Approaches to message authenticatio

List three approaches to message authentication.
What is a message authentication code?
What properties must a hash function have to be useful for message authentication?
In the context of a hash function, what is a compression function?
What are the principal ingredients of a public-key cryptosystem?
List and briefly define three uses of a public-key cryptosystem.
What is the difference between a private key and a secret key?
What is a digital signature?