[Ace] AD Evaluation of draft-ietf-ace-mqtt-tls-profile-12

Benjamin Kaduk <kaduk@mit.edu> Thu, 05 August 2021 22:39 UTC

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Date: Thu, 5 Aug 2021 15:39:31 -0700
From: Benjamin Kaduk <kaduk@mit.edu>
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Subject: [Ace] AD Evaluation of draft-ietf-ace-mqtt-tls-profile-12
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Hi all,

Sorry to have taken so long to get back to this, and thank you for
continuing to make updates in response to the changes in the framework and
other profiles.

In general, the protocol mechansisms defined here are in good shape;
thank you!

I made a github PR with some changes that seemed easier to phrase in the
form of a patch than a prose comment:

I did find a couple of significant issues that need to be addressed
before IETF LC, but I think any needed changes will be pretty localized.

Specifically, there's no requirement for ACE access tokens to be
self-deliniating, so we can't actually programmatically tell whether
there's content after the token in a CONNECT message; the mechanisms in
Sections seem to assume that we can do so for
determining whether the CONNECT is just providing a token or also
providing PoP over a TLS exporter value.  I think that this just means
we need to have an explicit "token length" field (or similar).

There are also a few places where we seem to be putting requirements on
Broker behavior that are in direct conflict with normative requirements
of the MQTT specification.  We can't override the external spec, so
we'll need to check and reword in any places where there are conflicts.
(I'm not an expert on MQTT and only read the spec as part of doing this
review, so it's entirely possible that I'm misinterpreting the MQTT spec
in some or all of these locations.)

A few other general notes before the section-by-section notes:

There is very little in this document about the HTTP-based interactions
with the AS.  I think the intent is to defer to the core framework for
that, but being a little more explicit about what is being pulled in and
how would be helpful.

If we're using TLS Exporters and allow TLS-not-1.3, we need to make some
additional requirements on TLS usage in order for the exporter values to
be safe.  Typically this takes the form of requiring the extended master
secret extension along with guidance on what cipher suites to use; I
guess RFC 7925 (rather than 7525) would be the default reference for
other TLS usage.

This document seems to mostly use British English.  AFAIR, that's okay,
but if it's inconsistent the RFC Editor will prefer American English.  I
didn't attempt to check for this (though there are tools like
https://github.com/larseggert/ietf-reviewtool that will do so).

Section 1

   their subscribers.  In the rest of the document the terms "RS", "MQTT
   Server" and "Broker" are used interchangeably.

We will probably get a reviewer asking why we can't pick a single term
and standardize on it.  However, I expect that there are places where we
want to emphasize on one aspect or another of its behavior, so don't
think we should actually do so.  Similarly for the places where we
mention that CoAP can be used (but don't reference a concrete

   Clients use MQTT PUBLISH message to publish to a topic.  This
   document does not protect the payload of the PUBLISH message 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

I suggest s/MAY/may/, which would avoid any need to make
ace-pubsub-profile a normative reference.

   reference or JSON Web Token (JWT) [RFC7519].  For JWTs, this document
   follows [RFC7800] for PoP semantics for JWTs.  The Client-AS and RS-
   AS 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.  Implementations MAY also use "application/ace+cbor"
   content type, and CBOR encoding [RFC8949], and CBOR Web Token (CWT)
   [RFC8392] and associated PoP semantics to reduce the protocol memory
   and bandwidth requirements.  For more information, see Proof-of-
   Possession Key Semantics for CBOR Web Tokens (CWTs) [RFC8747].

One thing that can be surprising to readers not versed in the ecosystem
is that RFCs 7800 and 8747 only talk about the token claims that are
used to build the PoP system, and don't actually define mechanisms for
providing the proof of possession.  We might want a forward-reference to
§ 2 where we do actually specify mechansisms to prove possession of the
indicated key.

Section 2.2.1

The way we name these authentication options with specific (quoted)
strings suggests that they will be used as a protocol element.  But
where?  Is the literal string "Known(RPK/PSK)" used in both cases (vs.
having distinct strings for RPK and PSK)?

Also, the hyphen character tends to more often be used as a joiner than
a separator, so it's easy to misread this as a triple of "TLS",
"Anon-MQTT", "None"/etc..  (I originally was going to ask why these all
had a "TLS" prefix...)  It might be better to use a semicolon or comma
instead of hyphen.

   o  "TLS:Anon-MQTT:None": This option is used only for the topics that
      do not require authorization, including the "authz-info" topic.

