4. (Polybius Cipher, 15pt) a) [3pt] Create the Polybius square for the keyword "cryptography". Note: we make the standard assumption that i = j so we have a 25 letter alphabet. b) [3pt] Use the Polybius square from a) to encrypt your first name (stop after the first six letters if your first name is longer; it it's shorter, add some letters from your last name; replace j with i if necessary). c) [3pt] Use the Polybius square from a) to decrypt the ciphertext" 4521415115342143" d) [6pt] You've intercepted the ciphertext 35 34 33 12 14 42 34 45 43 13 54 Based on the form of the ciphertext you suspect a Polybius cipher, but when you decrypt (with the standard square) you get nonsense. The sender must have used a Polybius cipher based on some keyword. You know that the sender is lazy and suspect that the keyword is short (length 4). By bribing the secretary of the sender you find out that the keyword used to construct the Polybius square did not contain letters in the second half of the alphabet (N-Z). Decrypt the message, reconstruct the Polybius square, and find the keyword used to create it. Hint: the fact that the keyword doesn't contain the letters N-Z tells you something about the square. Use that. Some trial/error, guessing may be involved. Include a brief description of how you managed to break the cipher.