and MQTT
2.2.2. authz-info: The Authorization Information Topic
2.2.3. Client Authentication over TLS
2.2.3.1. Raw Public Key Mode
2.2.3.2. Pre-Shared Key Mode
2.2.4. Client Authentication over MQTT
2.2.4.1. Transporting the Access Token inside the MQTT
CONNECT
2.2.4.2. Authentication Using the AUTH Property
2.2.5. Broker Token Validation
2.3. Token Scope and Authorization
2.4. Broker Response to Client Connection Request
2.4.1. Unauthorized Request and the Optional Authorization
Server Discovery
2.4.2. Authorization Success
3. Authorizing PUBLISH and SUBSCRIBE Packets
3.1. PUBLISH Packets from the Publisher Client to the Broker
3.2. PUBLISH Packets from the Broker to the Subscriber Clients
3.3. Authorizing SUBSCRIBE Packets
4. Token Expiration, Update, and Reauthentication
5. Handling Disconnections and Retained Messages
6. Reduced Protocol Interactions for MQTT v3.1.1
6.1. Token Transport
6.2. Handling Authorization Errors
7. IANA Considerations
7.1. TLS Exporter Labels Registration
7.2. Media Type Registration
7.3. ACE OAuth Profile Registration
7.4. AIF
8. Security Considerations
9. Privacy Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Checklist for Profile Requirements
Acknowledgments
Authors' Addresses

1. Introduction

This document specifies a profile for the ACE framework [RFC9200].
In this profile, Clients and Servers (Brokers) 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 OASIS Standard
[MQTT-OASIS-Standard-v5]. Since it is expected that MQTT deployments
will continue to support MQTT v3.1.1 Clients, this document also
describes a reduced set of protocol interactions for the MQTT v3.1.1
OASIS Standard [MQTT-OASIS-Standard-v3.1.1]. However, MQTT v5.0 is
the RECOMMENDED version, as it works more naturally with ACE-style
authentication and authorization.

MQTT is a publish-subscribe protocol, and after connecting to the
MQTT Server (Broker), a Client can publish and subscribe to multiple
topics. The Broker, which acts as the Resource Server (RS), is
responsible for distributing messages published by the publishers to
their subscribers. In the rest of the document, the terms "RS",
"MQTT Server", and "Broker" are used interchangeably.

Messages are published under a Topic Name, and subscribers subscribe
to the Topic Names to receive the corresponding messages. The Broker
uses the Topic Name in a published message to determine which
subscribers to relay the messages to. In this document, topics (more
specifically, Topic Names) 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 preconfigure policies at the Authorization
Server (AS) that give Clients publish or subscribe permissions to
different topics.

Clients prove their permission to publish and subscribe to topics
hosted on an MQTT Broker using an access token that is bound to a
Proof-of-Possession (PoP) key. This document describes how to
authorize the following exchanges between the Clients and the Broker.

* connection requests from the Clients to the Broker

* publish requests from the Clients to the Broker and from the
Broker to the Clients

* subscribe requests from the Clients to the Broker

Clients use the MQTT PUBLISH packet to publish to a topic. The
mechanisms specified in this document do not protect the Payload of
the PUBLISH packet from the Broker. Hence, the Payload is not signed
or encrypted specifically for the subscribers. This functionality
may be implemented using the proposal outlined in the ACE Pub-Sub
Profile [ACE-PUBSUB-PROFILE].

To provide communication confidentiality and Broker authentication to
the MQTT Clients, TLS is used, and TLS 1.3 [RFC8446] is RECOMMENDED.
This document makes the same assumptions as Section 4 of the ACE
framework [RFC9200] regarding Client and RS registration with the AS
for setting up the keying material. While the Client-Broker
exchanges are only over MQTT, the required Client-AS and RS-AS
interactions are described for HTTPS-based communication [RFC9110],
using the "application/ace+json" content type and, unless otherwise
specified, JSON encoding. The token MAY be an opaque reference to
authorization information or a JSON Web Token (JWT) [RFC7519]. For
JWTs, this document follows [RFC7800] for PoP semantics for JWTs, and
the mechanisms for providing and verifying PoP are detailed in
Section 2.2. The Client-AS and RS-AS exchanges MAY also use
protocols other than HTTP, e.g., Constrained Application Protocol
(CoAP) [RFC7252] or MQTT. It is recommended that TLS is used to
secure these communication channels between Client-AS and RS-AS. To
reduce the protocol memory and bandwidth requirements,
implementations MAY also use the "application/ace+cbor" content type,
Concise Binary Object Representation (CBOR) encoding [RFC8949], CBOR
Web Tokens (CWTs) [RFC8392], and associated PoP semantics. For more
information, see "Proof-of-Possession Key Semantics for CBOR Web
Tokens (CWTs)" [RFC8747]. A JWT uses JSON Object Signing and
Encryption (JOSE), while a CWT uses CBOR Object Signing and
Encryption (COSE) [RFC9052] for security protection.

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

Certain security-related terms, such as "authentication",
"authorization", "data confidentiality", "(data) integrity", "message
authentication code" (MAC), and "verify", are taken from [RFC4949].

The terminology for entities in the architecture is defined in OAuth
2.0 [RFC6749], such as "Client" (C), "Resource Server" (RS), and
"Authorization Server" (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. This document also
defines a Client Authorization Server for Clients that are not able
to support HTTP.

Client Authorization Server (CAS)
An entity that prepares and endorses authentication and
authorization data for a Client and communicates to the AS
using HTTPS.

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 (which are
labeled with their topics). For additional information, please refer
to the MQTT v5.0 OASIS Standard [MQTT-OASIS-Standard-v5] or MQTT
v3.1.1 OASIS Standard [MQTT-OASIS-Standard-v3.1.1].

Broker
The Server in MQTT. It acts as an intermediary between the
Clients that publish Application Messages and the Clients
that made Subscriptions. The Broker acts as the Resource
Server for the Clients.

Client
A device or program that uses MQTT.

Network Connection
A construct provided by the underlying transport protocol
that is being used by MQTT. It connects the Client to the
Server. It provides the means to send an ordered, lossless
stream of bytes in both directions. This document uses TLS
as the transport protocol.

Session
A stateful interaction between a Client and a Broker. Some
Sessions last only as long as the Network Connection; others
can span multiple Network Connections.

Application Message
The data carried by the MQTT protocol. The data has an
associated Quality-of-Service (QoS) level and Topic Name.

MQTT Control Packet
The MQTT protocol operates by exchanging a series of MQTT
Control Packets. Each packet is composed of a Fixed Header,
a Variable Header (depending on the Control Packet type), and
a Payload.

UTF-8-encoded string
A string prefixed with a two-byte-length field that gives the
number of bytes in a UTF-8-encoded string itself. Unless
stated otherwise, all UTF-8-encoded strings can have any
length in the range 0 to 65535 bytes.

Binary Data
Binary Data is represented by a two-byte-length field, which
indicates the number of data bytes, followed by that number
of bytes. Thus, the length of Binary Data is limited to the
range of 0 to 65535 bytes.

Variable Byte Integer
A Variable Byte Integer is encoded using an encoding scheme
that uses a single byte for values up to 127. For larger
values, the least significant seven bits of each byte encode
the data, and the most significant bit is used to indicate
whether there are bytes following in the representation.
Thus, each byte encodes 128 values and a "continuation bit".
The maximum number of bytes in the Variable Byte Integer
field is four.

QoS level
The level of assurance for the delivery of an Application
Message. The QoS level can be 0-2, where 0 indicates "At
most once delivery", 1 indicates "At least once delivery",
and 2 indicates "Exactly once delivery".

Property
The last field of the Variable Header is a set of properties
for several MQTT Control Packets (e.g., CONNECT and CONNACK).
A property consists of an Identifier that defines its usage
and data type, followed by a value. The Identifier is
encoded as a Variable Byte Integer. For example, the
"Authentication Data" property uses the identifier 22.

