4 In an attempt to prevent Kasiski’s attack on the Vigen`ere cipher, the
following modification has been proposed. Given the period t of the
cipher, the plaintext is broken up into blocks of size t. Recall that within
each block, the Vigen`ere cipher works by encrypting the ith character
with the ith key (using a basic cipher). Letting the key be k1, . . . , kt,
this means the ith character in each block is encrypted by adding ki to
it, modulo 26. The proposed modification is to encrypt the ith character
in the jth block by adding ki + j modulo 26.
(a) Show that decryption can be carried out.
(b) Describe the effect of the above modification on Kasiski’s attack.
(c) Devise an alternate attack that works better than Kasiski’s attack.