Re: [Cfrg] hpke encoding of DH output
Stephen Farrell <stephen.farrell@cs.tcd.ie> Thu, 20 August 2020 18:47 UTC
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To: Christopher Wood <caw@heapingbits.net>, cfrg@irtf.org
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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Thu, 20 Aug 2020 19:47:50 +0100
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Subject: Re: [Cfrg] hpke encoding of DH output
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On 20/08/2020 19:43, Christopher Wood wrote: > On Thu, Aug 20, 2020, at 11:29 AM, Richard Barnes wrote: >> Hi Stephen, >> >> I think you're right here. I just re-checked both the NIST and SECG >> specifications for ECDH [1][2], and they're in agreement that the >> secret value is the X-coordinate. I'll work with my coauthors to get >> this implemented. > > A proposed fix is here: > > https://github.com/cfrg/draft-irtf-cfrg-hpke/pull/148 LGTM, thanks both. S. > > Thanks, > Chris > >> >> --Richard >> >> [1] Section 5.7.1.2 of >> https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-186-draft.pdf >> [2] Section 3.3.1 of https://www.secg.org/sec1-v2.pdf >> >> >> >> On Thu, Aug 20, 2020 at 2:04 PM Stephen Farrell >> <stephen.farrell@cs.tcd.ie> wrote: >>> >>> 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. >>> _______________________________________________ >>> Cfrg mailing list >>> Cfrg@irtf.org >>> https://www.irtf.org/mailman/listinfo/cfrg >> _______________________________________________ >> Cfrg mailing list >> Cfrg@irtf.org >> https://www.irtf.org/mailman/listinfo/cfrg >> > > _______________________________________________ > Cfrg mailing list > Cfrg@irtf.org > https://www.irtf.org/mailman/listinfo/cfrg >
- [Cfrg] hpke encoding of DH output Stephen Farrell
- Re: [Cfrg] hpke encoding of DH output Richard Barnes
- Re: [Cfrg] hpke encoding of DH output Christopher Wood
- Re: [Cfrg] hpke encoding of DH output Stephen Farrell
- Re: [Cfrg] hpke encoding of DH output Dan Harkins