Identifying and authenticating users is perhaps the most important of these tasks. This is the process that ensures to the serving application or user that the client is who he claims to be. The field of authentication is very strong, and there are several very secure techniques to ensure a user's identity.
Data integrity refers to preventing data corruption.
Even changing one single character can be detected without a doubt. Data integrity ensures that the message received is the same message that was sent. Non-repudiation is a technique which does not allow a user to claim he did not send a message when in fact he did. Non-repudiation works on the principle that a user keeps his private key secret, and by signing a document, he acknowledges that he in fact signed the same document the other user reads. Data security , ensured by encryption algorithms and protocols, allows two or more parties to communicate in a way such that only the parties involved can read the communications.