Cryptography is the technical science that studies the preparation and deciphering of codes. The study of the objective laws of password changes, which is applied to compiling codes to keep communication secrets, is called cryptography; the application to deciphering codes to obtain communication intelligence is called deciphering, and is generally called cryptography.

Password is an important means of confidentiality for both communicating parties to perform special transformation of information according to agreed rules. According to these rules, changing plaintext into ciphertext is called encryption transformation; changing ciphertext into plaintext is called decryption transformation. In the early days, cryptography only performed encryption and decryption transformations on text or numbers. With the development of communication technology, encryption and decryption transformations can be implemented on voices, images, data, etc.

Cryptography was gradually developed in the practice of struggle between encoding and deciphering, and with the application of advanced science and technology, it has become a comprehensive and cutting-edge technical science. It has extensive and close connections with linguistics, mathematics, electronics, acoustics, information theory, computer science, etc. Its actual research results, especially the cryptography and deciphering methods currently used by governments around the world, are highly confidential.

The rules for transforming secrets into secrets are called cryptographic systems. The parameter indicating this transformation is called the key. They are an important part of cryptography. The basic types of cryptography systems can be divided into four types: confusion - changing the positions of plaintext letters or numbers to become ciphertext according to prescribed graphics and lines; substitution - replacing plaintext letters or numbers with one or more substitution tables. cipher text; cipher text - using a pre-programmed letter or number cipher group to replace certain phrases, words, etc. to turn plain text into cipher text; scrambling - using a sequence of finite elements as random numbers, according to a prescribed algorithm , combined with the plaintext sequence to become ciphertext. The above four cryptographic systems can be used alone or mixed to compile various practical passwords with high complexity.

Since the 1970s, some scholars have proposed a public key system, which uses the mathematical principle of one-way functions to achieve the separation of encryption and decryption keys. The encryption key is public and the decryption key is secret. This new cryptographic system has attracted widespread attention and discussion in the cryptography community.

Using the laws of text and passwords, and under certain conditions, adopting various technical means to analyze the intercepted ciphertext to obtain the plaintext and restore the password preparation, that is, to decipher the password. The requirements for deciphering passwords of different strengths are also different, or even very different.

The means of secret communication in ancient China already had some prototypes that were close to cryptography. According to the "Character Examination" of "Wu Jing Zong Yao" compiled by Zeng Gongliang, Ding Du and others of the Song Dynasty, in the early Northern Song Dynasty, 40 Chinese characters in a five-character rhyme poem were used to represent 40 situations or requirements respectively. This method has already had its own characteristics. Understand the characteristics of the secret book system.

In 1871, the Shanghai Dabei Waterline Telegraph Company selected 6899 Chinese characters and replaced them with four code numbers, which became China's first commercial clear code book. At the same time, it also designed the adaptation of the clear code book into a code book and carried out Way to add chaos. On this basis, it gradually developed into various more complex passwords.

In Europe, in 405 BC, the Spartan general Lysander used the original chaotic cipher; in the first century BC, the ancient Roman Emperor Caesar used an ordered single table instead of the cipher; later Gradually developed into various cryptographic systems such as secret edition, multi-table substitution and scrambling.

At the beginning of the 20th century, the first practical mechanical and electric cipher machines were produced, and commercial cipher machine companies and markets emerged at the same time. After the 1960s, electronic cryptography machines developed rapidly and were widely used, bringing the development of cryptography to a new stage.

Password deciphering is gradually produced and developed with the use of passwords. In 1412, the encyclopedia compiled by the Persian Kalakachandi contained methods for deciphering simple substitution ciphers. By the end of the 16th century, some European countries had full-time codebreakers to decipher intercepted secret messages. Code-breaking technology has developed considerably. "Cryptozoology and Decryption Techniques" written by the Prussian Kassky in 1863, and "Military Cryptozoology" written by the Frenchman Kerkhov in 1883 have made some discussions and discussions on the theory and methods of cryptography. Discuss. In 1949, American Shannon published the article "Communication Theory of Secret Systems", which applied the principles of information theory to analyze some basic issues in cryptography.

Since the 19th century, the widespread use of telegraphs, especially wireless telegraphs, has provided extremely favorable conditions for cryptographic communications and interception by third parties. Communication confidentiality and detection and deciphering have formed a hidden front with fierce fighting.

In 1917, Britain deciphered the telegram sent by German Foreign Minister Zimmermann, which prompted the United States to declare war on Germany. In 1942, the United States learned about the Japanese military's combat intentions and troop deployment in the Midway Islands area from deciphering Japanese naval secret reports, so that it could defeat the main force of the Japanese navy with inferior forces and reverse the war situation in the Pacific region. The success of code deciphering played an extremely important role in the defense of the British Isles and many other famous historical events. These examples also illustrate the important status and significance of code confidentiality.

The governments of major countries in the world today attach great importance to cryptography work. Some have established huge institutions, allocated huge sums of funds, concentrated tens of thousands of experts and scientific and technological personnel, and invested in a large number of high-speed computers and other Work with advanced equipment.

At the same time, various private enterprises and academic circles are paying increasing attention to cryptography. Many mathematicians, computer scientists and experts in other related disciplines have also devoted themselves to the research of cryptography, which has accelerated the development of cryptography.

Nowadays cryptography has become a separate discipline. Traditionally, cryptography is the study of how to convert information into a concealed way and prevent others from obtaining it.

Cryptozoology is an interdisciplinary subject derived from many fields: it can be seen as information theory, but uses a large number of tools from the field of mathematics, well-known ones such as number theory and finite mathematics.

The original information, that is, the information that needs to be password protected, is called plaintext. Encryption is the process of converting original information into an unreadable form, known as a password. Decryption is the reverse process of encryption, obtaining the original information from the encrypted information. cipher is the algorithm used for encryption and decryption.

The earliest steganography only required paper and pen, and is now called classical cryptography. The two main categories are substitution ciphering, which rearranges the order of letters; and substitution ciphering, which replaces a set of letters with other letters or symbols. Information from classical encryption methods is susceptible to statistical attacks. The more data there are, the easier it is to crack. Using frequency analysis is a good way to do this. Classical cryptography is still alive and often appears in mind games. In the early 20th century, a number of mechanical devices, including rotary machines, were invented for encryption, the most famous of which was the Enigma, a cipher machine used in World War II. The ciphers produced by these machines considerably increased the difficulty of cryptanalysis. For example, various attacks against Enigma were successful only after considerable effort.