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 QoS. A
Subscription is associated with a single Session.

Topic Filter
An expression that indicates interest in one or more Topic
Names. Topic Filters may include wildcards.

MQTT sends various Control Packets across a Network Connection. The
following is not an exhaustive list, and the Control Packets that are
not relevant for authorization are not explained. For instance,
these include the PUBREL and PUBCOMP packets used in the 4-step
handshake required for QoS level 2.

CONNECT
The Client requests to connect to the Broker. This is the
first packet sent by a Client.

CONNACK
The Broker connection acknowledgment. CONNACK packets
contain return codes that indicate either a success or an
error state in response to a Client's CONNECT packet.

AUTH
An AUTH Control Packet is sent from the Client to the Broker
or from the Broker to the Client as part of an extended
authentication exchange. AUTH properties include the
Authentication Method and Authentication Data. The
Authentication Method is set in the CONNECT packet, and
consequent AUTH packets follow the same Authentication
Method. The contents of the Authentication Data are defined
by the Authentication Method.

PUBLISH
Publish request sent from a publishing Client to the Broker
or from the Broker to a subscribing Client.

PUBACK
Response to a PUBLISH request with QoS level 1. PUBACK can
be sent from the Broker to a Client or from a Client to the
Broker.

PUBREC
Response to a PUBLISH request with QoS level 2. PUBREC can
be sent from the Broker to a Client or from a Client to the
Broker.

SUBSCRIBE
Subscribe request sent from a Client.

SUBACK
Subscribe acknowledgment from the Broker to the Client.

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 also alive. The "Keep Alive" period is
set in the CONNECT packet.

PINGRESP
Response sent by the Broker to the Client in response to
PINGREQ. It indicates the Broker is alive.

DISCONNECT
The DISCONNECT packet is the final MQTT Control Packet sent
from the Client or the Broker. It indicates the reason why
the Network Connection is being closed. If the Network
Connection is closed without the Client first sending a
DISCONNECT packet with reason code 0x00 (Normal
disconnection) and the MQTT Connection has a Will Message,
the Will Message is published.

Will
If the Network Connection is not closed normally, the Broker
sends a last Will Message for the Client if the Client
provided one in its CONNECT packet. Situations in which the
Will Message is published include, but are not limited to,
the following:

* an I/O error or network failure detected by the Broker,

* the Client fails to communicate within the Keep Alive
period,

* the Client closes the Network Connection without first
sending a DISCONNECT packet with reason code 0x00 (Normal
disconnection), and

* the Broker closes the Network Connection without first
receiving a DISCONNECT packet with reason code 0x00
(Normal disconnection).

If the Will Flag is set in the CONNECT flags, then the
Payload of the CONNECT packet includes information about the
Will. The information consists of the Will Properties, Will
Topic, and Will Payload fields.

2. Authorizing Connection Requests

This section specifies how Client connections are authorized by the
AS and verified by the MQTT Broker. Figure 1 shows the basic
protocol flows during connection setup. The token request and
response use the token endpoint at the AS, specified for HTTP-based
interactions in Section 5.8 of the ACE framework [RFC9200]. Steps
(D) and (E) are optional and use the introspection endpoint specified
in Section 5.9 of the ACE framework [RFC9200]. The discussion in
this document assumes that the Client and the Broker use HTTPS to
communicate with the AS via these endpoints. The Client and the
Broker use MQTT to communicate between them. The C-AS and Broker-AS
communications MAY be implemented using protocols other than HTTPS,
e.g., CoAP or MQTT. Whatever protocol is used for the C-AS and
Broker-AS communications MUST provide mutual authentication,
confidentiality protection, and integrity protection.

If the Client is resource constrained or does not support HTTPS, a
separate Client Authorization Server may carry out the token request
on behalf of the Client (Figure 1, steps (A) and (B)) and, later,
onboard the Client with the token. The interactions between a Client
and its Client Authorization Server for token onboarding and support
for MQTT-based token requests at the AS are out of the scope of this
document.

+---------------------+
| Client |
| |
+---(A) Token request------| Client - |
| | Authorization |
| +-(B) Access token-----> Server Interface |
| | | (HTTPS) |
| | |_____________________|
| | | |
+--v-------------+ | Pub/Sub Interface |
| Authorization | | (MQTT over TLS) |
| Server | +----------------^----+
|________________| | |
| ^ (C) Connection (F) Connection
| | request + response
| | access token |
| | | |
| | +---v--------------+
| | | Broker |
| | | (MQTT over TLS) |
| | |__________________|
| +(D) Introspection-----| |
| request (optional)| RS-AS interface |
| | (HTTPS) |
+-(E) Introspection-------->|__________________|
response (optional)

Figure 1: Connection Setup

2.1. Client Token Request to the Authorization Server (AS)

The first step in the protocol flow (Figure 1, step (A)) is the token
acquisition by the Client from the AS. The Client and the AS MUST
perform mutual authentication. The Client requests an access token
from the AS, as described in Section 5.8.1 of the ACE framework
[RFC9200]. The document follows the procedures defined in
Section 3.2.1 of the DTLS profile [RFC9202] for raw public keys
(RPKs) [RFC7250]) and in Section 3.3.1 of [RFC7250] for pre-shared
keys (PSKs). However, the content type of the request is set to
"application/ace+json", and the AS uses JSON in the Payload of its
responses to the Client and the RS. As explained earlier,
implementations MAY also use the "application/ace+cbor" content type.

On receipt of the token request, the AS verifies the request. If the
AS successfully verifies the access token request and authorizes the
Client for the indicated audience (i.e., RS) and scopes (i.e.,
publish/subscribe permissions over topics, as described in
Section 2.3), the AS issues an access token (Figure 1, step (B)).

The response includes the parameters described in Section 5.8.2 of
the ACE framework [RFC9200]. For RPKs, the parameters are as
described in Section 3.2.1 of the DTLS profile [RFC9202]. For PSKs,
the document follows Section 3.3.1 of the DTLS profile [RFC9202]. In
both cases, if the response contains an "ace_profile" parameter, this
parameter is set to "mqtt_tls". The returned token is a Proof-of-
Possession (PoP) token by default.

This document follows [RFC7800] for PoP semantics for JWTs (CWTs MAY
also be used). The AS includes a "cnf" (confirmation) parameter in
the PoP token to declare that the Client possesses a particular key
and the RS can cryptographically confirm that the Client has
possession of that key, as described in [RFC9201].

Note that the contents of the web tokens (including the "cnf"
parameter) are to be consumed by the RS and not the Client (the
Client obtains the key information in a different manner). The RPK
case is handled as described in Section 3.2.1 of the DTLS profile
[RFC9202]. For the PSK case, the referenced procedures apply, with
the following exceptions to accommodate JWT and JOSE use. In this
case, the AS adds a "cnf" parameter to the Access Information
carrying a JSON Web Key (JWK) [RFC7517] object that contains either
the symmetric key itself or a key identifier that can be used by the
RS to determine the secret key it shares with the Client. The JWT is
created as explained in Section 7 of [RFC7519], and the JWT MUST
include a JSON Web Encryption (JWE) [RFC7516]. If a CWT/COSE is
used, this information MUST be inside the "COSE_Key" object and MUST
be encrypted using a "COSE_Encrypt0" structure.

The AS returns error responses for JSON-based interactions following
Section 5.2 of [RFC6749]. When CBOR is used, the interactions MUST
implement the procedure described in Section 5.8.3 of the ACE
framework [RFC9200].

