Re: [TLS] Constraining ECH to HKDF-based HPKE ciphersuites

Christopher Wood <caw@heapingbits.net> Mon, 17 August 2020 23:06 UTC

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Date: Mon, 17 Aug 2020 16:05:40 -0700
From: "Christopher Wood" <caw@heapingbits.net>
To: "TLS@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Constraining ECH to HKDF-based HPKE ciphersuites
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On Mon, Aug 17, 2020, at 3:30 PM, Martin Thomson wrote:
> 
> 
> On Tue, Aug 18, 2020, at 07:55, Christopher Patton wrote:
> > Hi Martin, 
> >  
> > > Or maybe just running the HPKE KDF with a fixed input.
> > Do you mean something like this? Let `config_digest = KDF.extract("some 
> > salt", "some label", config)`, where `config` is the ECH configuration?
> 
> Sure.  I would probably use all of HKDF though, see below.

If we're going to use all of HKDF (extract + expand), then let's just use the HPKE's Extract+Expand functions in sequence. 

Best,
Chris