Re: [TLS] Barry Leiba's No Objection on draft-ietf-tls-tls13-cert-with-extern-psk-03: (with COMMENT)

[Russ Housley <[e-mail]>]

Barry:

You do not call for a response, but I want you to know that your comments are being addressed.
 
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
>> From the shepherd writeup:
> 
>   There was concern raised that no one has reported implementation
>   of this draft. The document has experimental status and that helped
>   gain working group consensus to move it forward.
> 
> ...and...
> 
>   The document has been reviewed and is supported by a few
>   working group members.  Not everyone in the group agrees
>   that it is needed,
> 
> This seems to imply that making it Experimental was a tactic to get it through
> the working group, and that concerns me a bit, though not enough to get to
> DISCUSS.  I would be happier if there were some discussion in the document
> about how we would determine that it is, indeed, needed and useful, and when we
> might know that we should move it to Standards Track or else abandon it.
> 
> Unfortunately, I suspect the answer to that is that we won’t know until we have
> quantum computers to mount attacks with, and that won’t be until certain places
> freeze over.  I realize that preparing for maybe someday having quantum
> computers and what they might someday do is an exercise that not everyone will
> want to spend time working on and implementing.

I do not think we can add anything to the document.  As was said on the email thread on the TLS mail list, there is a plan to use it by the US government.  Others have not said one way or the other.

> Some editorial comments, for which no reply is necessary:
> 
> — Section 4 —
> 
>   Since the
>   "tls_cert_with_extern_psk" extension is intended to be used only with
>   initial handshakes, it MUST NOT be sent alongside the "early_data"
>   extension.
> 
> What happens if it is?  Should this say that if they appear together the server
> aborts the handshake with an "illegal_parameter" alert?

I added a paragraph:

   If the client includes both the "tls_cert_with_extern_psk" extension
   and the "early_data" extension, then the server MUST terminate the
   connection with an "illegal_parameter" alert.

>   The hash algorithm MUST
>   be set when the PSK is established, with a default of SHA-256.
> 
> If it MUST be set, how is there a default?

This is stated in RFC 8446 in Section 4.2.11.

> — Section 5 —
> 
>   If the server responds with a HelloRetryRequest
>   message, then the client sends another ClientHello message as
>   described in Section 4.1.2 of [RFC8446], including the same
>   "tls_cert_with_extern_psk" extension as the original ClientHello
>   message or abort the handshake.
> 
> “, or aborts” (the comma closes the comma before “including”, and “aborts” is
> parallel to “sends”).

Fixed.

> — Section 5.1 —
> 
>   Most of those extension are
>   not impacted in any way.  This section discusses the impacts on the
>   other extensions.
> 
> Make it “those extensions”.  And I would rephrase the second sentence as, “This
> section discusses the impacts on the extensions that are affected.”

I suggest:

   Section 4 lists the extensions that are required to accompany the
   "tls_cert_with_extern_psk" extension.  Most of those extension are
   used in the usual manner.  This section discusses the impacts on the
   extensions that are affected the presence of the
   "tls_cert_with_extern_psk" extension.

>   The "psk_key_exchange_modes" extension restricts both the
>   use of PSKs offered in this ClientHello and those which the server
>   might supply via a subsequent NewSessionTicket.
> 
> “Use of” needs to be factored out of the “both” clause:
> NEW
> ...restricts the use of both the PSKs offered in this ClientHello
> and those that the server might supply...
> END

I accepted you better wording.

> — Section 7 —
> 
>   the external PSKs and searching the resulting small set of
>   possibilities, rather than brute force searching the whole key space.
> 
> “and search”, and “brute-force”

I accepted you better wording.

>   The reasoning is explained in [K2016] (see
>   Section 4.2).
> 
> I suggest “The reasoning is explained in Section 4.2 of [K2016].”  Otherwise it
> sounds like you should see 4.2 of this doc (and I think the html links will be
> generated better this way).

I accepted you better wording.

>   This specification does not require that external PSK is known only
> 
> “that the external PSK”

I added the missing article.

Russ