Re: [TLS] [Editorial Errata Reported] RFC8446 (6204)

Eric Rescorla <ekr@rtfm.com> Thu, 04 June 2020 01:45 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 3 Jun 2020 18:44:43 -0700
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To: Martin Thomson <mt@lowentropy.net>
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Subject: Re: [TLS] [Editorial Errata Reported] RFC8446 (6204)
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On Wed, Jun 3, 2020 at 6:07 PM Martin Thomson <mt@lowentropy.net> wrote:

> I think that this is a useful erratum and it should be approved/HFDU.  The
> extension to which this text alludes is RFC 8773, not post_handshake_auth.
>

Yes, although 8773 actually is not super-clear about post-handshake, so
that's actually something we should clarify there.



> There is one other piece to this that is very confusing, and less clear.
>
>  "Servers which are authenticating with a PSK MUST NOT send the
> CertificateRequest message in the main handshake, though they MAY send it
> in post-handshake authentication (see Section 4.6.2) provided that the
> client has sent the "post_handshake_auth" extension (see Section 4.2.6)."
>
> The motivation is the attack that Sam Scott et. al. found in their
> analysis of resumption:
> https://mailarchive.ietf.org/arch/msg/tls/TugB5ddJu3nYg7chcyeIyUqWSbA/
> However, this statement is unclear on whether it applies to external,
> resumption, or both types of PSK, but without qualification as it is you
> might be forgiven for thinking that it is both.
>
> However, the document already says:
>
> "It is unsafe to use certificate-based client authentication when the
> client might potentially share the same PSK/key-id pair with two different
> endpoints."-
>
> So I think that the right interpretation is that this statement applies to
> "a resumption PSK" only.
>
> If people agree with this interpretation, then I will file another erratum
> of the form:
>
> OLD:
> Servers which are authenticating with a PSK MUST NOT send the
> CertificateRequest message in the main handshake, [...]
> NEW:
> Servers which are authenticating with a resumption PSK MUST NOT send the
> CertificateRequest message in the main handshake, [...]
>

I see what you are trying to do, but I think if we are to restrict this to
resumption here it might leave the wrong impression.

I think it would help to be more explicit here:
Servers which are authenticating with a resumption PSK MUST NOT send the
CertificateRequest message in the main handshake, [...],
Servers which are authenticating with an external PSK MUST NOT send the
CertificateRequest message either in the main handshake or in the
post-handshake phase. Future specifications  MAY provide an extension to
permit this.

This would be consistent with the erratum Chris proposed


-Ekr


>
> On Thu, Jun 4, 2020, at 10:00, RFC Errata System wrote:
> > The following errata report has been submitted for RFC8446,
> > "The Transport Layer Security (TLS) Protocol Version 1.3".
> >
> > --------------------------------------
> > You may review the report below and at:
> > https://www.rfc-editor.org/errata/eid6204
> >
> > --------------------------------------
> > Type: Editorial
> > Reported by: Chris Wood <caw@heapingbits.net>
> >
> > Section: E.1
> >
> > Original Text
> > -------------
> > Implementations MUST NOT combine external PSKs with certificate-based
> > authentication of either the client or the server unless negotiated by
> > some extension.
> >
> > Corrected Text
> > --------------
> > Implementations MUST NOT combine external PSKs with certificate-based
> > authentication of either client or the server. Future specifications
> > MAY provide an extension to permit this.
> >
> > Notes
> > -----
> > The existing text can be misread as permitting this combination upon
> > negotiation of the "post_handshake_auth" extension, which would be
> > incorrect. [1] describes an attack that can occur based on this
> > misinterpretation. The proposed text aims to make clear that a *new*
> > extension is required for this combination.
> >
> > [1] https://link.springer.com/article/10.1007%2Fs11416-020-00352-0
> >
> > Instructions:
> > -------------
> > This erratum is currently posted as "Reported". If necessary, please
> > use "Reply All" to discuss whether it should be verified or
> > rejected. When a decision is reached, the verifying party
> > can log in to change the status and edit the report, if necessary.
> >
> > --------------------------------------
> > RFC8446 (draft-ietf-tls-tls13-28)
> > --------------------------------------
> > Title               : The Transport Layer Security (TLS) Protocol
> Version 1.3
> > Publication Date    : August 2018
> > Author(s)           : E. Rescorla
> > Category            : PROPOSED STANDARD
> > Source              : Transport Layer Security
> > Area                : Security
> > Stream              : IETF
> > Verifying Party     : IESG
> >
> > _______________________________________________
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> > https://www.ietf.org/mailman/listinfo/tls
> >
>
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