coding in cryptography

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.