[CFRG] Re: [lamps] Re: Re: Cross-testing update on LAMPS Composite and cfrg-concrete-hybrid-kems

[Thad Thompson <[e-mail]>]

Hey Martin,
> I have to admit, that in my HPKE implementation I did not even notice (or
have enforced) that X25519 keys generated through other means cannot be
used.
Is this a MUST requirement?

No, sorry for the confusion - I don't believe this is the case. As Watson
pointed out above, key generation is handled pretty well in
https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-hybrid-kems#section-5.2
which
describes the considerations for working without seeds. Also note that this
isn't in the context of an HPKE spec, but just hybrid KEMs at this point.

My earlier comment was aimed more generally toward the impression one gets
from various places is that seed based key generation is a 'should'
requirement, when it really isn't. For example,
https://www.ietf.org/archive/id/draft-irtf-cfrg-concrete-hybrid-kems-01.html#section-3.1.1-7
specifies:

> A hybrid KEM using these curves MUST specify the PRG that should be
used.  All of the hybrid KEMs in this document use SHAKE256 [FIPS202].

Which might make it appear that a PRG (and seed based key generation) is
required for 'in-production' implementations of the KEM, instead of really
being a requirement for interop testing.

Cheers,
-Thad



On Mon, Oct 27, 2025 at 6:04 AM Martin Schanzenbach <[e-mail]>
wrote:

> Am Mittwoch, dem 22.10.2025 um 07:46 -0500 schrieb Thad Thompson:
>
> > Implementations can obviously always go off-spec for keygen.
>
> This seems like the heart of the issue.
>
> Let's say I get my PQ-HPKE library implemented and tested and it works
> fine with all the test vectors. But in prod we're only allowed to use EC
> private keys forged in the fires of Mount Doom. One does not simply tell
> their boss (and customers) "no problem, we'll just go off spec."
>
> It would go a long way if there were a section clearly explaining the
> security impact of using EC keys generated elsewhere, making it an
> 'in-spec' option.
>
> Best
> -Thad
>
>
> +1
>
> I have to admit, that in my HPKE implementation I did not even notice (or
> have enforced) that X25519 keys generated through other means cannot be
> used.
> Is this a MUST requirement? Where is the reasoning in particlar for X25519
> and X448? If it is a MUST, somewhere in the document should be an
> explanation on why it is done such that other KEMs can make an informed
> decision how to place limitations on their (private) keys in a similar
> fashion.
>
> BR
>
>
>
> On Tue, Oct 21, 2025 at 6:06 PM Filippo Valsorda <[e-mail]>
> wrote:
>
> Hi Mike,
>
> Since this thread is about hybrid KEMs and not HPKE DHKEM, I think you are
> confusing the DeriveKeyPair function of RFC 9180 with the CG framework
> DeriveKeyPair function of draft-irtf-cfrg-hybrid-kems-06, Section 5.5,
> which references the RandomScalar function
> of draft-irtf-cfrg-concrete-hybrid-kems-01, Section 3.1.1. (I also find it
> hard to navigate the split in documents.)
>
> I have reproduced them below for you. I am certain you can implement them
> with any major cryptography library. There is nothing HPKE-specific about
> them. SP 800-56A, Section 5.6.1.2, only specifies a random key generation
> algorithm (a pretty silly one, IMHO, with the needless "d = c + 1" instead
> of checking "c > 0"), so I am not sure what relevance it has here: there is
> no such thing as a standard library function for SP 800-56A, Section
> 5.6.1.2.{1,2} key *derivation*. If the argument is that no one should be
> allowed to derive ECDH keys from a KDF because NIST doesn't like that, I
> doubt many folks will find that argument persuasive.
>
> Implementations can obviously always go off-spec for keygen if they need
> to appease a regulator (or e.g. use hardware keys), though! They will still
> interoperate.
>
> Cheers,
> Filippo
>
> def DeriveKeyPair(seed):
>     (ek_PQ, ek_T, dk_PQ, dk_T) = expandDecapsKeyG(seed)
>     return (concat(ek_PQ, ek_T), seed)
>
> def expandDecapsKeyG(seed):
>     seed_full = PRG(seed)
>     (seed_PQ, seed_T) = split(KEM_PQ.Nseed, Group_T.Nseed, seed_full)
>
>     (ek_PQ, dk_PQ) = KEM_PQ.DeriveKeyPair(seed_PQ)
>     dk_T = Group_T.RandomScalar(seed_T)
>     ek_T = Group_T.Exp(Group_T.g, dk_T)
>
>     return (ek_PQ, ek_T, dk_PQ, dk_T)
>
> def RandomScalar(seed):
>   state = XOF.Init(seed)
>   sk = OS2IP(XOF.Read(state, Nscalar))
>   while sk == 0 || sk >= order:
>     sk = OS2IP(XOF.Read(state, Nscalar))
>   return (sk, pk(sk))
>
> (The XOF is SHAKE256, Nseed/Nscalar is 32 for P-256, and 48 for P-384.)
>
> 2025-10-22 00:31 GMT+02:00 Mike Ounsworth <[e-mail]>:
>
> Hi Watson,
>
> SP 800-56A section 5.6.1 defines the FIPS-approved ways of deriving a ECDH
> key. You can't derive your keys as described in rfc9180 7.1.3 and claim
> that it is "ECDH" as defined in SP 800-56A, because it isn't. That's not a
> FIPS-allowed way to derive EC keys, and that's the whole story.
>
> On Tue, 21 Oct 2025 at 14:23, Watson Ladd <[e-mail]> wrote:
>
> On Tue, Oct 21, 2025 at 12:14 PM Mike Ounsworth
> <[e-mail]> wrote:
> >
> > Follow-up: I see that DHKEM.DeriveKeyPair(ikm) is defined in RFC9180:
> > https://datatracker.ietf.org/doc/html/rfc9180#derive-key-pair
> >
> > That makes liberal use of HPKE's LabelledExtract(), which fully explains
> why this is not compatible with the python API.
> >
> > That means that a seed-based key derivation is defined for P-256, P-384,
> and P-521, but only within the confines of the DHKEM primitive within the
> HPKE protocol. It is still the case that a seed-based key derivation is not
> defined for ECDH in general.
> >
> > So, on that grounds, I object to draft-irtf-cfrg-concrete-hybrid-kems
> RECOMMENDING a seed-based priv on exactly the same grounds that anyone else
> would object to it RECOMMENDING an ASN.1 based encoding: this is a CFRG
> document that normatively depends on a building block that only exists in
> one single IETF protocol, and most importantly to me, does not exist in
> FIPS or current cryptographic libraries.
>
> I don't understand: HKDF (or the underlying hash function worst case)
> absolutely does exist in current cryptographic libraries. If the
> argument is that a private key generated one way won't work when moved
> to a different system, that's a slightly different matter.
>
> >
> > On Tue, 21 Oct 2025 at 13:34, Mike Ounsworth <[e-mail]>