Are there topics other than "authz-info" that don't require
authorization?  We might need to add some heding language to the earlier
statement that "Client and the Broker MUST perform mutual
authentication" if so.

   o  "TLS:Anon-MQTT:ace": The token is transported inside the CONNECT
      message, 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.

We should probably look carefully at the "for AS discovery" phrasing, in
light of the late changes in how the framework talks about the AS
Request Creation Hints.

   o  "TLS:Known(RPK/PSK)-MQTT:none": For the RPK, the token MUST have
      been published to the "authz-info" topic.  For the PSK, the token
      MAY be, alternatively, provided as an "identity" in the
      "identities" field in the client's "pre_shared_key" extension in
      TLS 1.3.  The TLS session set-up is as described in DTLS profile
      for ACE [I-D.ietf-ace-dtls-authorize].

A couple notes here:  First, the DTLS profile primarily uses (D)TLS 1.2
terminology, with the DTLS 1.3 flow as an addendum.  I'm actually happy
for us to only or primarily talk about the TLS 1.3 idiom here in this
document, but we may want to think about what phrasing we use for the
relationship between our usage and the DTLS profile.  Second, the DTLS
profile is for, well, DTLS, not TLS.  We should acknowledge that and
state the (lack of) effect on the procedures to follow.

   It is RECOMMENDED that the Client implements "TLS:Anon-MQTT:ace" as a
   first choice when working with protected topics.  However, depending
   on the Client capability, Client MAY implement "TLS:Known(RPK/PSK)-
   MQTT:none", and consequently "TLS:Anon-MQTT:None" to submit its token
   to "authz-info".

It's good that we provide guidance on which of the authentication
schemes are preferred (since we offer both ACE-layer and TLS-layer
schemes).  However, we will surely be asked to defend why there are two
possible ways of doing it instead of just one, and this text doesn't
really do a good job of that.  What might cause a need to implement

   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, but
   fall back to "TLS:Anon-MQTT:ace" if the Client does not send a client
   certificate (i.e. it sends an empty Certificate message) during the
   TLS handshake.

[Just a potential nit about the wording: "fall back" implies some
ordering requirements, but IIRC use of RPK requires sending the token to
authz-info before starting the new TLS connection, which doesn't quite
match the steps as ordered in this description.]

   The Broker MUST be authenticated 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 a
   X.509 certificate or an RPK from the Broker against the 'rs_cnf'
   parameter in the token response.  The AS MAY include the thumbprint
   of the RS's X.509 certificate in the 'rs_cnf' (thumbprint as defined
   in [I-D.ietf-cose-x509]).  In this case, the client MUST validate the
   RS certificate against this thumbprint.

Just to confirm: we consciously chose to not reference RFC 6125 for
"normal" X.509 server certificate validation procedures, in favor of the
"constrained environment" procedures we describe here?

Section 2.2.2

   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

I see that this is covered in the framework (and we reference the
appropriate section already), but I'd consider reiterating that "not
valid" includes "the Broker is not an intended audience of the token".

   It must be noted that when the RS sends the 'Not authorized'
   response, this corresponds to the token being invalid, and not that
   the actual PUBLISH message was not authorized.  Given that the
   "authz-info" is a public topic, this response is not expected to
   cause confusion.

Thanks for including this note.  I assume that it would be challenging
to get OASIS to allocate a new reason code for us to use, and we didn't
attempt to pursue that path.

Section 2.2.3

   Similarly, the Broker MUST NOT process any packets before it has sent
   a CONNACK.  The only exceptions are DISCONNECT or an AUTH response
   from the Client.

nit: in some pedantic sense, these two sentences are in conflict, since
the latter violates the "MUST NOT".  Who cares and how much has varied
over time, especially on the IESG, so it's not clear that we actually
need to change anything right now.

          |CPT=1 | Rsvd.|Remaining len.| Protocol name len. = 4  |

Figure 2 appears to show that the Remaining Length field of the fixed
header occupies a single octet, but IIUC it's encoded as a variable byte
integer.  Fixing that would also let us put the separator that appears
after it on a proper byte boundary.

          |                      'M' 'Q' 'T' 'T'                 |

This figure does not show the two-byte length prefix for the string.

