Re: [TLS] Proposed change in TLS-Flags

Martin Thomson <mt@lowentropy.net> Wed, 01 July 2020 04:17 UTC

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Date: Wed, 01 Jul 2020 14:17:21 +1000
From: "Martin Thomson" <mt@lowentropy.net>
To: tls@ietf.org
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Subject: Re: [TLS] Proposed change in TLS-Flags
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More to the point, this makes it more difficult to analyze relative to an empty "flag" extension of the likes we currently use.

I haven't implemented this, but I imagine one strategy would be to rewrite these flags and pretend that they were empty extensions.  That would allow implementations to reuse a lot of infrastructure, like the stuff we added for custom extensions.  That would be more difficult if the server can speak first.

On Wed, Jul 1, 2020, at 14:03, Yoav Nir wrote:
> Yeah, the thread that Nick mentioned.
> 
> Also, since there are no such extensions defined in the base TLS 1.3 
> spec, the server can’t assume that the client knows what either the 
> specific flag means, or the entire flags extension means. 
> 
> So suppose we invent some new client authentication scheme for TLS, it 
> does make sense for the server to signal that it supports this so that 
> the client can do. But I don’t think it’s too onerous to require that 
> the client indicate support first.
> 
> Yoav
> 
> > On 1 Jul 2020, at 2:30, David Schinazi <dschinazi.ietf@gmail.com> wrote:
> > 
> > Hi Yoav,
> > 
> > Could you elaborate on the rationale for this change please?
> > I was assuming that the ability for servers to send extensions not requested by clients was useful.
> > 
> > Thanks,
> > David
> > 
> > On Mon, Jun 29, 2020 at 2:34 PM Yoav Nir <ynir.ietf@gmail.com> wrote:
> >> Hi
> >> 
> >> I’ve just submitted the following PR:
> >> 
> >> https://github.com/tlswg/tls-flags/pull/4
> >> 
> >> Three changes:
> >>  * It is no longer allowed to send an empty flags extension. If you don’t support any flags, don’t send the extension.
> >>  * The server is no longer allowed to respond with flag types that the client didn’t indicate support for first.
> >>  * I’ve split the extension description section into a format section and a rules section
> >> 
> >> Please comment. Barring any objections, I’ll merge the PR just before the submission deadline.
> >> 
> >> Yoav
> >> 
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