2.2. Client Connection Request to the Broker (C)

2.2.1. Overview of Client-RS Authentication Methods over TLS and MQTT

Unless the Client publishes and subscribes to only public topics, the
Client and the Broker MUST perform mutual authentication. The Client
MUST authenticate to the Broker either over MQTT or TLS before
performing any other action. For MQTT, the options are "None" and
"ace". For TLS, the options are "Anon" for an anonymous client, and
"Known(RPK/PSK)" for RPKs and PSKs, respectively. The "None" and
"Anon" options do not provide client authentication but can be used
either during authentication or in combination with authentication at
the other layer. When the Client uses TLS:Anon,MQTT:None, the Client
can only publish or subscribe to public topics. Thus, the client
authentication procedures involve the following possible
combinations:

TLS:Anon,MQTT:None:
This option is used only for the topics that do not require
authorization, including the "authz-info" topic. Publishing
to the "authz-info" topic is described in Section 2.2.2.

TLS:Anon,MQTT:ace:
The token is transported inside the CONNECT packet and MUST
be validated using one of the methods described in
Section 2.2.2. This option also supports a tokenless
connection request for AS discovery. As per the ACE
framework [RFC9200], a separate step is needed to determine
whether the discovered AS URI is authorized to act as an AS.

TLS:Known(RPK/PSK),MQTT:none:
This specification supports client authentication with TLS
with RPKs and PSKs, following the procedures described in the
DTLS profile [RFC9202]. For the RPK, the Client MUST have
published the token to the "authz-info" topic. For the PSK,
the token MAY be published to the "authz-info" topic or MAY
be, alternatively, provided as a "PSK identity" (e.g., an
"identity" in the "identities" field in the Client's
"pre_shared_key" extension in TLS 1.3).

TLS:Known(RPK/PSK),MQTT:ace:
This option SHOULD NOT be chosen as the token transported in
the CONNECT packet and overwrites any permissions passed
during the TLS authentication.

It is RECOMMENDED that the Client implements TLS:Anon,MQTT:ace as the
first choice when working with protected topics. However, MQTT
v3.1.1 Clients that do not prefer to overload the User Name and
Password fields for ACE (as described in Section 6) MAY implement
TLS:Known(RPK/PSK),MQTT:none and, consequently, TLS:Anon,MQTT:None to
submit their token to "authz-info".

The Broker MUST support TLS:Anon,MQTT:ace. To support Clients with
different capabilities, the Broker MAY provide multiple client
authentication options, e.g., support TLS:Known(RPK),MQTT:none and
TLS:Anon,MQTT:None, to enable RPK-based client authentication.

The Client MUST authenticate the Broker during the TLS handshake. If
the Client authentication uses TLS:Known(RPK/PSK), then the Broker is
authenticated using the respective method. Otherwise, to
authenticate the Broker, the Client MUST validate a public key from
an X.509 certificate or an RPK from the Broker against the "rs_cnf"
parameter in the token response, which contains information about the
public key used by the RS to authenticate if the token type is "pop"
and asymmetric keys are used as defined in [RFC9201]. The AS MAY
include the thumbprint of the RS's X.509 certificate in the "rs_cnf"
(thumbprint, as defined in [RFC9360]). In this case, the Client MUST
validate the RS certificate against this thumbprint.

2.2.2. authz-info: The Authorization Information Topic

In the cases when the Client must transport the token to the Broker
first, the Client connects to the Broker to publish its token to the
"authz-info" topic. The "authz-info" topic MUST only be published
(i.e., the Clients are not allowed to subscribe to it). "authz-info"
is not protected, and hence, the Client uses the TLS:Anon,MQTT:None
option over a TLS connection. After publishing the token, the Client
disconnects from the Broker and is expected to reconnect using client
authentication over TLS (i.e., TLS:Known(RPK/PSK),MQTT:none).

The Broker stores and indexes all tokens received to the "authz-info"
topic in its key store (similar to the DTLS profile for ACE
[RFC9202]). This profile follows the recommendation of
Section 5.10.1 of the ACE framework [RFC9200] and expects that the
Broker stores only one token per PoP key, and any other token linked
to the same key overwrites an existing token.

The Broker MUST verify the validity of the token (i.e., through local
validation or introspection if the token is a reference), as
described in Section 2.2.5. If the token is not valid, the Broker
MUST discard the token.

Depending on the QoS level of the PUBLISH packet, the Broker returns
the error response as a PUBACK, PUBREC, or DISCONNECT packet. If the
QoS level is equal to 0, and the token is not valid, or if the claims
cannot be obtained in the case of an introspected token, the Broker
MUST send a DISCONNECT packet with reason code 0x87 (Not authorized).
If the PUBLISH Payload does not parse to a token, the Broker MUST
send a DISCONNECT with reason code 0x99 (Payload format invalid).

If the QoS level of the PUBLISH packet is greater than or equal to 1,
and the token is not valid, or the claims cannot be obtained in the
case of an introspected token, the Broker MUST send reason code 0x87
(Not authorized) in the PUBACK or PUBREC. If the PUBLISH Payload
does not parse to a token, the PUBACK/PUBREC reason code is 0x99
(Payload format invalid).

When the Broker sends the "Not authorized" response, it must be noted
that this corresponds to the token being not valid and not that the
actual PUBLISH packet was not authorized. Given that the "authz-
info" is a public topic, this response is not expected to cause
confusion.

2.2.3. Client Authentication over TLS

This document supports TLS with raw public keys (RPKs) [RFC7250] and
with pre-shared keys (PSKs). The TLS session setup follows the DTLS
profile for ACE [RFC9202], as the profile applies to TLS equally well
[RFC9430]. When there are exceptions to the DTLS profile, these are
explicitly stated in the document. If TLS 1.2 is used, [RFC7925]
describes how TLS can be used for constrained devices, alongside
recommended cipher suites. Additionally, TLS 1.2 implementations
MUST use the "Extended Main Secret" extension (terminology adopted
from [TLS-bis]) to incorporate the handshake transcript into the main
secret [RFC7627]. TLS implementations SHOULD use the Server Name
Indication (SNI) [RFC6066] and Application-Layer Protocol Negotiation
(ALPN) [RFC7301] extensions so the TLS handshake authenticates as
much of the protocol context as possible.

2.2.3.1. Raw Public Key Mode

This document follows the procedures defined in Section 3.2.2 of the
DTLS profile for ACE [RFC9202] with the following exceptions. The
Client MUST upload the access token to the Broker using the method
specified in Section 2.2.2 before initiating the handshake.

2.2.3.2. Pre-Shared Key Mode

This document follows the procedures defined in Section 3.3.2 of the
DTLS profile for ACE [RFC9202] with the following exceptions.

To use TLS 1.3 with pre-shared keys, the Client utilizes the PSK
extension specified in [RFC8446] using the key conveyed in the "cnf"
parameter of the AS response. The same key is bound to the access
token in the "cnf" claim. The Client can upload the token, as
specified in Section 2.2.2, before initiating the handshake. When
using a previously uploaded token, the Client MUST indicate during
the handshake which previously uploaded access token it intends to
use. To do so, it MUST create a "COSE_Key" or "JWK" structure with
the "kid" that was conveyed in the "rs_cnf" claim in the token
response from the AS and the key type "symmetric". This structure is
then included as the only element in the "cnf" structure and the
encoded value of that "cnf" structure used as a PSK identity in TLS.
As an alternative to the access token upload, the Client can provide
the most recent access token, JWT or CWT, as a PSK identity.

In contrast to the DTLS profile for ACE [RFC9202], a Client MAY omit
support for the cipher suites TLS_PSK_WITH_AES_128_CCM_8 and
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8. For TLS 1.2, however, a client
MUST support TLS_ECDHE_PSK_WITH_AES_128_GCM_SHA256 for PSKs [RFC8442]
and TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 for RPKs [RFC8422], as
recommended in [RFC9325] (and adjusted to be a PSK cipher suite as
appropriate).

