the secret multiplier is limited by the order of the base point, that
also informs the size of elements for the signature (as they are
calculated modulo order), but the public point is a point, so its
elements are modulo prime from the curve. The same thing applies to the
shared secret: it's just one coordinate of the point, so it's modulo p
of the curve, not modulo order of generator.
for all curves up till now the size of order and size of the prime was
the same so it worked fine, but secp160r1 is different, so it showed the
bugs
so fix this bug and add secp160r1 as the test coverage for it