Let M=C=K={0,1,2,,25}. Consider the following cipher:

f(m,k)=m+k(mod26),g(c,k)=ck(mod26)

The key is generated uniformly at random from K. Does this cipher have perfect secrecy?

(i). No, there is a simple attack on this cipher.

(ii). Yes, it has perfect secrecy because the probability distribution of c is independent of m.

(iii). No, only OTP has perfect secrecy.

(iv). Yes, it has perfect secrecy because the key space and the message space have the same size.