2.2.4. Client Authentication over MQTT

2.2.4.1. Transporting the Access Token inside the MQTT CONNECT

This section describes how the Client transports the token to the
Broker inside the CONNECT packet. If this method is used, the Client
TLS connection is expected to be anonymous, and the Broker is
authenticated during the TLS connection setup. The approach
described in this section is similar to an earlier proposal by
Fremantle, et al. [Fremantle14].

After sending the CONNECT packet, the Client MUST wait to receive the
CONNACK packet from the Broker. The only packets it is allowed to
send are DISCONNECT or AUTH that are in response to the Broker AUTH.
Similarly, except for a DISCONNECT and AUTH response from the Client,
the Broker MUST NOT process any packets before sending a CONNACK
packet.

Figure 2 shows the structure of the MQTT CONNECT packet used in MQTT
v5.0. A CONNECT packet is composed of a Fixed Header, a Variable
Header, and a Payload The Fixed Header contains the Control Packet
Type (CPT), Reserved, and Remaining Length fields. The Remaining
Length is a Variable Byte Integer that represents the number of bytes
remaining within the current Control Packet, including data in the
Variable Header and the Payload. The Variable Header contains the
Protocol Name, Protocol Level, Connect flags, Keep Alive, and
Properties fields. The Connect flags in the Variable Header specify
the properties of the MQTT Session. It also indicates the presence
or absence of some 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 the flags in the
Variable Header. The Client Identifier identifies the Client to the
Broker and, therefore, is unique for each Client. It must be noted
that the Client Identifier is an unauthenticated identifier used
within the MQTT protocol and so is not bound to the access token.

0 8 16
+---------------------------+
|Protocol name length = 4 |
+---------------------------+
| 'M' 'Q' |
+---------------------------+
| 'T' 'T' |
+---------------------------+
|Proto.level=5|Connect flags|
+---------------------------+
| Keep alive |
+---------------------------+
| CONNECT Properties Length |
| (up to 4 bytes) |
+---------------------------+
| ( ..Other properties..) |
+---------------------------+
| Authentication Method |
| (0x15) | Len |
| Len | 'a' |
| 'c' | 'e' |
+---------------------------+
| Authentication Data |
| (0x16) | Len |
| Len | token |
| or token + PoP data |
+---------------------------+

Figure 2: MQTT v5 CONNECT Variable Header with Authentication
Method Property for ACE

The CONNECT flags are User Name, Password, Will Retain, Will QoS,
Will Flag, Clean Start, and Reserved. Table 1 shows how the flags
MUST be set to use AUTH packets for authentication and authorization,
i.e., the User Name Flag and Password Flag MUST be set to 0. An MQTT
v5.0 Broker MAY also support token transport using the User Name and
Password to provide a security option for MQTT v3.1.1 Clients, as
described in Section 6.

+===========+==========+========+======+======+=======+==========+
| User Name | Password | Will | Will | Will | Clean | Reserved |
| Flag | Flag | Retain | QoS | Flag | Start | |
+===========+==========+========+======+======+=======+==========+
| 0 | 0 | X | X X | X | X | 0 |
+-----------+----------+--------+------+------+-------+----------+

Table 1: CONNECT Flags for AUTH

The Will Flag indicates that a Will Message needs to be sent. The
Client MAY set the Will Flag as desired (marked as "X" in Table 1).
If the Will Flag is set to 1, the Broker MUST check that the token
allows the publication of the Will Message (i.e., the Will Topic
Filter is in the scope array). The check is performed against the
token scope described in Section 2.3. If the Will authorization
fails, the connection is refused, as described in Section 2.4.1. If
the Broker accepts the connection request, the Broker stores the Will
Message and publishes it when the Network Connection is closed
according to Will QoS, Will Retain parameters, and MQTT Will
management rules. To avoid publishing the Will Messages in the case
of temporary network disconnections, the Client specifies a Will
Delay Interval in the Will Properties. Section 5 explains how the
Broker deals with the retained messages in further detail.

In MQTT v5.0, the Client signals a new Session (i.e., that the
Session does not continue an existing Session) by setting the Clean
Start flag to 1 in the CONNECT packet. In this profile, the Client
SHOULD always start with a new Session. The Broker MAY also signal
that it does not support the continuation of an existing Session by
setting the Session Expiry Interval to 0 in the CONNACK. If the
Broker starts a new Session, the Broker MUST set the Session Present
flag to 0 in the CONNACK packet to signal this to the Client.

The Broker MAY support continuing an existing Session, e.g., if the
Broker requires it for QoS reasons. In this case, if a CONNECT
packet is received with Clean Start set to 0, and there is a Session
associated with the Client Identifier, the Broker MUST resume
communications with the Client based on the state from the existing
Session. In its response, the Broker MUST set the Session Present
flag to 1 in the CONNACK packet to signal the continuation of an
existing Session to the Client. The Session State stored by the
Client and the Broker is described in Section 5.

When reconnecting to a Broker that supports continuing existing
Sessions, the Client MUST still provide a token in addition to using
the same Client Identifier and setting the Clean Start to 0. The
Broker MUST still perform PoP validation on the provided token. If
the token matches the stored state, the Broker MAY skip introspecting
a token-by-reference and use the stored introspection result. The
Broker MUST also verify the Client is authorized to receive or send
MQTT packets that are pending transmission. When a Client connects
with a long Session Expiry Interval, the Broker may need to maintain
the Client's MQTT Session State after it disconnects for an extended
period. Brokers SHOULD implement administrative policies to limit
misuse.

Note that, according to the MQTT standard, the Broker uses the Client
Identifier to identify the Session State. In the case of a Client
Identifier collision, a Client may take over another Client's
Session. Given that the Broker MUST associate the Client with a
valid token, a Client will only send or receive messages to its
authorized topics. Therefore, while this issue is not expected to
affect security, it may affect QoS (i.e., PUBLISH or QoS messages
saved for Client A may be delivered to a Client B). In addition, if
this Client Identifier represents a Client already connected to the
Broker, the Broker sends a DISCONNECT packet to the existing Client
with reason code 0x8E (Session taken over) and closes the connection
to the Client.

2.2.4.2. Authentication Using the AUTH Property

Figure 2 shows the Authentication Method and Authentication Data
fields when the client authenticates using the AUTH property. 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 Broker does not support
this profile, it sends a CONNACK packet with reason code 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, the Broker MUST support both options
below:

* proof of possession using a challenge from the TLS session

* proof of possession via a Broker-generated challenge/response

2.2.4.2.1. Proof of Possession Using a Challenge from the TLS Session

+-----------------------------------------------------------------+
|Authentication|Token Length|Token |MAC or Signature |
|Data Length | | |(over TLS exporter content) |
+-----------------------------------------------------------------+

Figure 3: Authentication Data for PoP Based on TLS Exporter Content

For this option, the Authentication Data inside the Client's CONNECT
packet MUST contain the two-byte integer token length, the token, and
the keyed message digest (MAC) or the Client signature (as shown in
Figure 3). The Proof-of-Possession key in the token is used to
calculate the keyed message digest (MAC) or the Client signature
based on the content obtained from the TLS exporter ([RFC5705] for
TLS 1.2 and Section 7.5 of [RFC8446] for TLS 1.3). This content is
exported from the TLS session using the exporter label "EXPORTER-ACE-
MQTT-Sign-Challenge", an empty context, and a length of 32 bytes.
The token is also validated, as described in Section 2.2.5, and the
Broker responds with a CONNACK packet with the appropriate response
code. The Client cannot reauthenticate using this method during the
same TLS session (see Section 4).

