$($a$)$ The ciphertext PHFHTI is obtained from a Playfair cipher with the key BLOCKCIPHER. Find the plaintext.

$($b$)$ A password consists of $8$ characters, and each character is randomly chosen from the lower case letters $($a to z$),$ the upper case letters $($A to Z $),$ and ten numbers $(0$ to $9).$ What is the entropy of the password?

$($c$)$ An unknown classical cipher was used to encrypt an English message. The ciphertext character frequencies are shown in the following figure. The ciphertext is different from the plaintext. What is the classical cipher being used? Please justify your answer.

Standard English character frequency