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

Martin Thomson <mt@lowentropy.net> Mon, 17 August 2020 23:28 UTC

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Date: Tue, 18 Aug 2020 09:27:39 +1000
From: "Martin Thomson" <mt@lowentropy.net>
To: "Christopher Patton" <cpatton@cloudflare.com>
Cc: "<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 Tue, Aug 18, 2020, at 09:04, Christopher Patton wrote:
> Just to be clear, you're proposing something like this? Referring to 
> the KDF API called for in draft-irtf-cfrg-hpke-05:
> 
> config_digest = Expand(PRK=Extract("some_salt", "some_label", 
> IKM=config), "some_info", 16)
> It's maybe more hashing than necessary, but I'd be good with this.

Something like that yeah.  And yes, that's a lot of hashing.  But that's a lower-order concern.  Maybe if we try to find a KDF that doesn't cost so much to operate we won't feel so bad about this.