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Re: [TLS] ServerHello extensions

Eric Rescorla <ekr@rtfm.com> Thu, 18 January 2018 18:26 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 18 Jan 2018 10:25:27 -0800
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To: R du Toit <r@nerd.ninja>
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Subject: Re: [TLS] ServerHello extensions
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Thanks. These are good points. I would welcome a PR.

On Thu, Jan 18, 2018 at 10:21 AM, R du Toit <r@nerd.ninja> wrote:

> Issue#1: Section "4.1.3 Server Hello" currently states:
>
> *extensions   A list of extensions. The ServerHello MUST only include
> extensions which are required to establish the cryptographic context.
> Currently the only such extensions are “key_share” and “pre_shared_key”.
> All current TLS 1.3 ServerHello messages will contain one of these two
> extensions, or both when using a PSK with (EC)DHE key establishment. The
> remaining extensions are sent separately in the EncryptedExtensions
> message.*
>
>
>
> "supported_versions" should be added to the list of required extensions
> for a session that negotiates TLS 1.3.
>
>
>
>
>
> Issue#2: Section "4.1.4 Hello Retry Request" currently states:
>
> *Upon receiving the ServerHello, clients MUST check that the cipher suite
> supplied in the ServerHello is the same as that in the HelloRetryRequest
> and otherwise abort the handshake with an “illegal_parameter” alert.*
>
>
>
> There is no rule about checking that
> *SH.supported_versions.selected_version* matches
> *HRR.supported_versions.selected_version*.   I am currently adding draft
> 23 support, and want to enforce that rule to make sure the protocol state
> machine does not have to jump back and forth between TLS 1.2 and TLS 1.3.
>
>
>
> I can add a PR for both issues, if you agree.
>
>
>
> --Roelof
>
>
>
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