In the quest for a more secure method of protecting information, the introduction of a key adds another level of security. A key is a piece of information that allows only those that hold it to encode and decode a message.
Keys come in many different forms such as passwords, numbers generated by an algorithm, digital fingerprints and even electronic devices that work like door keys. It is a series of numbers or symbols that are used to encode a message so that it can only be read by someone in possession of that key or a related key. A key allows both the sender and the recipient of the message to understand how the message has been encrypted and assures them that nobody else knows how it has been encrypted. It is the key that enables the recipient to properly decode the message.
Using the previous example of a substitution cipher, anyone who knows the Caesar Cipher can decrypt all messages encrypted with it, regardless of who actually encrypted the message. One could strengthen the substitution cipher with a key, by choosing an arbitrary number and using that as the number of letters by which to shift when creating their alternate alphabet. That number therefore becomes the key by which the message is unlocked.
The individual who is sending the message communicates the key to the recipient of the message, allowing them to unlock it. One disadvantage of this system is that an attacker can decrypt the message if the key is intercepted.
To protect the key, encryption can be used during communication or the key can be sent in a separate communication.
