[Cfrg] hpke which HKDF to use for eae_prk?

Stephen Farrell <stephen.farrell@cs.tcd.ie> Thu, 20 August 2020 18:09 UTC

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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Thu, 20 Aug 2020 19:09:25 +0100
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Subject: [Cfrg] hpke which HKDF to use for eae_prk?
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I also have a 2nd interop issue: when calculating
the eae_prk value my code uses the HKDF from the HPKE
ciphersuite. So if I'm using x25519,hkdf-sha512,aesgcm128
in the base mode, then my eae_prk value is 64 octets
long.

In contrast, the test vectors use the HKDF that is
associated with the KEM and so in the above case the
eae_prk value is only 32 octets.

I don't think there's a real security difference here,
in which case I'd argue that using the HPKE ciphersuite
is more natural and easily understood (IOW, I prefer my
code:-).

Either way, we should clarify this and maybe again also
include that intermediate value in the test vectors.

Cheers,
S.