   The Will Flag indicates that a Will message needs to be sent if the
   Interval in the Will Properties.  Section 5 explains how the Broker
   deals with the retained messages in further detail.

We might be able to get away with leaving the description of Will
operation to the MQTT spec, and not have to say so much about it here.

   In MQTT v5.0, the Client signals a clean session (i.e. the session
   does not continue an existing session), by setting the Clean Start
   Flag to 1 and, the Session Expiry Interval to 0 in the CONNECT
   message.  [...]

I don't understand why setting the Session Expiry Interval to 0 is
needed to produce a clean session.  As I understand it, setting this
interval to 0 merely directs the server to not store session state after
the client disconnects from this session, which is unrelated to whether
or not this is a new session or a reused session.

             In this profile, the Broker SHOULD always start with a
   clean session regardless of how these parameters are set.  Starting a
   clean session helps the Broker avoid keeping unnecessary session
   state for unauthorised clients.  If the Broker starts a clean

This SHOULD seems highly problematic to me.  It looks like it
contradicts a hard requirement of MQTT 5.0 ("If a CONNECT packet is
received with Clean Start set to 0 and there is a Session associated
with the Client Identifier, the Server MUST resume communications with
the Client based on state from the existing Session").  If we want to
recommend that the broker does not maintain session state, that should
be implemented by setting the Session Expiry Interval in the CONNACK,
not as part of the CONNECT processing.

   When reconnecting to a Broker that supports session continuation, the
   Client MUST still provide a token, in addition to using the same
   Client identifier, setting the Clean Start to 0 and supplying a
   Session Expiry interval in the CONNECT message.  The Broker MUST

(As above, setting a Session Expiry interval seems to relate to the
subsequent connection, not the current one, so this feels

   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.  [...]

Just to confirm: the ACE token is not used to provide authorization to
use a given client identifer; the client identifier is just used as an
unauthenticated identifier?  We might consider calling that out

   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).  [...]

I think this is just an aside and not something we need to cover in this
document, but what happens if (e.g.) PUBCOMP goes to client B when
PUBREC went to client A?  Does the message actually get delivered?  Does
anything deadlock?


(Expounding on the high-level comment from above,)
It seems that we're using the presence/absence of extra data after the
token to indicate whether or not the MAC/Signature over exporter content
is present and thus which message flow is being used for authentication.
However, this is only possible if the token itself is self-describing,
which I do not think is guaranteed.  Consider, for example, the case of
a token that's a reference value that must be introspected in order to
retrieve claim information.  Such tokens can be arbitrary byte strings,
so I think we need some other in-band way to differentiate between
authentication methods.

Section 2.2.4

   To use AUTH, 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'.  [...]

(I assume there's not an MQTT registry that we can register "ace" in as
an Authentication Method.)


   For this option, the Authentication Data MUST contain the two-byte
   integer token length, the token, and the keyed message digest (MAC)
   or the Client signature (as shown in Figure 4).  [...]

Does this go in the AUTH (which is what §2.2.4 claims to cover) or the
initial CONNECT?  (Hint: later in the paragraph the broker replies to it
with a CONNACK.)

   Section 7.5 of [RFC8446]).  This content is exported from the TLS
   session using the exporter label 'EXPORTER-ACE-MQTT-Sign-Challenge',
   an empty context, and length of 32 bytes.  [...]

While an empty context should provide ample protection, it seems to me
that we could consider using the client identifier as the context.
There can also be value in incorporating information on the server
identity in the output.  If the SNI extension is used, that information
would already be included in the key schedule, though we do not
currently seem to mandate SNI usage.  Current best practices for new
deployments are to always use SNI and always use ALPN, so we should
probably consider both of those.

   a CONNACK with the appropriate response code.  The client cannot
   reauthenticate using this method during the same session ( see
   Section 4).

Depending on what "session" means, this restriction may be too strict.
We should probably be more clear about what "session" means...if it's
the MQTT session, I think it's okay to use this method when
re-connecting to take over the session.


I a little bit wonder if we need to hardcode the nonce length or could
let it be variable, with the corresponding change in level of protection
provided.  In non-constrained setups we would typically use a 128- or
even 256-bit bit nonce for this purpose, not a 64-bit one, and it's
somewhat surprising to preclude the stronger usage.  (Especially so
since we use a 256-bit value from the TLS exporter.)  If we do allow for
length variation, we'll need to add length prefixes to the MAC/signature
input (we should probably do that anyway since the client nonce is

Section 2.2.5

   To authenticate the Client, the RS validates the signature or the
   MAC, depending on how the PoP protocol is implemented.  HS256 (HMAC-
   SHA-256) [RFC6234] and Ed25519 [RFC8032] are mandatory to implement
   depending on the choice of symmetric or asymmetric validation.

