Re: [TLS] WG: New Version Notification for draft-bruckert-brainpool-for-tls13-00.txt

[Eric Rescorla <[e-mail]>]

On Tue, Sep 4, 2018 at 5:46 AM, Hubert Kario <[e-mail]> wrote:

> On Monday, 3 September 2018 21:26:06 CEST Eric Rescorla wrote:
> > On Mon, Sep 3, 2018 at 12:19 PM, Hubert Kario <[e-mail]> wrote:
> > > On Monday, 3 September 2018 17:30:15 CEST Eric Rescorla wrote:
> > > > On Mon, Sep 3, 2018 at 8:20 AM, Hubert Kario <[e-mail]>
> wrote:
> > > > > On Monday, 3 September 2018 17:15:24 CEST Eric Rescorla wrote:
> > > > > > On Mon, Sep 3, 2018 at 7:28 AM, Hubert Kario <[e-mail]>
> > >
> > > wrote:
> > > > > > > On Monday, 3 September 2018 16:01:22 CEST Eric Rescorla wrote:
> > > > > > > > On Mon, Sep 3, 2018 at 4:18 AM, Hubert Kario <
> [e-mail]>
> > > > > > >
> > > > > > > > wrote:
> > > > > > > not
> > > > > > > abort connection, so I still think it will create less
> confusion
> > > > > > > to
> > > > > > > re-allow
> > > > > > > them than to re-assign new codepoints
> > > > > >
> > > > > > The issue is that it's not possible to distinguish a
> non-compliant
> > >
> > > TLS
> > >
> > > > > 1.3
> > > > >
> > > > > > implementation which is inappropriately sending these code points
> > >
> > > from
> > >
> > > > > > one which actually supports Brainpool with TLS 1.3. Using new
> code
> > > > > > points makes this clear.
> > > > >
> > > > > and why having that distinction is that important?
> > > >
> > > > Because otherwise you are risking interop problems:
> > > >
> > > > 1. A stack which supports TLS 1.2 and TLS 1.3 but only supports
> > > > Brainpool
> > > > for TLS 1.2 (the only kind you can write at this point), and
> > >
> > > inappropriately
> > >
> > > > advertises the Brainpool curves in violation of the MUST above.
> > > > 2. A stack which supports TLS 1.2 and TLS 1.3 and supports Brainpool
> for
> > > > both (assuming that we adopt your proposal and reactivate these code
> > > > points).
> > > >
> > > > If stack 2 receives a CH from stack 1 and responds by selecting a
> > >
> > > Brainpool
> > >
> > > > curve, then there will be an interop issue when it sends an HRR [0]
> > > > selecting
> > > > the Brainpool curve.
> > > >
> > > > -Ekr
> > > >
> > > > [0] I'm assuming that the client doesn't offer a Brainpool KeyShare.
> > >
> > > ah, yes, missed this case. That does taint all those codepoints for TLS
> > > 1.3
> > >
> > > but while the server may abort the connection upon receiving them in
> TLS
> > > 1.3
> > > CH (as it is violation of the MUST clause), I don't think it actually
> > > should
> > > abort it...
> > >
> > > For one, and I think we can agree on that, is the server MUST ignore
> them
> > > if
> > > it doesn't support them in TLS 1.2.
> >
> > I don't think I agree with this. Why would that be the case?
>
> because when servers don't do it we have "TLS version intolerance", "TLS
> extension intolerance" and so on.
>
> From RFC 5246:
>
>    If the list contains cipher
>    suites the server does not recognize, support, or wish to use, the
>    server MUST ignore those cipher suites, and process the remaining
>    ones as usual.
>
> and:
>
>    the rules specified in [TLSEXT]
>    require servers to ignore extensions they do not understand.
>
> Not to mention the most explicit statement on this topic, from RFC 8446:
>
>    Future extensions or additions to the protocol may define new values.
>    Implementations need to be able to parse and ignore unknown values
>    unless the definition of the field states otherwise.
>
> In general, if the servers weren't required to ignore values they don't
> recognise, we would never be able to extend the value lists (be it
> SupportedGroups, or SignatureSchemes/SignatureAlgorithms).
>

This isn't an unknown value. It's a specifically prohibited value. Those
are different cases.



> > > Given that TLS 1.3 server usually implement both TLS 1.2 and TLS 1.3,
> > > having
> > > code that does ignore them in TLS 1.2 and doesn't ignore them in TLS
> 1.3
> > > is
> > > only inviting bugs.
> >
> > We already have other special case code that enforces such rules. For
> > instance,
> > compression:
> >
> >       For every TLS 1.3 ClientHello, this vector
> >       MUST contain exactly one byte, set to zero, which corresponds to
> >       the "null" compression method in prior versions of TLS.  If a
> >       TLS 1.3 ClientHello is received with any other value in this
> >       field, the server MUST abort the handshake with an
> >       "illegal_parameter" alert.  Note that TLS 1.3 servers might
> >       receive TLS 1.2 or prior ClientHellos which contain other
> >       compression methods and (if negotiating such a prior version) MUST
> >       follow the procedures for the appropriate prior version of TLS.
>
> the difference is that we we don't want people to use compression in TLS
> 1.2,
> while brainpool in TLS 1.2 is just not recommended, not provably dangerous
> even when implemented correctly
>

That's reading stuff into the spec that's not there. They're both
prohibited from
being offered by TLS 1.3 clients.

-Ekr


> --
> Regards,
> Hubert Kario
> Senior Quality Engineer, QE BaseOS Security team
> Web: www.cz.redhat.com
> Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic
>