Mr. Cipher is a deep undercover agent from the Republic of Cryptonia. Every day, he sends one of two messages back to base:
- M1 =“Nothing to report”,
- M2 =“Meet me tomorrow at the rendez-vous point, I have information”.
2
To send these messages, Mr. Cipher uses counter-mode encryption based on AES with a 128-bit key. In particular, it encrypts the current date (using 8 bytes), followed by the ASCII encoding of  one of the above two messages. Mr. Cipher uses the same secret key every day.
You work for the counterintelligence – you know the messages M1 and M2, you know the exact location of the rendez-vous point, and you are intercepting Mr. Cipher’s ciphertexts. Your task is to predict when Mr. Cipher will show up at the rendez-vous location.
 
a) Describe a strategy to obtain this information. 

b) How could Mr. Cipher protect himself from your attack strategy?
 
1Note that we are not asking whether the block cipher is secure!