The cryptographic purpose of quantum cryptography

The purpose of cryptography is to change the original form of information and make it difficult for outsiders to read. The information code in cryptography is called cipher, the text information that has not been converted into cipher is called plaintext, the information represented by cipher is called ciphertext, the conversion process from plaintext to ciphertext is called encryption, and the opposite process is called decryption, which is carried out through the so-called key. Therefore, the security of a cryptographic system depends only on the confidentiality of its key. When designing and establishing a cryptosystem, it must be assumed that the decryption opponent can know all the information about the cryptosystem, and the only thing he doesn't know is which encryption map is used to encrypt the specific ciphertext. In the traditional cryptosystem, as long as the encryption map is known, the decryption map is also known. Therefore, the traditional cryptographic system requires both parties to agree and transmit the key through a "secure channel" in order to conduct secure communication. In addition, under the traditional cryptosystem, each pair of users needs a key. In this way, in a communication network with n users, to ensure that any two users can communicate safely, many "secure channels" are needed to transmit n(n- 1)/2 keys. If n is large, it is difficult to ensure safety.