Previously, one could construct an RSA key pair "successfully" even if
the bit length of the private moduli was not evenly divisible by 512.
However, using such a key to sign something would (usually) fail on
x86-64 because the x86-64 signing code requires the modulus to be an
even multiple of 8 limbs.
Now, all keys that have private primes with lengths that aren't
multiples of 512 bits will be rejected at key construction time with
error "PrivateModulusLenNotMultipleOf512Bits".
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