[Tutorial] Understand the encryption principle of public key cryptography

This is a good metaphor for an article in Chinese: https://www.thenewslens.com/article/31591
Let use my word to mark it down in English.


Imagine this is a box with lock for transfer information.When somebody use public key and turn it to the left can lock, use private key and turn it to the left can unlock.

Photo Credit: Panayotis Vryonis, Koppas, CC BY-SA 3.0
image of John and Sarah is edited from : https://www.youtube.com/watch?v=71mSYkuGZtk

Case

John want send a message "Hi Sarah!" to Sarah via internet.

Photo edit from : https://www.youtube.com/watch?v=71mSYkuGZtk

However , some authorized person may able to read what John sent, so that why we need cryptography.

Step 1

Sarah use large random number and key generation program such as PGP created 2 keys,  one is  public key and the other is private key:

also a box (although there is no box in Public key cryptography, imagine with a box can help understand the cryptography) with a lock shown in the first figure in this article (envelope in this figure) .

*Remark : John want send message but not Sarah ! but the keys and box are created by receiver.

Step2

Sarah send the box and a copy of public key to John.

Step3

John put the message into box from Sarah, and then use public key from Sarah turn the key to the left lock the box.

Step4

John send the box with message inside back to Sarah.

Step5

When Sarah received the box from John, she use the her private key to unlock this box.

In public key cryptography (asymmetrical cryptography) is any cryptographic system that uses pairs of keys: public keys and private keys. Where private key is own by message receiver for decryption, purpose (unlock box in this article). Public key is for encryption and can be own by many people.

Part of wikipedia definition:
public keys which may be disseminated widely, and private keys which are known only to the owner.

In this example, Sarah can made 100 copies of her public key and box to Tom, Timmy, Joe, Sam etc... for send encrypted message to her but only Sarah can use private key owned by her to decrypted and read the message . That's why wiki said public keys which may be disseminated widely, and private keys which are known only to the owner.

An overview what John and Sarah do.

Photo edit from : https://www.youtube.com/watch?v=71mSYkuGZtk

Reference

不用數學，一張圖了解公鑰加密法原理 - The News Lens 關鍵評論網
https://zh.wikipedia.org/wiki/%E5%85%AC%E5%BC%80%E5%AF%86%E9%92%A5%E5%8A%A0%E5%AF%86