Re: [TLS] [ECH] Reverting the config ID change

Martin Thomson <mt@lowentropy.net> Tue, 16 February 2021 23:01 UTC

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Date: Wed, 17 Feb 2021 10:00:45 +1100
From: "Martin Thomson" <mt@lowentropy.net>
To: tls@ietf.org
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Subject: Re: [TLS] [ECH] Reverting the config ID change
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On Wed, Feb 17, 2021, at 08:31, Christopher Wood wrote:
> That's true, but I'd personally prefer one tracking vector to two. This 
> structure also better aligns with other proposed use cases for HPKE 
> configurations. I also don't see an immediate need for flexibility in 
> this value given that there are extensions in ECHConfigContents already.

I don't see the tracking angle as relevant here.  The only things that might matter is size, collision probability (for greasing), and consistency.  Size doesn't matter, because it's a handful of bytes at most; collisions matter little because the cost is a resource the server is prepared to spend anyway; consistency with something that can also change isn't worth much.

The primary argument I would have in support of this is YAGNI.  The number of active keys should be much smaller than 256, and there's a slot for extensions should that need arise.