In sending and receiving messages, parties are often interested in three prop- erties of the communication other than confidentiality. Integrity means that the sender and receiver should be able to verify that a message has not been modified in transit. As a consequence, this means that an intruder should not be able to sub- stitue a false message for a legitimate one without being detected. Authentication means that the receiving party should be able to ascertain the origin of a message. Nonrepudiation means that the sender should not be able to falsely deny that she sent a message