2.2.4.2.2. Proof of Possession via Broker-generated Challenge/Response

+------------------------------------+
|Authentication|Token Length|Token |
|Data Length | | |
+------------------------------------+

Figure 4: Authentication Data to Initiate PoP Based on Challenge/
Response

+--------------------------+
|Authentication|RS Nonce |
|Data Length |(8 bytes) |
+--------------------------+

Figure 5: Authentication Data for Broker Challenge

For this option, the Broker follows a Broker-generated challenge/
response protocol. If the Authentication Data inside the Client's
CONNECT contains only the two-byte integer token length and the token
(as shown in Figure 4), the Broker MUST respond with an AUTH packet
with the authenticated reason code set to 0x18 (Continue
Authentication). The Broker also uses this method if the
Authentication Data does not contain a token, but the Broker has a
token stored for the connecting Client.

The Broker continues authentication using an AUTH packet that
contains the Authentication Method and the Authentication Data. The
Authentication Method MUST be set to "ace", and the Authentication
Data MUST NOT be empty and MUST contain an 8-byte RS nonce as a
challenge for the Client (Figure 5).

+---------------------------------------------------------+
|Authentication|Client Nonce |MAC or Signature |
|Data Length |(8 bytes) |(over RS nonce+Client nonce)|
+---------------------------------------------------------+

Figure 6: Authentication Data for the Client Challenge Response

The Client responds to this with an AUTH packet with reason code 0x18
(Continue Authentication). Similarly, the Client packet sets the
Authentication Method to "ace". The Authentication Data in the
Client's response is formatted as shown in Figure 6 and includes the
8-byte Client nonce and the signature or MAC computed over the RS
nonce concatenated with the Client nonce using PoP key in the token.

Next, the token is validated as described in Section 2.2.5. The
success case is illustrated in Figure 7. The Client MAY also
reauthenticate using this challenge-response flow, as described in
Section 4.

Client Broker
| |
|<===========>| TLS connection setup
| |
| |
+------------>| CONNECT with Authentication Data
| | contains only token
| |
<-------------+ AUTH 0x18 (Cont. Authentication)
| | 8-byte RS nonce as challenge
| |
|------------>| AUTH 0x18 (Cont. Authentication)
| | 8-byte Client nonce + signature/MAC
| |
| |---+ Token validation
| | | (may involve introspection)
| |<--+
| |
|<------------+ CONNACK 0x00 (Success)

Figure 7: PoP Challenge/Response Flow - Success

2.2.5. Broker Token Validation

The Broker MUST verify the validity of the token either locally
(e.g., in the case of a self-contained token) or MAY send a request
to the introspection endpoint of the AS (as described for HTTP-based
interactions in Section 5.9 of the ACE framework [RFC9200]). The
Broker MUST verify the claims in the access token according to the
rules set in Section 5.10.1.1 of the ACE framework [RFC9200].

To authenticate the Client, the Broker validates the signature or the
MAC, depending on how the PoP protocol is implemented. For self-
contained tokens, the Broker MUST process the security protection of
the token first, as specified by the respective token format, i.e., a
CWT uses COSE, while a JWT uses JOSE. For a token-by-reference, the
Broker uses the "cnf" structure returned as a result of token
introspection, as specified in [RFC7519]. HMAC-SHA-256 (HS256)
[RFC6234] and Ed25519 [RFC8032] are mandatory to implement for the
Broker. The Client MUST implement at least one of them depending on
the choice of symmetric or asymmetric validation. Validation of the
signature or MAC MUST fail if the signature algorithm is set to
"none" when the key used for the signature algorithm cannot be
determined or the computed and received signature/MAC do not match.

The Broker MUST check if the access token is still valid, if it is
the intended destination (i.e., the audience) of the token, and if
the token was issued by an authorized Authorization Server. If the
Client is using TLS RPK mode to authenticate to the Broker, the AS
constructs the access token so that the Broker can associate the
access token with the Client's public key. The "cnf" claim MUST
contain either the Client's RPK or, if the key is already known by
the Broker (e.g., from previous communication), a reference to it.

2.3. Token Scope and Authorization

The scope field contains the publish and subscribe permissions for
the Client. Therefore, the token or its introspection result MUST be
cached to allow a Client's future PUBLISH and SUBSCRIBE messages.
During the CONNECT, if the Will Flag is set to 1, the Broker MUST
also authorize the publication of the Will Topic and Will Message
using the token's scope field. The Broker uses the scope to match
against the Topic Name in a PUBLISH packet (including Will Topic in
the CONNECT) or a Topic Filter in a SUBSCRIBE packet.

The scope in the token is a single value. For a JWT, the single
scope is a base64url-encoded string with any padding characters
removed, which has an internal structure of a JSON array. For a CWT,
this information is represented in CBOR. The internal structure
follows the Authorization Information Format (AIF) for ACE [RFC9237].
Using the Concise Data Definition Language (CDDL) [RFC8610], the
specific data model for MQTT is:

AIF-MQTT = AIF-Generic<mqtt-topic-filter, mqtt-permissions>
AIF-Generic<Toid, Tperm> = [* [Toid, Tperm]]
mqtt-topic-filter = tstr ; as per Section 4.7 of MQTT v5.0
mqtt-permissions = [+permission]
permission = "pub"/"sub"

Figure 8: AIF-MQTT Data Model

Topic Filters are implemented according to Section 4.7 of the MQTT
v5.0 OASIS Standard [MQTT-OASIS-Standard-v5]. By default, Wildcard
Subscriptions are supported, and so, the Topic Filter may include
special wildcard characters. The multi-level wildcard, "#", matches
any number of levels within a topic, and the single-level wildcard,
"+", matches one topic level. The Broker MAY signal in the CONNACK
explicitly whether Wildcard Subscriptions are supported by returning
a CONNACK property "Wildcard Subscription Available". A value of 0
means that Wildcard Subscriptions are not supported. A value of 1
means Wildcard Subscriptions are supported.

Following this model, an example scope may contain:

[["topic1",["pub","sub"]],["topic2/#",["pub"]],["+/topic3",["sub"]]]

Figure 9: Example Scope

This access token gives publish ("pub") and subscribe ("sub")
permissions to the "topic1", publish permission to all the subtopics
of "topic2", and subscribe permission to all "topic3", skipping one
level.

If the scope is empty, the Broker records no permissions for the
Client for any topic. In this case, the Client is not able to
publish or subscribe to any protected topics. The non-empty scope is
used to authorize the Will Topic, if provided, in the CONNECT packet,
during connection setup and, if the connection request succeeds, the
Topic Names or Topic Filters requested in the future PUBLISH and
SUBSCRIBE packets. For the authorization to succeed, the Broker MUST
verify that the Topic Name or Topic Filter in question is either an
exact match to or a subset of at least one "topic_filter" in the
scope.

2.4. Broker 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 packet to the Client.

2.4.1. Unauthorized Request and the Optional Authorization Server
Discovery

Authentication can fail for the following reasons:

* if the Client does not provide a valid token,

* the Client omits the Authentication Data field and the Broker has
no token stored for the Client,

* the token or Authentication data are malformed, or

* if the Will Flag is set, the authorization checks for the Will
Topic fails.

The Broker responds with the CONNACK reason code 0x87 (Not
Authorized) or any other applicable reason code.

The Broker MAY also trigger AS discovery and include a User Property
(identified as property type 38 (0x26)) in the CONNACK for the AS
Request Creation Hints. The User Property is a UTF-8 string pair,
composed of a name and a value. The name of the User Property MUST
be set to "ace_as_hint". The value of the User Property is a UTF-
8-encoded JSON object containing the mandatory "AS" parameter and the
optional parameters "audience", "kid", "cnonce", and "scope", as
defined in Section 5.3 of the ACE framework [RFC9200].

2.4.2. Authorization Success

On success, the reason code of the CONNACK is 0x00 (Success). If the
Broker starts a new Session, it MUST also set Session Present to 0 in
the CONNACK packet to signal a new Session to the Client. Otherwise,
it MUST set Session Present to 1.

