On Tuesday, 4 September 2018 15:32:19 CEST Eric Rescorla wrote: > On Tue, Sep 4, 2018 at 5:46 AM, Hubert Kario <hka...@redhat.com> wrote: > > On Monday, 3 September 2018 21:26:06 CEST Eric Rescorla wrote: > > > On Mon, Sep 3, 2018 at 12:19 PM, Hubert Kario <hka...@redhat.com> wrote: > > > > On Monday, 3 September 2018 17:30:15 CEST Eric Rescorla wrote: > > > > > On Mon, Sep 3, 2018 at 8:20 AM, Hubert Kario <hka...@redhat.com> > > > > wrote: > > > > > > On Monday, 3 September 2018 17:15:24 CEST Eric Rescorla wrote: > > > > > > > On Mon, Sep 3, 2018 at 7:28 AM, Hubert Kario <hka...@redhat.com> > > > > > > > > wrote: > > > > > > > > On Monday, 3 September 2018 16:01:22 CEST Eric Rescorla wrote: > > > > > > > > > On Mon, Sep 3, 2018 at 4:18 AM, Hubert Kario < > > > > hka...@redhat.com> > > > > > > > > > > > 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.
yes, that's why I said "doesn't support them in TLS 1.2.". I meant there the protocol in general, not a specific implementation (i.e. a server that implements TLS 1.2 as the highest version) > > > > 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. Yes, it's not there, and I already agreed that it's acceptable for a server to abort when it sees such behaviour. But the reason why a behaviour (any behaviour) is prohibited influences the urgency of the fix (or presence of a check). -- 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
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