Case Study: Consider a sender A and a receiver B.

A should send a message M to B. A want to give a proof for B that the message has not been modified during transmission. A uses a hashing function H. He uses also a symmetric encryption E which needs a shared key K. Let the concatenation operation be noted as ||

The receiver B uses a decryption algorithm D which needs the same key K as the sender. He also uses the same hash function H to produce a digest.

There are many ways to fulfill this objective. We here propose two cases and you will help drawing and explain them.

1. Case 1 : Draw with explanation the use of message authentication using symmetric key encryption and a hash function. In this communication, only the digest should be encrypted. The message is sent as a clear plaintext. States what security objectives has been achieved by this scheme.
2. Case 2 : Draw with explanation the use of message authentication using symmetric key encryption and a hash function. In this communication, both the message and the digest should be encrypted. States what security objectives has been achieved by this scheme.