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Re: [TLS] GREASE ECH repeated value after HRR

David Benjamin <davidben@chromium.org> Tue, 17 August 2021 20:15 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Tue, 17 Aug 2021 16:15:28 -0400
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Subject: Re: [TLS] GREASE ECH repeated value after HRR
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It's because of the rules in RFC8446. If the server doesn't utter an
extension in HelloRetryRequest, the client is not allowed to change the
corresponding ClientHello extension. We found an implementation which
actually enforces this.
https://github.com/tlswg/draft-ietf-tls-esni/issues/358

David

On Tue, Aug 17, 2021 at 4:03 PM Stephen Farrell <stephen.farrell@cs.tcd.ie>
wrote:

>
> Hiya,
>
> (I'm just getting around to playing with draft-13 ECH and
> HRR and have a question...)
>
> In 6.2 talking about GREASEd ECH, the draft says:
>
>     If sending a second ClientHello in response to a
>     HelloRetryRequest, the client copies the entire
>     "encrypted_client_hello" extension from the first
>     ClientHello.  The identical value will reveal to an
>     observer that the value of "encrypted_client_hello" was
>     fake, but this only occurs if there is a
>     HelloRetryRequest.
>
> I don't object to that, but can't recall why we wanted
> the same value re-tx'd. (My code just naturally generated
> a new GREASE ECH value and it all worked fine, so being
> the lazy person I am, I'm wondering if doing nothing is
> a good option:-)
>
> Ta,
> S.
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