We seperate encryption into two part: updateKey and process. Each time we got an message with a new RSA key, we have to call updateKey before get into encryption/decryption. If we process messages with the same key continuously, updateKey only need be called once at the beginning.
It should be notice that we provide actually two implementation of function keydateKey. One have two inputs: modulus and exponent. The other one has three inputs: modulus, exponent and rMod. The extract argument “rMod” is actually 2^(2*N) mod modulus. This is a key parameters in the encryption/decryption calculation. If you has pre-calculated this arguments, you could call the second updateKey and directly set it up. If you don’t have it, you could call the first one and we will do the calculation on chip, with extract resource.
RSA encryption and decryption are basically the same calculation: big integer modulus exponential calculation. Instead of doing straight calculation, we convert the big integer into its Montgomery field and do exponential calculation. Finally we convert the result back to normal representation. In such we, we could avoid most integer division and multiplication to save resource and have higher frequency.