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Re: [TLS] handling of duplication of extensions between ServerHello and EncryptedExtensions

Eric Rescorla <ekr@rtfm.com> Fri, 02 September 2016 19:23 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 02 Sep 2016 12:23:03 -0700
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To: David Benjamin <davidben@chromium.org>
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Subject: Re: [TLS] handling of duplication of extensions between ServerHello and EncryptedExtensions
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I agree. FWIW, this is how NSS behaves. A clarifying PR would be welcome
here.

On Fri, Sep 2, 2016 at 12:21 PM, David Benjamin <davidben@chromium.org>
wrote:

> Huh. I agree that text is weird. We should probably remove it. I think we
> actually get something akin to what you suggest basically implicitly:
>
> Servers aren't allowed to send unsolicited extensions, so your SH and EE
> parsers should be rejecting any unknown extensions. If you combine that
> with not accepting known extensions in the wrong context (unencrypted ALPN
> or encrypted key_share) since extensions already specify which messages
> they may live in, this all works out.
>
> So even if we explicitly say this rule, I don't think reflecting it in
> code is the cleanest implementation strategy. (It requires one keep a list
> of SH extensions and compare against it.) It might be better to say that
> extensions in unexpected contexts should be rejected like any other
> unexpected extension.
>
> David
>
>
> On Fri, Sep 2, 2016 at 1:31 PM Hubert Kario <hkario@redhat.com> wrote:
>
>> So, the draft has following text:
>>
>>     The same extension types MUST NOT appear in both the ServerHello and
>>     EncryptedExtensions.  If the same extension appears in both locations,
>>     the client MUST rely only on the value in the EncryptedExtensions
>>     block.
>>
>> if the extension "MUST NOT" be in both ServerHello and
>> EncryptedExtensions,
>> why the client should continue with the handshake if a server makes such a
>> major mistake? Why aborting the connection in such situation isn't safer?
>> --
>> Regards,
>> Hubert Kario
>> Senior Quality Engineer, QE BaseOS Security team
>> Web: www.cz.redhat.com
>> Red Hat Czech s.r.o., Purkyňova 99/71, 612 45, Brno, Czech
>> Republic_______________________________________________
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>> https://www.ietf.org/mailman/listinfo/tls
>>
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