(a) Find the linear recursion defining the sequence 0101110 0101110. . . of period 7. (b) Let Ek(m); Dk(c) be a block cipher. The special mode of this block cipher encrypts a sequence of message blocks m; m2; : : : ; by the sequence of ciphertext blocks c1; C2; : : : using the following method: C = m i-1 xor Ek(mixor c i-1); i 1, mo and co are fixed (public) initialization vectors. C i. Describe how decryption is performed. ii. Suppose ciphertext block c; is damaged in transit. Which plaintext blocks become undecipherable as a result? Explain. (c) We use the affine cipher for encryption and decryption. However, we would like to be able to distinguish between uppercase and lowercase characters, so that we have a cleartext or plaintext and ciphertext space of size 52. How large is the key space of the affine cipher in this case?