Re: [Cfrg] hpke which HKDF to use for eae_prk?

Stephen Farrell <stephen.farrell@cs.tcd.ie> Thu, 20 August 2020 18:53 UTC

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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Thu, 20 Aug 2020 19:53:31 +0100
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Subject: Re: [Cfrg] hpke which HKDF to use for eae_prk?
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On 20/08/2020 19:42, Richard Barnes wrote:
> I think the spec is right here.  The DHKEM instances are defined with a
> curve and a KEM, so assuming your descriptor "x25519,hkdf-sha512,aesgcm128"
> is meant to be "KEM,KDF,AEAD", it's either incomplete or it represents a
> DHKEM instance that isn't defined in the document.  In the schemes in the
> document, the KDF in DHKEM is matched to the size of the curve, so x25519
> goes with SHA-256.

Could be. Lemme delve a bit more into the openssl APIs
to see if that's easily extracted or needs some new
config before I give up :-) That'll be tomorrow. That
said it still seems unnatural to have an IANA registry
for KDFs as part of the ciphersuite and yet use something
else at this point in the process.

> The current test vectors wouldn't catch this misunderstanding, though, so
> maybe it would be worth adding a case.

Probably worthwhile either way - including pretty much
every possible internal value in the test vectors would
be good I think.

Cheers,
S.

> 
> 
> On Thu, Aug 20, 2020 at 2:10 PM Stephen Farrell <stephen.farrell@cs.tcd.ie>
> wrote:
> 
>>
>> 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.
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>