I am studying my Cryptology exam and one question I got stuck below;
Recall the Diffie-Hellman key establishment protocol. Alice runs this protocol over the Internet to communicate with Bob. Assume that the agreed key after running the protocol is k = g^ab (mod p). Alice then computes SHA256(k) and uses this as the key to encrypt a message m using AES. The resulting ciphertext is then sent to Bob. a) Does this protocol guarantee the secrecy of m?
I know that perfect secrecy is if the ciphertext provides no information about the plain text. Can somebody explain the above question to me?