Re: [TLS] [EXTERNAL] [CFRG] X-Wing: the go-to PQ/T hybrid KEM?

[Bas Westerbaan <[e-mail]>]

> Speaking for myself (not for my co-authors), this feels like friendly,
> complementary work to draft-ounsworth-cfrg-kem-combiners;
>

I agree.


> We could consider adding a section with concrete instantiations, and the
> first one would be X-Wing 😊 (followed by ML-KEM + P-256, Brainpool, and
> RSA variants).
>
>
>
> I guess that leads to the following question: @Bas Westerbaan
> <bas=[e-mail]>, @Deirdre Connolly
> <[e-mail]>, Peter, would you be open to merging X-Wing
> into the generic combiner draft, or is there value in it being standalone?
>

X-Wing explicitly trades genericity for simplicity. We will not get such a
simple and efficient construction if it is the instantiation of an
easy-to-use generic construction.

Best,

 Bas


>
> ---
>
> *Mike* Ounsworth
>
>
>
> *From:* CFRG <[e-mail]> *On Behalf Of *Bas Westerbaan
> *Sent:* Wednesday, January 10, 2024 2:14 PM
> *To:* IRTF CFRG <[e-mail]>; <[e-mail]> <[e-mail]>
> *Cc:* [e-mail]
> *Subject:* [EXTERNAL] [CFRG] X-Wing: the go-to PQ/T hybrid KEM?
>
>
>
> Dear tls and cfrg working groups, With ML-KEM (née Kyber) expected to be
> finalized this year, it’s time to revisit the question of which PQ/T hybrid
> KEMs to standardize, and which to recommend. # Status quo For TLS at the
> time of writing there
>
> Dear tls and cfrg working groups,
>
> With ML-KEM (née Kyber) expected to be finalized this year, it’s time to
> revisit the question of which PQ/T hybrid KEMs to standardize, and which to
> recommend.
>
> # Status quo
>
> For TLS at the time of writing there are two PQ/T hybrids registered:
> X25519Kyber768 [1] and P256Kyber768 [2]. The former has been deployed
> widely [3]. Both are instances of the hybrid-design draft [4], which use
> the simple combiner ss_ECC || ss_Kyber, which is suitable for TLS, but not
> for other applications such as HPKE, as it’s not IND-CCA2 robust [5].
>
> For HPKE, there is a different KEM called X25519Kyber768 [6], which uses a
> different combiner that mixes in the X25519 ephemeral key, by using HPKE’s
> DHKEM construction instead of raw X25519.
>
> There is also the ounsworth-kem-combiners I-D [7] that informed by [5]
> proposes the generic combiner
>
>   KDF( counter || ct1 || ss1 || ct2 || ss2 || fixedInfo, outputBits )
>
> From a security standpoint that would be suitable for HPKE and TLS. To TLS
> it is somewhat unattractive as it requires hashing the typically large PQ
> ciphertexts, and adds some extra hashing in the conversion of the ECDH into
> a KEM. On the other hand, for TLS it would be nice to have a KEM that is
> also suitable for HPKE, as HPKE is used in ECH.
>
> From a usability perspective, ounsworth-kem-combiners requires the user to
> make several choices: which KEMs and in particular which method to use to
> turn ECDH into a KEM, which security levels, which KDF, etc.
>
> # The proposal: X-Wing
>
> Let us introduce X-Wing [0]. The goal of X-Wing is to be *the* go-to PQ/T
> hybrid KEM for the majority of use cases (including TLS and HPKE): no need
> to make choices, or understand the subtleties.
>
> X-Wing aims for 128-bit security, and for that combines the time-tested
> X25519 with ML-KEM-768 [8]. X-Wing uses the combiner
>
>   SHA3-256( xwing-label || ss_ML-KEM || ss_X25519 || ct_X25519 ||
> pk_X25519 )
>
> Here ss_X25519 is the plain X25519 shared secret; ct_X25519 is the
> ephemeral public key; xwing-label a 6-byte label. Note that it doesn’t hash
> in the ML-KEM ciphertext. For a generic KEM one cannot leave out the
> ciphertext, but in the case of ML-KEM we can, assuming we can model
> SHA3/SHAKE as a random oracle. This is proven in [0]. The gist is that FO
> transform in ML-KEM makes it “ciphertext collision resistant”: even if the
> underlying lattice problem is broken, it’s infeasible to create from one
> ciphertext another different ciphertext with the same shared secret.
>
> # Not final
>
> We would love to hear your input: X-Wing is not final. For one, ML-KEM
> itself might still change (presumably only in minor ways) before final
> standardization. We think the CFRG would be a good venue to standardize
> X-Wing — do you concur?
>
> Best,
>
> Bas, Deirdre, Karolin, Manuel, Peter
>
>
> PS. We want to mention explicitly that we see value in the kem-combiners
> and hybrid-design drafts as generic safe methods to construct hybrids for
> those use cases where X-Wing would not suffice.
>
>
> [0] Spec: https://datatracker.ietf.org/doc/draft-connolly-cfrg-xwing-kem/
> <https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-connolly-cfrg-xwing-kem/__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-Y-JP2DY$>
> Proof: https://eprint.iacr.org/2024/039
> <https://urldefense.com/v3/__https:/eprint.iacr.org/2024/039__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-Xl0zY2C$>
> [1] Full name X25519Kyber768Draft00.
> https://datatracker.ietf.org/doc/draft-tls-westerbaan-xyber768d00/
> <https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-tls-westerbaan-xyber768d00/__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-bUDJTlz$>
> [2] Full name SecP256r1Kyber768Draft00.
> https://datatracker.ietf.org/doc/draft-kwiatkowski-tls-ecdhe-kyber/
> <https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-kwiatkowski-tls-ecdhe-kyber/__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-cpge9_6$>
> [3]
> https://blog.chromium.org/2023/08/protecting-chrome-traffic-with-hybrid.html
> <https://urldefense.com/v3/__https:/blog.chromium.org/2023/08/protecting-chrome-traffic-with-hybrid.html__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-X2cJwvg$>
> https://twitter.com/bwesterb/status/1734586155868287457
> <https://urldefense.com/v3/__https:/twitter.com/bwesterb/status/1734586155868287457__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-agVitjD$>
> [4] https://datatracker.ietf.org/doc/draft-stebila-tls-hybrid-design/
> <https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-stebila-tls-hybrid-design/__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-axrezMz$>
> [5] https://link.springer.com/chapter/10.1007/978-3-319-76578-5_7
> <https://urldefense.com/v3/__https:/link.springer.com/chapter/10.1007/978-3-319-76578-5_7__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-U_tyIdl$>
> [6]
> https://datatracker.ietf.org/doc/draft-westerbaan-cfrg-hpke-xyber768d00/
> <https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-westerbaan-cfrg-hpke-xyber768d00/__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-V-p_aAA$>
> [7] https://datatracker.ietf.org/doc/draft-ounsworth-cfrg-kem-combiners/
> <https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-ounsworth-cfrg-kem-combiners/__;!!FJ-Y8qCqXTj2!YuGyk3egE_PIU03oVixCUPtatL8PHtv4HwoB1vN5giqCIDkH6AQcs-lATDzPlozu91nN60pT2kp1AwmLESgzB4xc58lF-bx4gLTn$>
> [8] Following earlier deployment of X25519Kyber768, despite targeting 128
> bits, we use ML-KEM-768 instead of ML-KEM-512 to hedge against advances in
> lattice attacks.
>