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Consider IND-CCA2 #16

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ounsworth opened this issue Oct 2, 2023 · 0 comments

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ounsworth commented Oct 2, 2023

https://datatracker.ietf.org/doc/html/draft-westerbaan-cfrg-hpke-xyber768d00-02 has this very similar security consideration.

Does the same apply here?

We aim for IND-CCA2 robustness: that means that if either constituent KEM is not IND-CCA2 secure, but the other is, the combined hybrid remains IND-CCA2 secure.In general [GHP18] [COMBINERS] this requires a combiner that mixes in the cipher texts, such as, assuming fixed-length cipher texts and shared secrets:

HKDF(concat(ss1, ss2, enc1, enc2)).

In the present case, DHKEM(X25519, -) and Kyber768Draft00 already mix in the respective cipher texts into their shared secrets. Thus we can forgo mixing in the cipher texts a second time.Furthermore, in HPKE, the shared secret is never used directly, but passed through HKDF (via KeySchedule), and thus we can forgo the call to HKDF as well.

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