What potential flaws or downsides are there with using a Vigenre cipher? (There may be more than one correct answer.)

A. There is no downside as it provides perfect security.

B. If the key is shorter than the message, then patterns will emerge in the cipher text that make it possible to attack the message using frequency analysis.

C. If the key is shorter than the alphabet being used, then patterns will emerge in the cipher text that make it possible to attack the message using frequency analysis.

D. The key must be exchanged between the sender and the recipient.

E. No key exchange is required.