alice and bob want to establish a secure communication channel between them. they do not care about the confidentiality of the messages being transmitted, but they do want to ensure the integrity and authenticity of the messages. assume a and b share a common key k. answering the following questions. (15 points) a. (5 points) how can they achieve their goal only with secret key cryptography? b. (5 points) how can they achieve their goal with hash function h and the key? c. (5 points) can they get non-repudiation? if yes, how? if no, why?