Subject: [Ace] Comments on draft-ietf-ace-mqtt-tls-profile

From: Jim Schaad <[email-masked]>
Date: Wed, 22 May 2019 14:58:28 -0700

Thanks for the updates from my last message.  This has helped quite a bit.

1.  A discussion of the use of raw public keys rather than certificates for
the server may be in order.  This can refer to the same RPK issues from the
current DTLS document.  It may also be that this just uses normal
certificate processing and that should be noted as well, but some discussion
of deciding if the subject name/alt name matches the token returned from the
AS.

For the connect message there are a couple of issues that need to be thought
about.

2.  What items are required to be in the connect message.  The response to
my last message suggested that a client identifier is required to be present
but that is not documented.

3.  It is not completely clear what portions of the CONNECT message are
being covered by the signature/MAC value.  As an example, is the password
field omitted entirely or is it set to a zero length password.  In addition
to this, from the couple of implementations that I have looked at the entire
packet is not passed out of the base server code for authentication
purposes.  This might need to be taken into account in terms of what is used
for the body and how it is constructed.  (As a side note, the
implementations that I have looked at so far also think that the password is
a text string rather than a binary value which is going to be a short term
issue as well.)

4.  I must admit that I am disappointed that there is no challenge response
mechanism in the MQTT specifications.  I don't know that anything can be
done at this point about it but there are some security issues that need to
be highlighted because of this in the security considerations section.  Only
the v3 seems to have this problem.  Also doing the channel binding would
deal with this problem as well.  Might just need some general discussions
around the channel binding text.

5.  Is there an intention to provide a "standard" format for the scope field
or just to leave it as ad hoc?

6.  It might be reasonable in section 2.1.3 to note that even if a result is
cached, that cached check should be limited for a specific amount of time to
recheck if the token is still active.  More of an issue in terms of how long
to cache for introspection.

7.  In section 2.1.4 - I would presume that the last paragraph should be
extended to say that the token is stored only for the length of the
connection.  That is the token always needs to be supplied every time a
connect message is sent.
 
Jim

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From: Ludwig Seitz <[email-masked]>
Date: Thu, 23 May 2019 08:39:42 +0200

> or just to leave it as ad hoc?

I would be very much in favor of this, or at least provide guidelines to 
avoid adding to this: https://www.brandur.org/oauth-scope

/Ludwig

-- 
Ludwig Seitz, PhD
Security Lab, RISE
Phone +46(0)70-349 92 51

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From: Cigdem Sengul <[email-masked]>
Date: Thu, 23 May 2019 12:20:40 +0100

Thank you, Jim, for the comments.
I have created issues corresponding to each one in the GitHub repository.

We will start working on them, and specifically clarify the issues 1-3
around the CONNECT message.

For 4, MQTT v5 can support a challenge-response; not possible with v3
indeed. Will expand the security considerations section on that.

5.  We described a convention in the draft with SHOULD. Do we turn that
into a MUST?

6. We thought that the AS would dictate the token expiration time. It can
also be done that RS times out tokens even if they have not expired. But,
security-performance trade-off should be considered.
Token caching is important even if the token does not need introspection.
The only time the MQTT client can push a token is with the CONNECT message
(all subsequent publish/subscribe messages rely on this). In MQTT v5, a
client can push a new token via reauthentication during an ongoing session
(enabled by MQTT v5). If RS times out the token, this means a disconnection
in MQTT v3. We mention this  in Security considerations as "it is expected
that AS follows a reasonable expiration strategy for access tokens"

-->There may be workarounds designed especially MQTT v5 which has better
support for request-response style communication. But need to think about
it.

7. That is correct. Will add the clarification.

Thanks,
--Cigdem

On Wed, May 22, 2019 at 10:58 PM Jim Schaad <[email-masked]> wrote:

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From: Jim Schaad <[email-masked]>
Date: Thu, 23 May 2019 10:40:48 -0700

From: Cigdem Sengul <[email-masked]> 
Sent: Thursday, May 23, 2019 4:21 AM
To: Jim Schaad <[email-masked]>
Cc: [email-masked]; [email-masked]
Subject: Re: [Ace] Comments on draft-ietf-ace-mqtt-tls-profile

We will start working on them, and specifically clarify the issues 1-3 around the CONNECT message. 

For 4, MQTT v5 can support a challenge-response; not possible with v3 indeed. Will expand the security considerations section on that. 

5.  We described a convention in the draft with SHOULD. Do we turn that into a MUST? 

[JLS] I missed this somehow.  What is there is fine.

6. We thought that the AS would dictate the token expiration time. It can also be done that RS times out tokens even if they have not expired. But, security-performance trade-off should be considered. 

Token caching is important even if the token does not need introspection. The only time the MQTT client can push a token is with the CONNECT message (all subsequent publish/subscribe messages rely on this). In MQTT v5, a client can push a new token via reauthentication during an ongoing session (enabled by MQTT v5). If RS times out the token, this means a disconnection in MQTT v3. We mention this  in Security considerations as "it is expected that AS follows a reasonable expiration strategy for access tokens" 

-->There may be workarounds designed especially MQTT v5 which has better support for request-response style communication. But need to think about it. 

[JLS] For introspection, but not for a published token, the token could be “revoked” by the RS.  In this case a new introspection check would lead to that information.  There may be ways to deal with this in the introspection dialog and not need to be called up here.  The question would be does the exi field mean – recheck the token or discard the token.  

Jim

Thanks,

--Cigdem 

 

On Wed, May 22, 2019 at 10:58 PM Jim Schaad <[email-masked] <mailto:[email-masked]> > wrote:

Jim

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From: Cigdem Sengul <[email-masked]>
Date: Fri, 24 May 2019 09:56:52 +0100

Hello,

Thanks, Jim, this was helpful, and it also triggered that I go back and
read the introspection section of the core draft again.

>
>
> [JLS] For introspection, but not for a published token, the token could be
> “revoked” by the RS.  In this case a new introspection check would lead to
> that information.  There may be ways to deal with this in the introspection
> dialog and not need to be called up here.  The question would be does the
> exi field mean – recheck the token or discard the token.
>

Understood. We definitely need to clarify here what we do with exi field if
it exists, and if it doesn't exist. The MAY language we used in the draft
is not appropriate if exi does not exist.
We considered the expiration time in the introspection response as a
revocation signal and hence RS discards the token.  (The basic flow was: RS
receives a token, introspects it, caches the result and then when it
expires, it revokes the token - which may next lead to different steps
depending on the version of the MQTT.)
But if introspection is done to handle the case of dynamic authorisation
decisions, and exi would signal a recheck indeed.
Will clarify.

Thanks,
--Cigdem