Q6: A blockcipher normally can be considered as a pseudorandom permutation, i.e. E: 0,1}128 {0,1}128, when encrypting a picture m, say with n = 105 blocks (each with 128 bits, total about 1.5 megabytes), and we use an ECB mode, i.e., C1 = E(k, m), ...,Cn = E(k, mn). It is very common that the picture has many identical blocks, e.g., mi = m2 = ...,= mk, explain why ECB mode will reveal this pattern; and why other modes such as counter mode will not? A counter mode encryption generates ciphertext as ci = E(k, 1) m1, C2 = E(k, 2) m2, ..., n = E(k,n) mn and here 1,2,3,...,n are all treated as a 128-bit binary string, assuming n