Alice and Bob wish to communicate over an insecure link and

have agreed to use the AES encryption algorithm with a key

length of $128$ bits. However, they need to agree on a

cryptographic mode of operation.

$,$ are two possible implementations of the AES

algorithm in the cipher block chaining and counter modes,

respectively, in both cases assuming the standard AES block of

$128$ bits. Which of the above two modes do you recommend in

order to reduce the number of times the key needs to be

changed? Explain and justify your answer.

For your chosen mode, calculate, with full explanation, the

maximum number of bits that can be transmitted over this link

using the same encryption key such that the advantage of an

adversary capable of carrying out a chosen plaintext attack

does not exceed $2.$ It is assumed that the advantage of the

adversary over AES is $2$ in the case of one time key use.