Let n be a large integer and let k = m = c = z/nz. for each of the
functions

e : k×m→c

listed in (a), (b), and (c), answer the following questions:
• is e an encryption function?
• if e is an encryption function, what is its associated decryption function d?
• if e is not an encryption function, can you make it into an encryption function
by using some smaller, yet reasonably large, set of keys?
(a) ek(m) ≡ k − m (mod n).
(b) ek(m) ≡ k · m (mod n).
(c) ek(m) ≡ (k + m)2 (mod n).