[Cfrg] hpke encoding of DH output

Stephen Farrell <stephen.farrell@cs.tcd.ie> Thu, 20 August 2020 18:03 UTC

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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Thu, 20 Aug 2020 19:03:26 +0100
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Subject: [Cfrg] hpke encoding of DH output
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Hi,

I ran into an interop problem with draft-05 that I think is
worth bringing to the list.

Draft-05 says:

"For the variants of DHKEM defined in this document, the
size Ndh of the Diffie-Hellman shared secret is equal to
Npk, and the size Nsecret of the KEM shared secret is equal
to the output length of the hash function underlying the
KDF."

What that means is that, for the NIST curves, the DH
value (used to be zz I think) is represented as a public
key in uncompressed form. My code uses the OpenSSL
EVP_PKEY_derive() function (same as it did for draft-02)
which only gives me the X co-ordinate, and OpenSSL doesn't
seem to have an easy way to get the uncompressed version
from that. I don't know, but I'd guess that other libraries
might be similar. In draft-02 only the X co-ordinate was
used btw, and I don't recall this change being brought
up on the list.

I don't think there's any security benefit in treating
the output of the DH operation as a public key. If there
were, then I'd be fine with changing to use lower level
calls to do the DH operation. But that seems a bit wrong,
so I'd argue that we'd be better to not treat the DH
shared secret value as a public key when encoding that.

Separately, it'd be good to add those values to the
test vectors - took me a while to find this - in the
end I had to add more tracing to the go implementation
to spit out these values.

Lastly, even if we don't make a change, it'd be good
to add text to clarify this, but I think I'd prefer we
make the change if there's no security downside.

Cheers,
S.