Overview of military science, shooting, ballistics, internal ballistics, external ballistics, intermediate ballistics, terminal ballistics, missile ballistics, military geography, military topography, military engineering, military meteorology, military medicine, military operations research, war science, cryptography and cryptography of chemical warfare all come from the Greek word "krypto". Its use can be traced back to about four thousand years ago. In 2000, the Egyptians carved a monument on the tombstone. From then on, people used to write on paper to convey secret information. Cryptography played an important role in the Second World War. Many people believe that the Allies won the war entirely because the digital computer invented in World War II cracked the passwords of Germany and Japan. 1949, Shannon put forward the first paper to discuss the communication theory of cryptographic systems. Modern cryptography can be said to have originated in Sri Lanka. It was not until 1975 that Diffie and Hellman put forward the concept of public key cryptosystem, and the research direction of modern cryptography officially broke away from the pattern of secret key cryptosystem and flourished, which has been nearly 20 years. So far, there are two kinds of cryptographic systems. The first is symmetric key cryptosystem, and the second is asymmetric key cryptosystem. First of all, cryptography is studied by hippies on the World Wide Web, which involves key transmission. The sixties? 6? 8 ask the security guard to hand over the key; In the early 1970s, it was found that this method was not easy to distinguish, and it was time-consuming and laborious. From 65438 to 0965, the Department of Electrical Engineering of Stanford University in the United States-Merkel, Duffy and Herman-studied cryptography, but unfortunately they did not find it. In addition, Cerna, the secret organization of the British Communications Electronic Security Group (CESG), discovered the password reduction model, but it cannot be made public because it belongs to a secret organization. It was not until Levi's, a graduate student at MIT (1977), that Aderman discovered a formula similar to selman's. They set up RSA Security Company (RSA is the prefix of their name), which is now worth $2.5 billion, and has played a huge role in delivering credit cards. RSA has installed 500 million products. IE and the small lock under Netscape are RSA products. It was not the United States that first discovered the hanging needle in mathematics, but? Was the first to be made public. Mathematical padlocks are easy to lock and difficult to recover, so they are widely used, that is, information coding is confidential. General example of mathematical padlock: one-way type of mathematical padlock: n = pxq