Re: [TLS] ESNI padding the Certificate message

Eric Rescorla <ekr@rtfm.com> Thu, 13 December 2018 15:10 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 13 Dec 2018 07:09:32 -0800
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Subject: Re: [TLS] ESNI padding the Certificate message
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On Thu, Dec 13, 2018 at 5:10 AM Stephen Farrell <stephen.farrell@cs.tcd.ie>;
wrote:

>
> Hiya,
>
> Was just adding code for this and I noticed that the draft says
> a server: "SHOULD pad the Certificate message, via padding at
> the record layer, such that its length equals the size of the
> largest possible Certificate (message) covered by the same ESNI
> key."
>
> I think that ought also mention the CertificateVerify as that
> could also tell you something about the ESNI being used if the
> key lengths in the various certificates differ.
>

Sure. Send a PR.

-Ekr


More specific to openssl though, there isn't really a terribly
> easy way to do such fine-grained padding (that I've found so
> far), so to cover the above, and as there are likely many
> other ways that the ESNI could be exposed, it might be good to
> add that clients and servers could reasonably decide to pad all
> handshake messages or even all records. (That last being what
> is currently easiest with openssl, and hence what I've done
> for now with my proof-of-concept code:-)
>
> Cheers,
> S.
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