Consider the following scheme to provide message authentication. B gets a message supposedly from A, and B wants to make sure that the message was sent by A and not modified by BG along the way. A generates a secret key K for DES, calculates C = E_K (M), and sends M, C, K to B. B checks it M = D_K (C) and if it is, accepts that M was sent by A. Is this is a good scheme? i. Give a YES/NO answer. ii. If you said NO explain what you think is the single biggest weakness of the scheme