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Re: [TLS] ECH AAD for HRR

David Benjamin <davidben@chromium.org> Wed, 01 September 2021 17:13 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Wed, 01 Sep 2021 13:13:03 -0400
Message-ID: <CAF8qwaBWndRS5-HAfyTYHeLyM_i-DZwG5tUmnZeWNnOjXHK55w@mail.gmail.com>
To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Subject: Re: [TLS] ECH AAD for HRR
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That's right. The AAD and actual CH should be exactly the same, apart from
the payload being zeroed in place. You don't need to reserialize the
structure as a server, or serialize ClientHelloOuter twice as a client.

On Wed, Sep 1, 2021 at 1:01 PM Stephen Farrell <stephen.farrell@cs.tcd.ie>
wrote:

>
> (Apologies for the acronym laden subject:-)
>
> I'm more or less at the "code complete" stage of
> implementing draft-13 incl. HRR. (If anyone wants
> to try interop, for now please contact me, but I
> should have a server up in a few days.) I'm sure
> as usual I'll have gotten some details wrong, but
> I wasn't clear about one thing:
>
> - When sending the 2nd CH following HRR, the spec
> calls for omitting the "enc" field of the ECH
> extension ("enc" holds the sender's public HPKE
> value that's re-used from the 1st CH).
> - The AAD for that ECH encryption is the outer
> CH with zeros replacing where the ciphertext will
> go.
> - I concluded that the sender's ECH public value
> (the "enc" field) ought not be present in the
> AAD in that case, as well as being omitted in
> the ECH value, but it wasn't entirely clear to me
> from the spec (and it'd work either way).
>
> So my question is: did I get that right or not?
>
> Thanks in advance,
> S.
>
> _______________________________________________
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