[CFRG] Re: Silithium - A Compact, Efficient and Non-separable Hybrid Signature

["D. J. Bernstein" <[e-mail]>]

Mothma has the big advantage of using PQ signatures _and_ ECC signatures
as black boxes. Silithium uses the PQ signature as a black box but needs
new ECC-signing code. Is there a compensating advantage of Silithium? (I
don't find it plausible that 2452 bytes vs. 2484 bytes matters.)

For purposes of obtaining ECC+PQ SUF-CMA assuming PQ SUF-CMA---and in
particular stopping the argument that one should retreat from ECC+PQ to
solo PQ to achieve SUF-CMA---it suffices to follow the suggestion of

    https://pqcrypto.eu.org/deliverables/d2.5.pdf#subsection.3.3

to sign with ECC and then sign the message-signature pair with PQ. This
also has a benefit far beyond protocols that need SUF-CMA: it simplifies
the use of signed-message APIs, which are safer than detached-signature
APIs. Mothma does this, plus randomization for more obscure properties.

The main argument I've noticed for unwrapping an ECC identification
protocol to insert a PQ signature is to aim for achieving ECC+PQ SUF-CMA
not just in the above scenario of the PQ part achieving SUF-CMA but also
in a further scenario, namely when the ECC part achieves SUF-CMA while
the PQ part fails to do so. But this is a smaller benefit that has to be
compared to the costs that the unwrapping imposes on implementors and
auditors. Anyway, my understanding is that Silithium doesn't aim for
this extra feature to begin with.

---D. J. Bernstein


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