A foreign navy has implemented a secure communications system in which submarine commanders transmit a single 5-letter message every day. The messages may only use capital (English) letters, and all messages are equally probable. A crypto device encrypts each message into 20 bits of ciphertext.
a. What is the entropy of the plaintext message?
b. What is the entropy of the ciphertext message?
c. Is this a good cryptosystem? Why or why not?