Re: [PATCH 16/18] crypto: dh - calculate Q from P for the full public key verification

Stephan Müller [off-list ref] writes:

Am Mittwoch, 1. Dezember 2021, 01:48:56 CET schrieb Nicolai Stange:

quoted

As the ->q in struct dh_ctx gets never set anywhere, the code
in dh_is_pubkey_valid() for doing the full public key validation in
accordance to SP800-56Arev3 is effectively dead.

However, for safe-prime groups, Q = (P - 1)/2 by definition and this
enables dh_is_pubkey_valid() to calculate Q on the fly for these groups.
Implement this.

With this change, the last code accessing struct dh_ctx's ->q is now gone.
Remove this member from struct dh_ctx.

Isn't it expensive to always calculate Q for a-priori known values? Why not 
add Q to the safe-prime definitions and do not do this operation here?

I actually considered this when writing the patch: it's basically a
time-memory tradeoff and I opted for time here. The reason is that I'd
expect the rather trivial Q calculation to be negligible when compared
to the subsequent mpi_powm() operation in dh_is_pubkey_valid(). OTOH, as
the size of Q is (almost) equal to that of P, the space needed for
storing all the the individual groups' precomputed Qs would be
significant. So I'd say let's wait and see whether the dynamic Q
calculation does actually show up in profiles before thinking about
optimizations like e.g. precomputations.

Thanks,

Nicolai

If you need Q for all of those safe-primes, you may get them from [1] and 
following lines.

[1] https://github.com/smuellerDD/acvpparser/blob/master/parser/
safeprimes.h#L346

Ciao
Stephan

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