Re: [TLS] TLS ECH, how much can the hint stick out?

Martin Thomson <mt@lowentropy.net> Sat, 12 September 2020 12:58 UTC

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Date: Sat, 12 Sep 2020 22:58:14 +1000
From: "Martin Thomson" <mt@lowentropy.net>
To: tls@ietf.org
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/Bk3XHREmmKIZsWrPjUL1PP50vI8>
Subject: Re: [TLS] TLS ECH, how much can the hint stick out?
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On Sat, Sep 12, 2020, at 21:55, Karthik Bhargavan wrote:
> > Any big issue keeping N=8
> 
> Regarding the length of N, I gather that the trade-off is that if it is 
> too short, the probability of collisions between the signal and 
> randomly generated server randoms becomes significant,
> and so does the probability of an active MitM forging the signal. Is 
> there some other concern? 
> 8 bytes seems fine for these considerations. Is the idea that we would 
> reuse the downgrade sentinel?

If they collide, you could test with trial decryption, but 8 bytes seems like it might be enough that you might choose not to bother.