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Re: [TLS] "selected_group" field in HelloRetryRequest in TLS 1.3

Martin Thomson <martin.thomson@gmail.com> Sat, 28 November 2015 02:25 UTC

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Date: Fri, 27 Nov 2015 18:25:00 -0800
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From: Martin Thomson <martin.thomson@gmail.com>
To: Xuelei Fan <xuelei.fan@vimino.com>
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Subject: Re: [TLS] "selected_group" field in HelloRetryRequest in TLS 1.3
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On 26 November 2015 at 18:38, Xuelei Fan <xuelei.fan@vimino.com> wrote:
> What's the consideration to place selected_group out of the extensions filed
> in HelloRetryRequest?

An extension would work, except that I believe that extensions in
HelloRetryRequest are going to carry somewhat different semantics to
those in other Hello messages.

If we go to dynamically generated groups, then we can easily define a
new FFDHE code point to signal the use of a dynamic group.  Though I
think that I'd be sad about having to always spend an extra round trip
if it came to that.

Also, it's not much, but the explicit field keeps the message (a tiny
bit) smaller and easier to process.

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