I think there is a decent argument (and that it's likely some other AD
will make it) that we need to make both HS256 and Ed25519 mandatory to
implement for the Broker, leaving only clients with the choice.
Otherwise we can get into scenarios where interop is impossible.

   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.

Where would the "none" appear?  We haven't said anything about a COSE
encoding for the signature or MAC value, or anything like that...I
assumed it was going to be the "raw" output from the relevant primitive
(EdDSA, HMAC, etc.).


   On success, the reason code of the CONNACK is "0x00 (Success)".  The
   AS informs the client that selected profile is "mqtt_tls" using the
   "ace_profile" parameter in the token response.  If the Broker starts

The line about the AS returning "mqtt_tls" as the selected profile feels
out of place here, where we're discussing successful MQTT

   a new session, it MUST also set Session Present to 0 in the CONNACK
   packet to signal a clean session to the Client.  Otherwise, it MUST
   set Session Present to 1.

(As above,) my understanding is that the Broker does not have agency
over whether a new session is started, and must honor the client's
request.  So these "MUST set" seem out of place.

   If the Broker accepts the connection, it MUST store the token until
   the end of the connection.  On Client or Broker disconnection, the
   Client is expected to transport a token again on the next connection

This seems to deviate somewhat from the framework, that expects the RS
to be prepared to store at least one token for future use, and
recommends storing one token per PoP key.

                                                             The Broker
   SHOULD also use a cache time out to introspect tokens regularly.

We will surely be asked to provide guidance on what timescale
"regularly" indicates, if we do not proactively provide some guidance.

Section 3

   The scope field contains the publish and subscribe permissions for
   the Client.  The scope is a JSON array, each item following the
   Authorization Information Format (AIF) for ACE [I-D.ietf-ace-aif].
   Using the Concise Data Definition Language (CDDL) [RFC8610], the
   specific data model for MQTT is:

This seems a little dicey, since we claim to allow JWT tokens as well as
CWT.  JWT "scope" is pretty tightly nailed down to be a space-separated
list (though CWT gives much greater freedom).  We probably need to have
some text about this situation, with a phrase about how "in order to be
compatible with the JWT scope format, we use a single scope value with
internal structure", that this structure is also compatible with the CWT
rules, and noting that our AIF usage prevents any internal spaces, so
the interpretation of the scope value is unambiguous even when
unmodified JWT libraries are used.

   If the Will Flag is set, then 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).

We might want to say a little more about when this check happens.  My
intuition is that it occurs during the CONNECT processing where the
actual Will message is provided, and that the connection would be
rejected if the Will message is unauthorized.  But this section is
titled "Authorizing PUBLISH and SUBSCRIBE Messages", so one might be
forgiven for assuming that CONNECT is out of scope...

Section 3.2

   subscription to the particular topic).  The Broker sends a PUBLISH
   message with the Topic name to all the valid subscribers.

(nit) It seems a little strange to mention specifically the Topic name
but say nothing about there being a payload along with it.

Section 3.3

   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 match against the scope field of the stored token or introspection
   result.  The Topic Filters MUST be equal or a subset of at least one
   of the 'topic_filter' fields in the scope array found in the Client's

I suggest being very explicit about whether the wildcards in the token
scope are expanded as part of matching the topic filter in the request,
or if the SUBSCRIBE message must use topic filters that match
byte-for-byte the permissions granted by the token.

Section 4

   Authentication Data.  The Broker accepts reauthentication requests if
   the Client has already submitted a token (may be expired) and
   validated via the challenge-response PoP.  Otherwise, the Broker MUST
   deny the request.  If the reauthentication fails, the Broker MUST
   send a DISCONNECT with the reason code "0x87 (Not Authorized)".

Is this correct?  It seems to say that if the initial CONNECT used the
TLS exporter for PoP, then it's forbidden to use the challenge-response
method for PoP and thus impossible to reauthenticate on that connection.
I don't understand why such a limitation would be needed or useful.