Having accepted the connection, the Broker MUST be prepared to store
the token during the connection and after disconnection for future
use. 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 packets. PUBLISH and SUBSCRIBE
packets, which are sent after a connection setup, do not contain
access tokens. If the introspection result is not cached, the Broker
needs to introspect the saved token for each request. The Broker
SHOULD also use a cache timeout to introspect tokens regularly. The
timeout value is specific to the application and should be chosen to
reduce the risk of using stale introspection responses.

3. Authorizing PUBLISH and SUBSCRIBE Packets

Using the cached token or its introspection result, the Broker uses
the scope field to match against the Topic Name in a PUBLISH packet
or a Topic Filter in a SUBSCRIBE packet.

3.1. PUBLISH Packets from the Publisher Client to the Broker

On receiving the PUBLISH packet, the Broker MUST use the type of
packet (i.e., PUBLISH) and the Topic Name in the packet header to
match against the scope array items in the cached token or its
introspection result. Following the example in Section 2.3, the
Client sending a PUBLISH packet for "topic2/a" would be allowed, as
the scope array includes the ["topic2/#",["pub"]].

If the Client is allowed to publish to the topic, the Broker
publishes the message to all valid subscribers of the topic. In the
case of an authorization failure, the Broker MUST return an error if
the Client has set the QoS level of the PUBLISH packet to greater
than or equal to 1. Depending on the QoS level, the Broker responds
with either a PUBACK or PUBREC packet with reason code 0x87 (Not
authorized). On receiving an acknowledgment with 0x87 (Not
authorized), the Client MAY reauthenticate by providing a new token,
as described in Section 4.

For QoS level 0, the Broker sends a DISCONNECT packet with reason
code 0x87 (Not authorized) and closes the Network Connection. 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).

For all QoS levels, the Broker MAY return 0x80 (Unspecified error) if
they do not want to leak the Topic Names to unauthorized clients.

3.2. PUBLISH Packets from the Broker to the Subscriber Clients

To forward PUBLISH packets to the subscribing Clients, the Broker
identifies all the subscribers that have valid matching Topic
Subscriptions to the Topic Name of the PUBLISH packet (i.e., the
tokens are valid, and token scopes allow a Subscription to this
particular Topic Name). The Broker forwards the PUBLISH packet to
all the valid subscribers.

The Broker MUST NOT forward messages to unauthorized subscribers. To
avoid silently dropping messages, the Broker MUST close the Network
Connection and SHOULD inform the affected subscribers. In this case,
the only way to inform a client would be sending a DISCONNECT packet.
Therefore, the Broker SHOULD send a DISCONNECT packet with reason
code 0x87 (Not authorized) before closing the Network Connection to
these clients.

3.3. Authorizing SUBSCRIBE Packets

In MQTT, a SUBSCRIBE packet is sent from a Client to the Broker to
create one or more Subscriptions to one or more topics. The
SUBSCRIBE packet may contain multiple Topic Filters. The Topic
Filters may include wildcard characters.

On receiving the SUBSCRIBE packet, the Broker MUST use the type of
packet (i.e., SUBSCRIBE) and the Topic Filter in the packet header to
match against the scope field of the stored token or introspection
result. The Topic Filters MUST be an exact match to or be a subset
of at least one of the "topic_filter" fields in the scope array found
in the Client's token. For example, if the Client sends a SUBSCRIBE
request for topic "a/b/*" and has a token that permits "a/*", this is
a valid SUBSCRIBE request, as "a/b/*" is a subset of "a/*". (The
process is similar to a Broker matching the Topic Name in a PUBLISH
packet against the Subscriptions known to the Server.)

As a response to the SUBSCRIBE packet, the Broker issues a SUBACK
packet. 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. The Broker MAY return 0x80 (Unspecified
error) if they do not want to leak the Topic Names to unauthorized
clients. 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.

4. Token Expiration, Update, and Reauthentication

The Broker MUST check for token expiration whenever a CONNECT,
PUBLISH, or SUBSCRIBE packet is received or sent. The Broker SHOULD
check for token expiration on receiving a PINGREQ packet. The Broker
MAY also check for token expiration periodically, e.g., every hour.
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 AS, as described in Section 5.9 of the ACE framework
[RFC9200]. Token expirations may trigger the Broker to send PUBACK,
SUBACK, and DISCONNECT packets with the return code set to "Not
authorized". After sending a DISCONNECT packet, the Network
Connection is closed, and no more messages can be sent.

The Client MAY reauthenticate a response to PUBACK and SUBACK, which
signal loss of authorization. The Clients MAY also proactively
update their tokens, i.e., before they receive a packet with a "Not
authorized" return code. To start reauthentication, the Client MUST
send an AUTH packet with reason code 0x19 (Reauthentication). The
Client MUST set the Authentication Method as "ace" and transport the
new token in the Authentication Data. If reauthenticating during the
current TLS session, the Client MUST NOT use the method described in
Section 2.2.4.2.1, i.e., proof of possession using a challenge from
the TLS session, to avoid reusing the same challenge value from the
TLS-Exporter. Note that this means that servers will either need to
record in the session ticket or database entry whether the TLS-
Exporter-derived challenge was used or always deny use of the TLS-
Exporter-derived challenge for resumed sessions. In TLS 1.3, the
resumed connection would have a new exporter value, but the
requirement is phrased this way for simplicity. For
reauthentications in the same TLS-session, the Client MUST use the
challenge-response PoP, as defined in Section 2.2.4.2.2. The Broker
accepts reauthentication requests if the Client has already submitted
a token (may be expired), for which it performed proof of possession.
Otherwise, the Broker MUST deny the request. If the reauthentication
fails, the Broker MUST send a DISCONNECT packet with reason code 0x87
(Not Authorized).

5. Handling Disconnections and Retained Messages

In the case of a Client DISCONNECT, if the Session Expiry Interval is
set to 0, the Broker doesn't store the Session State but MUST keep
the retained messages. If the Broker stores the Session State, the
state MAY include the token and its introspection result (for
reference tokens) in addition to the MQTT Session State. The MQTT
Session State is identified by the Client Identifier and includes the
following:

* the Client Subscriptions,

* messages with QoS levels 1 and 2, which have not been completely
acknowledged or are pending transmission to the Client, and

* if the Session is currently not connected, the time at which the
Session will end and the Session State will be discarded.

The token/introspection state is not part of the MQTT Session State,
and PoP validation is required for each new connection, regardless of
whether existing MQTT Sessions are continued.

The messages to be retained are indicated to the Broker by setting a
RETAIN flag in a PUBLISH packet. This way, the publisher signals to
the Broker to store the most recent message for the associated topic.
Hence, the new subscribers can receive the last sent message from the
publisher for that particular topic without waiting for the next
PUBLISH packet. The Broker MUST continue publishing the retained
messages as long as the associated tokens are valid. In the MQTT
standard, if QoS is 0 for the PUBLISH packet, the Broker may discard
the retained message any time. For QoS > 1, the message expiry
interval dictates how long the retained message is kept. However, it
is important that the Broker avoids sending messages indefinitely for
the Clients that never update their tokens (i.e., the Client connects
briefly with a valid token, sends a PUBLISH packet with the RETAIN
flag set to 1 and QoS > 1, disconnects, and never connects again).
Therefore, the Broker MUST use the minimum of the token expiry and
message expiry interval to discard a retained message.

In case of disconnections due to network errors or server
disconnection due to a protocol error (which includes authorization
errors), the Will Message is sent if the Client supplied a Will in
the CONNECT packet. The Client's token scope array MUST include the
Will Topic. The Will Message MUST be published to the Will Topic,
regardless of whether the corresponding token has expired (as it has
been validated and accepted during CONNECT).

6. Reduced Protocol Interactions for MQTT v3.1.1

This section describes a reduced set of protocol interactions for the
MQTT v3.1.1 Clients. An MQTT v5.0 Broker MAY implement these
interactions for the MQTT v3.1.1 Clients; the flows described in this
section are NOT RECOMMENDED for use by MQTT v5.0 Clients. Brokers
that do not support MQTT v3.1.1 Clients return a CONNACK packet with
reason code 0x84 (Unsupported Protocol Version) in response to the
connection requests.

