A military unit uses a Vigen$$re cipher to encrypt classified information. The adversary

intercepts the following encrypted message: "VPEH PFLK IUBT FYJI UQBQ". The adversary

knows that the message starts with the phrase "SECRET". Using this information:

a$)$ Break the encryption and recover the full plaintext using cryptanalysis techniques.

b$)$ Discuss how the known$-$plaintext attack was used in this scenario and explain why Vigen$$re

ciphers are vulnerable to such attacks.