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

Martin Thomson <mt@lowentropy.net> Mon, 17 August 2020 21:42 UTC

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Date: Tue, 18 Aug 2020 07:42:09 +1000
From: "Martin Thomson" <mt@lowentropy.net>
To: 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 07:09, Christopher Wood wrote:
> HPKE recently removed the Hash() interface from the KDF, which means we 
> need to either (a) change how the config_digest (formerly 
> record_digest) is computed, or (b) constrain ECH to HKDF-based HPKE 
> ciphersuites. This PR takes approach (b):
> 
>    https://github.com/tlswg/draft-ietf-tls-esni/pull/271
> 
> Please have a look and provide feedback. 

Was there discussion about just defining a set of hash function identifiers and adding those to the config?

Or maybe just running the HPKE KDF with a fixed input.  Unless I've missed something critical, you don't need any sort of preimage resistance for this, it's only for identification purposes.