Confidentiality is the passing of information between two parties without a third party being able to understand it. Encryption is the main way of achieving confidentiality using computers. Encryption transforms a message into a ciphertext using an encryption key. The ciphertext can only be transformed back into the original message using the encryption key and therefore can only be read by someone who has the key. DES, RSA, and IDEA are examples of encryption schemes.
Authentication is the process of verifying that a message was sent by a given person.
This MAC is then appended to the message. A MAC is similar to private key encryption because the sender and receiver both share the same secret key. When using encryption, the secret key allows the receiver to read the message. If the message was changed or the wrong secret key was used, the MAC will be wrong. This tells the receiver that the message is not authentic.