Now cryptography has become an independent discipline. Cryptography in the traditional sense is to study how to transform information into a hidden way to prevent others from obtaining it.

Cryptography is an interdisciplinary subject, which comes from many fields: it can be regarded as information theory, but it uses tools in many mathematical fields, such as number theory and finite mathematics.

Original information, that is, information that needs password protection, is called plaintext. Encryption is the process of transforming the original information into an unreadable form, which is a password. Decryption is the reverse process of encryption, and the original information is obtained from the encrypted information. Password is an algorithm used for encryption and decryption.

The earliest steganography only needed pen and paper, and now it is called classical cryptography. Its two categories are permutation encryption and rearranging the order of letters; Replace encryption and replace a group of letters with other letters or symbols. The information of classical encryption methods is easily cracked by statistics. The more information, the easier it is to crack. It is a good method to use analysis frequency. Classical cryptography has not disappeared, and it often appears in intelligence games. At the beginning of the 20th century, some mechanical devices were invented for encryption, including the rotary wheel machine, the most famous of which was Enigma, a cipher machine used in World War II. The passwords generated by these machines greatly increase the difficulty of password analysis. For example, all kinds of attacks on Ingmar were successful only after considerable efforts.