6.1. Token Transport

As in MQTT v5.0, the token MAY either be transported before, by
publishing to the "authz-info" topic, or inside the CONNECT packet.
If the Client provided the token via the "authz-info" topic and will
not update the token in the CONNECT packet, it MUST authenticate over
TLS. The Broker SHOULD still be prepared to store the Client access
token for future use (regardless of the method of transport).

In MQTT v3.1.1, after the Client has published to the "authz-info"
topic, the Broker cannot communicate the result of the token
validation because PUBACK reason codes or server-side DISCONNECT
packets are not supported. In any case, the subsequent TLS handshake
would fail without a valid token, which can prompt the Client to
obtain a valid token.

To transport the token to the Broker inside the CONNECT packet, the
Client uses the User Name and Password fields. Figure 10 shows the
structure of the MQTT CONNECT packet.

0 8 16
+---------------------------+
|Protocol name length = 4 |
+---------------------------+
| 'M' 'Q' |
+---------------------------+
| 'T' 'T' |
+---------------------------+
|Proto.level=5|Connect flags|
+---------------------------+
| Keep alive |
+---------------------------+
| Payload |
| Client Identifier |
| (UTF-8-encoded string) |
| User Name as access token |
| (UTF-8-encoded string) |
| Password for signature/MAC|
| (Binary Data) |
+---------------------------+

Figure 10: MQTT CONNECT Variable Header Using a User Name and
Password for ACE

Table 2 shows how the MQTT connect flags MUST be set to initiate a
connection with the Broker.

+================+==========+========+======+======+=======+=======+
| User Name Flag | Password | Will | Will | Will | Clean | Rsvd. |
| | Flag | Retain | QoS | Flag | | |
+================+==========+========+======+======+=======+=======+
| 1 | 1 | X | X X | X | X | 0 |
+----------------+----------+--------+------+------+-------+-------+

Table 2: MQTT CONNECT Flags (Rsvd. = Reserved)

The Client SHOULD set the Clean flag to 1 to always start a new
Session. If the Clean flag is set to 0, the Broker MUST resume
communications with the Client based on the state from the current
Session (as identified by the Client Identifier). If there is no
Session associated with the Client Identifier, the Broker MUST create
a new Session. The Broker MUST set the Session Present flag in the
CONNACK packet accordingly, i.e., 0 to indicate a new Session to the
Client and 1 to indicate that the existing Session is continued. The
Broker MUST still perform PoP validation on the provided Client
token. MQTT v3.1.1 does not use a Session Expiry Interval, and the
Client expects that the Broker maintains the Session State after it
disconnects. However, the stored Session State can be discarded as a
result of administrator action or policies (e.g., defining an
automated response based on storage capabilities), and Brokers SHOULD
implement administrative policies to limit misuse.

The Client MAY set the Will Flag as desired (marked as "X" in
Table 2). User Name and Password flags MUST be set to 1 to ensure
that the Payload of the CONNECT packet includes both the User Name
and Password fields. The MQTT User Name is a UTF-8-encoded string,
and the MQTT Password is Binary Data.

The CONNECT in MQTT v3.1.1 does not have a field to indicate the
Authentication Method. To signal that the User Name field contains
an ACE token, this field MUST be prefixed with the keyword "ace",
i.e., the User Name field is a concatenation of 'a', 'c', 'e', and
the access token represented as:

'U+0061'||'U+0063'||'U+0065'||UTF-8(access token)

Figure 11: User Name in CONNECT

To this end, the access token MUST be encoded with base64url,
omitting the "=" padding characters [RFC4648].

The Password field MUST be set to the keyed message digest (MAC) or
signature associated with the access token for PoP. The Client MUST
apply the PoP key on the challenge derived from the TLS session, as
described in Section 2.2.4.2.1.

6.2. Handling Authorization Errors

Error handling is more primitive in MQTT v3.1.1 due to not having
appropriate error fields, error codes, and server-side DISCONNECTs.
Therefore, the Broker will disconnect on almost any error and may not
keep the Session State, necessitating that clients make a greater
effort to ensure that tokens remain valid and do not attempt to
publish to topics that they do not have permissions for. The
following lists how the Broker responds to specific errors.

CONNECT without a token:
The tokenless CONNECT attempt MUST fail. This is because the
challenge-response-based PoP is not possible for MQTT v3.1.1.
It is also not possible to support AS discovery since 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.

Client-Broker PUBLISH authorization failure:
In the case of a failure, it is not possible to return an
error in MQTT v3.1.1. Acknowledgment messages only indicate
success. In the case of an authorization error, the Broker
MUST ignore the PUBLISH packet and disconnect the Client.
Also, as DISCONNECT packets are only sent from a Client to
the Broker, the server disconnection needs to take place
below the application layer.

SUBSCRIBE authorization failure:
In the SUBACK packet, the return code is 0x80, indicating
failure for the unauthorized topic(s). Note that, in both
MQTT versions, a reason code is returned for each Topic
Filter.

Broker-Client PUBLISH authorization failure:
When the Broker is forwarding PUBLISH packets to the
subscribed Clients, it may discover that some of the
subscribers are no longer authorized due to expired tokens.
These token expirations MUST lead to disconnecting the Client
rather than silently dropping messages.

7. IANA Considerations

7.1. TLS Exporter Labels Registration

This document registers "EXPORTER-ACE-MQTT-Sign-Challenge"
(introduced in Section 2.2.4.2.1 in this document) in the "TLS
Exporter Labels" registry [RFC8447].

Recommended: N

DTLS-OK: N

Reference: RFC 9431

7.2. Media Type Registration

This document registers the "application/ace+json" media type for
messages of the protocols defined in this document carrying
parameters encoded in JSON.

Type name: application

Subtype name: ace+json

Required parameters: N/A

Optional parameters: N/A

Encoding considerations: Encoding considerations are identical to
those specified for the "application/json" media type.

Security considerations: Section 8 of RFC 9431

Interoperability considerations: none

Published specification: RFC 9431

Applications that use this media type: This media type is intended
for Authorization-Server-Client and Authorization-Server-Resource-
Server communication as part of the ACE framework using JSON
encoding, as specified in RFC 9431.

Fragment identifier considerations: none

Additional information:

Deprecated alias names for this type: none

Magic number(s): none

File extension(s): none

Macintosh file type code(s): none

Person & email address to contact for further information:
Cigdem Sengul <[email protected]>

Intended usage: COMMON

Restrictions on usage: none

Author: Cigdem Sengul <[email protected]>

Change controller: IETF

7.3. ACE OAuth Profile Registration

The following registrations have been made in the "ACE Profiles"
registry, following the procedure specified in [RFC9200].

Name: mqtt_tls

Description: Profile for delegating Client authentication and
authorization using MQTT for the Client and Broker (RS)
interactions and HTTP for the AS interactions. TLS is used for
confidentiality and integrity protection and server
authentication. Client authentication can be provided either via
TLS or using in-band PoP validation at the MQTT application layer.

CBOR Value: 3

Reference: RFC 9431

7.4. AIF

For the media types "application/aif+cbor" and "application/
aif+json", defined in Section 5.1 of [RFC9237], IANA has registered
the following entries for the two media type parameters Toid and
Tperm in the respective subregistry defined in Section 5.2 of
[RFC9237] within the "Media Type Sub-Parameter Registries".

For Toid:
Name: mqtt-topic-filter

Description/Specification: Topic Filter, as defined in
Section 2.3 of RFC 9431.

Reference: RFC 9431, Section 2.3

For Tperm:
Name: mqtt-permissions

Description/Specification: Permissions for the MQTT Client, as
defined in Section 2.3 of RFC 9431. Tperm is an array of one
or more text strings that each have a value of either "pub" or
"sub".

