In a secure network, users communicate with each other, but they are still concerned about their confidentiality. Both the parties need to make sure that a third party is not listening to them. This is the privacy concept.
The message needs to be interpreted in such way that it becomes meaningful for the transmitter and the receiver. To all others, it must appear to be garbage, or meaningless. To achieve privacy, we use a technique called encryption. The message must be encrypted before it is transmitted by the sender. We have two techniques to achieve message privacy.
- Privacy with symmetric key encryption
- Privacy with public key encryption.
In symmetric key encryption we have one key used for both encryption and decryption. Imagine a door that can lock and open with a single key, so both parties share one key. This is very common technique to achieve privacy.
Privacy can also be achieved using public key encryption. Here we have two keys: a private key and a public key. The receiver uses the private key, while the public key is given to public.
Let's say that Smith sends a message to Alek. Both are now sure that their conversation is not intercepted by a third person, as their message is encrypted. However, Alek is still doubtful. He needs to make sure that the message he got is actually from Bob (or else somebody has sent him a fake message.) Alek needs to authenticate the source of his message.
Alek also needs to make sure that the message he got remained unchanged during transmission (by either accidental or malicious means). Integrity plays a vital role in secure communications. For example, integrity is crucial when it comes to online banking systems. It would be a system failure if a request for transferring $1,000 changed to $100,000. We use a technique called digital signatures to provide message integrity.
Bob sends a message to Alek. Now Alek must be able to prove that the message he got is from Bob. This is called nonrepudiation. For another example, consider a customer requesting that her bank manager transfer money from one account to the other. The bank manager must be able to prove that the customer did request the transaction in the event she later tries to deny it.