On some architectures, including AMD Zen CPUs, dividing a secret
by a constant denominator may not be a constant-time operation.
And most Kyber implementations, including ours, could leak the
hamming weight of the shared secret because of this. See:
https://kyberslash.cr.yp.to
Multiplications aren't guaranteed to be constant-time either, but
at least on the CPUs we currently support, it is.
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