I am doing practice problems for my upcoming final exam, and am having trouble with this RSA encryption problem. If any one could check to see if i did these correctly, it would be greatly appreciated.
For (a) I got $\mathbb{Z}^*_8 = \{1,2,3,4,5,6,7\}$,
(b) i got $3*3 = 1 \text{ mod } 8 = 3^{-1} = 3$
(c) this one i am not sure how to do
(d) $M = N^d \text{ mod } 15$, i got $d = 3$, so $M = 4^3 \text{ mod } 15 = 4$