Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

Eric Rescorla <ekr@rtfm.com> Fri, 03 June 2016 17:33 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 3 Jun 2016 10:32:37 -0700
Message-ID: <CABcZeBP57pVnKwQYCLKQGzDMKyA41PYUsHzBxGpjnF8qVCoibQ@mail.gmail.com>
To: Andrei Popov <Andrei.Popov@microsoft.com>
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Subject: Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]
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That's a clearer version of what I was trying to say.

-Ekr


On Fri, Jun 3, 2016 at 10:28 AM, Andrei Popov <Andrei.Popov@microsoft.com>;
wrote:

> It’s not that the existing version negotiation mechanism doesn’t work; the
> problem is that implementers got it wrong. Similarly, implementers can mess
> up the new negotiation mechanism. Especially since we have to support it at
> the same time as the old one.
>
>
>
> Cheers,
>
>
>
> Andrei
>
>
>
> *From:* TLS [mailto:tls-bounces@ietf.org] *On Behalf Of *Eric Rescorla
> *Sent:* Friday, June 3, 2016 6:40 AM
> *To:* Dave Garrett <davemgarrett@gmail.com>;
> *Cc:* tls@ietf.org
> *Subject:* Re: [TLS] no fallbacks please [was: Downgrade protection,
> fallbacks, and server time]
>
>
>
> My opinion on this hasn't really changed since the last time. This seems
> like it's more complicated and it's not clear to me why it won't lead to
> exactly the same version intolerance problem in future.
>
>
>
> -Ekr
>
>
>
>
>
> On Thu, Jun 2, 2016 at 9:17 PM, Dave Garrett <davemgarrett@gmail.com>;
> wrote:
>
> Allrighty then; time to dust off and rebase an old changeset I was
> fiddling with last year on this topic:
>
> https://github.com/davegarrett/tls13-spec/commit/058ff1518508b094b8c9f1bd4096be9393f20076
> (I cleaned up a bit when rebasing, but it probably needs some work; was
> just a WIP branch, never a PR)
>
> This was the result of prior discussions on-list about TLS version
> intolerance. The gist of the proposal:
> 1) Freeze all the various version number fields.
> 2) Send a list of all supported versions in an extension. (version IDs
> converted to 16-bit ints instead of 8-bit pairs)
> 3) Use short (1 or 2 value, based on hello version) predefined lists for
> hellos from old clients not sending the extension.
> 4) Compare lists to find highest overlap, avoiding guesswork or problems
> with noncontinuous lists.
> 5) Forget the old mess of version intolerance existed.
>
> Do we want to consider scrapping the old version negotiation method again?
>
>
> Dave
>
>
>