When you want to communicate with a lot of people, you need to manage one key for each partner, the storage of these keys becomes a huge problem.
This can be solved by Public Key Crypto-System.

A Public Key Crypto-System is based on more advanced mathematics: each person uses a "key-pair", consisting of a public key and a private key public key is used for encryption (the lock in the figure below), and private key is used for decryption (the key in the figure below)

Some important points:

• Each public key has a corresponding private key.
• A public key is different from its corresponding private key.
• Each user keeps his/her own private key (In the figure below, there is only one private key kept by its owner).
• A user's public key is known to all others (In the figure below, there is a public person, Miss C.A., who keeps unlimited supplies of locks (i.e. public keys) for everybody.  Everyone can ask Miss C.A. for a lock of some person.)
• Most importantly, it is almost impossible to find the private key from the corresponding public key.

   Chinese Version