Hello,

So

p=5; q=23

Because hecubesthe message he wants to send, e=3.

(n,e) is the public key.

C=3, it's the coded message you receive.

M is the message he sent.

Hence, we have

Let d be such that

(you should know that..)

So if you want to get M, calculate modulo n.

~~~~~~~~

Calculating d :

We know that

Therefore,

So

~~~~~~~~

Either you use a calculator, either you do it by hand. I've tried parts but this yields to numbers I don't like and that are not kind to deal with...

This looks like a draft, but I just tried to make it the simpliest way xD