> wrote:
> >>
> >> Hi CFRG and HPKE,
> >>
> >> I just want to give an update on this. Richard and I did some more
> cross-testing this week, and we are at an impasse on the SEC-P curves, and
> it has to do with the seeds. The CFRG /HPKE draft stores the hybrid private
> keys as a seed, so you have to re-derive the P256 half from a seed. My
> problem is that ECDH is a thing that has been standardized for like 20
> years in [SEC1] and [SP.800-56A], and those two documents do not specify or
> allow a seed-based EC keygen, and as a consequence, many crypto libraries
> don't support a seed-based EC keygen API. For example, I'm doing my
> reference implementation for Composites on top of the core python
> cryptography library, and I cannot implement the CRFG / HPKE draft that
> way. Python crypto has this:
> >>
> https://cryptography.io/en/latest/hazmat/primitives/asymmetric/ec/#cryptography.hazmat.primitives.asymmetric.ec.derive_private_key
> >> but that appears to not derive the same keys as Richard's
> implementation.
> >>
> >> The issue is that EC.derive_priv_from_seed() is not a standardized
> function. You can probably infer from a careful read of [SEC1] or
> [SP.800-56A] what it needs to be, but it is not actually standardized or
> FIPS-allowed. It requires mucking around with EC point multiplication. The
> fact that it's not a standardized thing means that I will likely not be the
> last person to have interop problems with this. Maybe my issues are as
> simple as not using that python ec.derive_private_key() properly, maybe his
> scalar is 'big' rather than 'little' or something. If that's the case, I'd
> be happy -- but this is the kind of mess you get into when you use
> non-standardized APIs for existing 20 year-old primitives.
> >>
> >> So at this point, I think we have to declare non-compatibility between
> LAMPS and HPKE Hybrid KEMs. Since our X-Wings are compatible, we know that
> our QSF frameworks are identical, so in principle our P256's should work
> too, but we're been incapable of testing it because this off-spec seed
> stuff is getting in the way.
> >>
> >> On Thu, 16 Oct 2025 at 17:40, Tushar Patel <[e-mail]>
> wrote:
> >>>
> >>> Sorry, jumping in a running train and
> >>> many years ago while working through JSON bindings for the tests, I
> ran into a sign bit conversion error on big numbers and had to rewrite part
> of the library, the fixed code is with a previous employer, 64-bit and
> 128-bit in JSON libs are a mess or sign bit issues in general.
> >>>
> >>> Also, please make sure that you are parsing the compressed notations
> for ECC correctly, some libraries pack those incorrectly and also wrote my
> own ASN.1 parsers for some elements, you might be surprised where I hit
> these areas, can’t disclose, though both were from reputed organizations.
> >>>
> >>> Last, please make sure you have matching generators and incorporate
> the affine and infinity checks, these may translate to all sorts of weird
> issues that are impossible to debug through OSS macros in most cases.
> >>>
> >>> There is substandard ECC, ECDSA and ECDHE at many places in OSS libs
> and you may have to debug these.
> >>>
> >>> Personally, I think NIST should mandate pristine implementations as
> companies spend a lot more time fixing other implementations than their own.
> >>>
> >>> Hope it’s Useful,
> >>> Tushar
> >>>
> >>> On Thu, Oct 16, 2025 at 8:45 AM Mike Ounsworth <
> [e-mail]> wrote:
> >>>>
> >>>> Thanks Filippo,
> >>>>
> >>>> Richard and I did some debugging last night. He gave me new test
> vectors that include that bug fix to the P256 / P384 constants.
> >>>>
> >>>> My current theory is that the code I quickly whipped up to do a P256
> private key expansion from seed does not match Richard's. Since this is
> wholly out of scope for LAMPS Composite (we store the full P256 private
> key), I'm hoping that Richard can give me a dump of intermediate values
> that includes the expanded P256 / P384 private key so that we don't need to
> debug through the P256-from-seed stuff. Anyway, this is all good validation
> of both our test vectors.
> >>>>
> >>>> Anyway, making progress!
> >>>>
> >>>> On Thu, 16 Oct 2025 at 10:29, Filippo Valsorda <[e-mail]>
> wrote:
> >>>>>
> >>>>> 2025-10-16 03:55 GMT+02:00 Mike Ounsworth <[e-mail]
> >:
> >>>>>
> >>>>> Results: we are cross-compatible on X-Wing, but not on P256 (and
> that's after hacking my script to use the CFRG label not the LAMPS label,
> so there's clearly some other bug at play, but I have no idea what, so of
> course I assume it's on the cfrg reference impl side 😋).
> >>>>>
> >>>>>
> >>>>> Could be
> https://github.com/cfrg/draft-irtf-cfrg-concrete-hybrid-kems/pull/21.
> >>>>
> >>>> _______________________________________________
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> >
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>
>
> --
> Astra mortemque praestare gradatim
>
>
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