Re: [TLS] Binding imported PSKs to KDFs rather than hash functions

"Martin Thomson" <mt@lowentropy.net> Tue, 17 September 2019 02:19 UTC

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Date: Tue, 17 Sep 2019 12:18:58 +1000
From: "Martin Thomson" <mt@lowentropy.net>
To: tls@ietf.org
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/Nv7msE4xm5lP6is4HFm5ZjPOiFI>
Subject: Re: [TLS] Binding imported PSKs to KDFs rather than hash functions
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On Tue, Sep 17, 2019, at 11:26, Martin Thomson wrote:
> What we learned from TLS 1.3 is that HKDF is effectively a completely 
> different KDF when it is used with a different hash function.

Hugo points out that I should clarify this to add:

One should not use HKDF with two hash functions (or more generally two KDF functions) and the same IKM. The same way as you should not use two different cryptographic functions with the same key.

This is an idea that this draft exists to support, so it needs to be very careful about how it does the same itself.