Which of the following statements are correct regarding the Diffie-Helman key exchange? (Select all that apply) a. Diffe-Hellman requires a pre-shared key be transmitted before the exchange can begin b. The discrete logarithm problem is a major vulnerability in the Diffie-Hellman protocol c. 27mod6=3 d. Diffie-Hellman key exchange is vulnerable to man-in-the-middle attacks