Security needs on networks

Confidentiality: Only authorized people - e.g., the sender and recipient of a message, and not any eavesdroppers - can know the message.

Authentication: When Bob receives a message that purports to be sent by Alice, Bob can be sure that the message was really sent by Alice.

Integrity: When Bob receives a message, he can be sure that it was not modified en route after Alice sent it.

Non-repudiation: Alice cannot later deny that the message was sent. Bob cannot later deny that the message was received.