Reference: RFC 9431, Section 2.3

8. Security Considerations

This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework [RFC9200].
Therefore, the security considerations outlined in [RFC9200] apply to
this work.

In addition, the security considerations outlined in the MQTT v5.0
OASIS Standard [MQTT-OASIS-Standard-v5] and MQTT v3.1.1 OASIS
Standard [MQTT-OASIS-Standard-v3.1.1] apply. Mainly, this document
provides an authorization solution for MQTT, the responsibility of
which is left to the specific implementation in the MQTT standards.
In the following, we comment on a few relevant issues based on the
current MQTT specifications.

After the Broker validates an access token and accepts a connection
from a client, it caches the token to authorize a Client's publish
and subscribe requests in an ongoing Session. The Broker does not
cache any tokens that cannot be validated. If a Client's permissions
get revoked, but the access token has not expired, the Broker may
still grant publish/subscribe to revoked topics. If the Broker
caches the token introspection responses, then the Broker SHOULD use
a reasonable cache timeout to introspect tokens regularly. The
timeout value is application specific and should be chosen to reduce
the risk of using stale introspection responses. When permissions
change dynamically, it is expected that the AS also follows a
reasonable expiration strategy for the access tokens.

The Broker may monitor Client behavior 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 Broker supports the public
"authz-info" topic, described in Section 2.2.2, then this may be
vulnerable to a DDoS attack, where many Clients use the "authz-info"
public topic to transport tokens that are not meant to be used and
that the Broker may need to store until they expire.

For MQTT v5.0, when a Client connects with a long Session Expiry
Interval, the Broker may need to maintain the Client's MQTT Session
State after it disconnects for an extended period. For MQTT v3.1.1,
the Session State may need to be stored indefinitely, as it does not
have a Session Expiry Interval feature. The Broker SHOULD implement
administrative policies to limit misuse by the Client resulting from
continuing existing Sessions.

9. Privacy Considerations

The privacy considerations outlined in [RFC9200] apply to this work.

In MQTT, the Broker is a central trusted party and may forward
potentially sensitive information between Clients. The mechanisms
defined in this document do not protect the contents of the PUBLISH
packet from the Broker, and hence, the content of the PUBLISH packet
is not signed or encrypted separately for the subscribers. This
functionality may be implemented using the proposal outlined in the
ACE Pub-Sub Profile [ACE-PUBSUB-PROFILE]. However, this solution
would still not provide privacy for other fields of the packet, such
as Topic Name.

10. References

10.1. Normative References

[MQTT-OASIS-Standard-v3.1.1]
Banks, A., Ed. and R. Gupta, Ed., "MQTT Version 3.1.1 Plus
Errata 01", OASIS Standard, December 2015,
<https://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., "MQTT Version 5.0", OASIS Standard, March
2019, <https://docs.oasis-open.org/mqtt/mqtt/v5.0/mqtt-
v5.0.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>.

[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.

[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.

[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>.

[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>.

[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.

[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, DOI 10.17487/RFC7516, May 2015,
<https://www.rfc-editor.org/info/rfc7516>.

[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<https://www.rfc-editor.org/info/rfc7517>.

[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.

[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/info/rfc7627>.

[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>.

[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.

[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>.

[RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier", RFC 8422,
DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>.

[RFC8442] Mattsson, J. and D. Migault, "ECDHE_PSK with AES-GCM and
AES-CCM Cipher Suites for TLS 1.2 and DTLS 1.2", RFC 8442,
DOI 10.17487/RFC8442, September 2018,
<https://www.rfc-editor.org/info/rfc8442>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.

[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.

[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/info/rfc9052>.

[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/info/rfc9200>.

[RFC9201] Seitz, L., "Additional OAuth Parameters for Authentication
and Authorization for Constrained Environments (ACE)",
RFC 9201, DOI 10.17487/RFC9201, August 2022,
<https://www.rfc-editor.org/info/rfc9201>.

[RFC9202] 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)", RFC 9202,
DOI 10.17487/RFC9202, August 2022,
<https://www.rfc-editor.org/info/rfc9202>.

[RFC9237] Bormann, C., "An Authorization Information Format (AIF)
for Authentication and Authorization for Constrained
Environments (ACE)", RFC 9237, DOI 10.17487/RFC9237,
August 2022, <https://www.rfc-editor.org/info/rfc9237>.

[RFC9360] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Header Parameters for Carrying and Referencing X.509
Certificates", RFC 9360, DOI 10.17487/RFC9360, February
2023, <https://www.rfc-editor.org/info/rfc9360>.

[RFC9430] Bergmann, O., Preuß Mattsson, J., and G. Selander,
"Extension of the Datagram Transport Layer Security (DTLS)
Profile for Authentication and Authorization for
Constrained Environments (ACE) to Transport Layer Security
(TLS)", RFC 9430, DOI 10.17487/RFC9430, July 2023,
<https://www.rfc-editor.org/info/rfc9430>.

10.2. Informative References

[ACE-PUBSUB-PROFILE]
Palombini, F., Sengul, C., and M. Tiloca, "Publish-
Subscribe Profile for Authentication and Authorization for
Constrained Environments (ACE)", Work in Progress,
Internet-Draft, draft-ietf-ace-pubsub-profile-06, 13 March
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
ace-pubsub-profile-06>.

[Fremantle14]
Fremantle, P., Aziz, B., Kopecky, J., and P. Scott,
"Federated Identity and Access Management for the Internet
of Things", International Workshop on Secure Internet of
Things, DOI 10.1109/SIoT.2014.8, September 2014,
<https://dx.doi.org/10.1109/SIoT.2014.8>.

[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>.

[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.

[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.

[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.

[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.

[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>.

[RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/info/rfc9325>.

[TLS-bis] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", Work in Progress, Internet-Draft, draft-
ietf-tls-rfc8446bis-09, 7 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
rfc8446bis-09>.

Appendix A. Checklist for Profile Requirements

Based on the requirements on profiles for the ACE framework
[RFC9200], this document fulfills the following:

* Optional AS discovery: AS discovery is supported with the MQTT
v5.0 described in Section 2.2.

* The communication protocol between the Client and Broker (RS):
MQTT

* The security protocol between the Client and RS: TLS

* Client and RS mutual authentication: Several options are possible
and described in Section 2.2.1.

* Proof-of-possession protocols: Both symmetric and asymmetric keys
are supported, as specified in Section 2.2.4.2.

* Content-Format: For the HTTPS interactions with AS, "application/
ace+json".

* Unique profile identifier: mqtt_tls

* Token introspection: The RS uses the HTTPS introspection interface
of the AS.

* Token request: The Client or its Client AS uses the HTTPS token
endpoint of the AS.

* authz-info endpoint: It MAY be supported using the method
described in Section 2.2.2 but is not protected other than by the
TLS channel between the Client and RS.

* Token transport: Via the "authz-info" topic, TLS with PSKs
(provided as a PSK identity), or in the MQTT CONNECT packet for
both versions of MQTT. The AUTH extensions can also be used for
authentication and reauthentication for MQTT v5.0, as described in
Sections 2.2 and 4.

Acknowledgments

The authors would like to thank Ludwig Seitz for his review and his
input on the authorization information endpoint; Benjamin Kaduk for
his review, insightful comments, and contributions to resolving
issues; and Carsten Bormann for his review and revisions to the AIF-
MQTT data model. The authors would like to thank Paul Fremantle for
the initial discussions on MQTT v5.0 support.

Authors' Addresses

Cigdem Sengul
Brunel University
Dept. of Computer Science
Uxbridge
UB8 3PH
United Kingdom
Email: [email protected]

Anthony Kirby
Oxbotica
1a Milford House
Mayfield Road, Summertown
Oxford
OX2 7EL
United Kingdom
Email: [email protected]