Using shared secret keys, it is possible to calculate Integrity check values from a message to achieve integrity. The integrity check value should depend on all bits of the plaintext. Should any bits of the message be changed between the sender and recipient, the recipient would calculate a different integrity check value. An adversary modifying a message might as well modify the check value too, but without knowledge of the secret key she cannot duplicate the correct in- tegrity check value. If the receiver correctly verifies the integrity check value, she knows the message was generated by someone who knew the key. .
