The main idea of cryptography is that a group of people can use private knowledge to keep written messages secret from everyone else.

The original message sent is called PLAINTEXT. The message encrypted is called the CIPHERTEXT . In both encryption and decryption process, we need a KEY to be incorporated into the process.
What is KEY?

Your own door lock is mass-produced by a company. The point is that you own a key which is different from the others who also own a door lock from the same company (the same model). Therefore, even though the mechanism to build the door lock and the internal structure of the door lock is well known to the others, your own house is secured for you own the door lock with the specific key combination design in it that only you in the World who owns the particular key can open it (theoretically !).

This is the security concept that Leon Battista Alberti, the famous Italian Renaissance architect, brought to cryptography in 1466 where he invented the cryptographic key. Everyone can have the same brand lock but with different key.

KEY solves the problem of moving in and out of a private group. If Tom and May share a key, they want to let Philip to join their conversation, they can simply pass the key to him. If later on, they find Philip is un-trustworthy, they can simply change the key without telling Philip and Philip would be immediately out of the subsequent conversation.

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