Previous implementation assumed the moduli of 65537 instead of reading
the real value from the public key.

Adjust the code and test it against a key using 257 for the modulus.

Fixes: #72
See: https://github.com/wiktor-k/gen-rsa-exp-pgp


Signed-off-by: Wiktor Kwapisiewicz <wiktor@metacode.biz>