Re: [TLS] Genart last call review of draft-ietf-tls-exported-authenticator-09

Nick Sullivan <> Wed, 18 September 2019 21:50 UTC

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From: Nick Sullivan <>
Date: Wed, 18 Sep 2019 14:49:56 -0700
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To: Christer Holmberg <>
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Subject: Re: [TLS] Genart last call review of draft-ietf-tls-exported-authenticator-09
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Hello Christer,

Thank you for this thorough review and for waiting so long for my reply. My
responses to your comments are inline below and a PR is included here:


On Sun, Jul 7, 2019 at 3:58 AM Christer Holmberg via Datatracker <> wrote:

> Reviewer: Christer Holmberg
> Review result: Ready with Issues
> I am the assigned Gen-ART reviewer for this draft. The General Area
> Review Team (Gen-ART) reviews all IETF documents being processed
> by the IESG for the IETF Chair.  Please treat these comments just
> like any other last call comments.
> For more information, please see the FAQ at
> <>.
> Document: draft-ietf-tls-exported-authenticator-09
> Reviewer: Christer Holmberg
> Review Date: 2019-07-07
> IETF LC End Date: 2019-07-16
> IESG Telechat date: Not scheduled for a telechat
> Summary: The document is well written. However, I have found some issues
> that
> the author may want to consider clarifying in the document.
> Major issues: N/A
> Minor issues:
> MIN_1:
> The last sentence of Section 1 says that the mechanism requires TLS
> version 1.2
> or later. Would it be useful to state that in a dedicated Applicability
> section?

I'm disinclined to include an applicability section considering how short
it would be. I'm open to being convinced otherwise with examples.

> MIN_2:
> Can the mechanism be used also for DTLS?

I think the answer is yes. I don't see any reason to disallow the use of
Exported Authenticators in DTLS.

> MIN_3:
> The documents talk about additional certificates. If I only have one
> additional
> certificate, can I use that for multiple authenticators throughout the TLS
> session?

Yes, there is nothing disallowing the creation of multiple exported
authenticators with the same certificate.

> MIN_4:
> Section 3 and 4 say that the authenticator request and authenticator
> sent using TLS, and Section 1 says that the proof of authentication can be
> sent
> out-of-band. I think it would be useful to clarify whether both the
> authenticator request and authenticator can be sent out-of-band ( i.e., not
> using the TLS connection that the additional authentication is associated
> with), and also to state whether it IS allowed to send the authenticator
> request and authenticator on the TLS connection they are associated with.

Good point. I can clarify this a bit. Both the authenticator and
authenticator request can be sent through either the TLS connection they
were derived from or another TLS connection altogether. The important part
is that they are sent over an encrypted and authenticated connection.

> MIN_5:
> Section 5 talks about an endpoint sending an empty authenticator. But,
> what if
> the sender of the authenticator request does not receive anything?  Does it
> simply move on? Does it terminate the TLS session? Is the action based on
> local
> policy?

The TLS layer is stateless. The decision to time out or fail the connection
if an authenticator request is not responded to is left to higher-level
protocols that leverage EAs.

> MIN_6:
> Related to MIN_5, I can't find text about how endpoints inform each other
> about
> the support of the mechanism, so maybe a few words about that would be
> useful.
> And some words about backward compatibility with endpoints that don't
> support
> the mechanism.

Adding an extension to signal support for exported authenticators was
discussed at some point, but it was decided that the higher-layer
application should handle the signaling.

> MIN_7:
> What happens if the validation of an authenticator fails? Does the
> requester
> simply move on? Does it terminate the TLS session? Is the action based on
> local
> policy?

This is also up to the application-layer protocol. I'll add text covering
MIN_5, MIN_6 and MIN_7.

> Nits/editorial comments:
> ED_1:
> The document uses "session", "TLS connection" and "TLS communication"
> terminology. Is that intentional, or wouuld it be possible to use
> consistent
> terminology?
Good note. I'll standardize on "connection".

> ED_2:
> Section 3 says: "The authenticator request is a structured message that
> can be
> created..." Section 4 says: "The authenticator is a structured message
> that can
> be exported..."
> In the 2nd paragraph of Section 4 it is stated that "authenticator" is sent
> based on an "authenticator request". I wonder if that could be stated
> already
> in the beginning of Section 4, to further clarify the difference between
> them.
> E.g.,
> "The authenticator is a structured message, triggered by an authenticator
> request, that can be exported from either party of a TLS connection."
> The issue is that servers can generate spontaneous exported authenticators
without an authenticator request. Given this, the updated sentence would be:
"The authenticator is a structured message, sometimes triggered by an
request, that can be exported from either party of a TLS connection."
Which I'm not sure is an improvement to readability.