Section 5

   In the case of a Client DISCONNECT, the Broker deletes all the
   session state but MUST keep the retained messages.  By setting a

As written, this seems to imply that all client DISCONNECT messages
result in the loss of session state.  I didn't (quickly) find a clear
statement one way or the other in the MQTT spec, but it does seem that
it's allowed to send a nonzero session expiry interval in a DISCONNECT,
which seems to imply that such DISCONNECT messages do not cause all
session state to be discarded.

   Hence, the new subscribers can receive the last sent message from the
   publisher for 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.

This MUST seems to be repeating a requirement from the MQTT spec, which
may not merit normative language from us.

   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 message.  The Client's token scope array MUST include the

This "if the Client supplied a Will in the CONNECT message" implies that
authorization checks are performed at time of CONNECT...

   Will Topic.  The Will message MUST be published to the Will Topic

... but "scope array MUST include the Will Topic" suggests an
authorization check when the Will message is actually sent.  Is it one,
the other, or both checks that must pass?

Section 6.1

[My earlier comments about the MQTT header layout apply to Figure 11 as

   The Broker SHOULD NOT accept session continuation.  To this end, the
   Broker ignores how the Clean Session Flag is set, and on connection
   success, the Broker MUST set the Session Present flag to 0 in the
   CONNACK packet to indicate a clean session to the Client.  [...]

As above, this seems to violate the MQTT normative requirements (but I
mostly only read about MQTT 5, not 3.1.1).

   The CONNECT in MQTT v3.1.1 does not have a field to indicate the
   authentication method.  To signal that the Username field contains an
   ACE token, this field MUST be prefixed with 'ace' keyword, which is
   followed by the access token.  [...]

An example of what this looks like would be helpful.  It sounds like we
just take the first three bytes for the sentinel and then go directly to
the access token, vs having some kind of "separator" as part of the
sentinel value.

   In MQTT v3.1.1, the MQTT Username is 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.)

Data-dependent encoding transformation without explicit signaling is a
really bad idea.  I think we need to always base64-encode the token.
We should also specify a section reference to RFC 4648 (for plain base64
vs base64url) and probably make a statement about whether padding
characters are retained or omitted.

Section 6.2

   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.

(I'm not actually sure how much the MQTT spec says about this type of
scenario, and thus whether the "SHOULD" is the right term to use.)

Section 7.1

   This document registers 'EXPORTER-ACE-MQTT-Sign-Challenge'
   (introduced in Section in this document) in the TLS Exporter
   Label Registry [RFC8447].

We need to specify the other columns in the registry.
I think we can have:
Recommended: Y
Reference: [this document]

Section 7.2

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

Thanks for sending this to the media-types list for review
It's unfortunate that nobody replied to that thread, but I don't know
that there's more that we can do.

Section 7.3

   The following registrations are done for the ACE OAuth Profile
   Registry following the procedure specified in
   o  CBOR Value:

I think it's clearer for reviewers if we put "TBA" or "to be assigned by
IANA" here.

   o  Description: Profile for delegating Client authentication and
      authorization using MQTT as the application protocol and TLS For
      transport layer security.

It seems like it might be preferred to talk about using MQTT for the
C/RS interactions and HTTP for the interactions with the AS.
Separately, mention that TLS is used for confidentiality/integrity
protection and server authentication; also, client authentication can be
provided either via TLS or using in-band proof of possession at the MQTT
application layer.

Section 8

   revoked topics.  If the RS caches the token introspection responses,
   then the RS SHOULD use a reasonable cache timeout to introspect

("reasonable cache timeout" again)

                                      If the RS 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 fictitious tokens, which RS may need to
   store indefinitely.

We do say that the RS only stores "valid" tokens, which includes being
generated by a trusted AS and having RS as the audience.  So it's not
clear that this statement is accurate if the attack is to involve
"fictitious" tokens.  Similar attacks are possible, though.

Section 10.1

The normative use of AIF means that we'll have to wait for AIF to catch
up to us at some point, whether at the RFC Editor or sooner.

RFC 8447 probably does not need to be normative; it is just mentioned as
the thing that is the reference for the IANA registry of TLS exporter

Section 10.2

I think the DTLS profile needs to be a normative reference, since we
defer to it for TLS session set-up.

Likewise, RFC 6234 and RFC 8032 specify MTI algorithms for
authentication, which would make them normative.

Appendix A

I think we need to update the checklist to match the current template