Let S,V be a secure MAC defined over KMT where K={0,1}^128, M={0,1}^n, and T={0,1}^128. Which of the following is a secure MAC? (As usual, we use "|" to denote string concatenation.)
1) S_k,m = S_k,m || m) and V_k,m,t = V_k,m || m,t
2) S'_k,m = S_k,m[0,n-2] || 0 and V'_k,m,t = V_k,m[0,n-2] || 0,t
V_k,m,t if m ≠0
3) S_k,m = S(k,m) and V_k,m,t = V_k,m,t if m ≠0, otherwise 1
4) S'_k1,k,m = S_k1,m || S_k2,m and V'_k1,k,m,t1,t = [V_k1,m,t1 and V_k2,m,t]
i.e. V_k1,k,m,t1,t outputs 1 if both t1 and t2 are valid tags
5) S_k,m = t || t, where t = S_k,m and V_k,m,t = 1 if t1 = t2 and V_k,m,t1,t = 0 otherwise (i.e. V_k,m,t1,t only outputs 1 if t1 and t2 are equal and valid)
S_k,m = S_k,m1 and V_k,m,t = V_k,m1,t
S_k1 if m = 0, V_k,1,t if m = 0
7) S'_k,m and V_k,m,t = S(k,m) and V_k,m,t if m ≠0, otherwise V_k,m,t otherwise
