In the security analysis of ciphers, it is generally assumed that the cryptanalysis knows the cryptosystem, which is kerckhoffs hypothesis. Once the password system is established, it will be used for a long time, so it is very difficult to maintain its confidentiality. The key can be changed frequently, and the secret can be embedded in the key, which makes the security of the password easier to realize. Therefore, the key of cryptanalysis is to obtain the key.
Under certain conditions, any of the classical cryptosystems can be deciphered. Mobile cipher, affine cipher, Virginia cipher and permutation cipher are very vulnerable to known plaintext attacks. That is to say, most classical cryptosystems are easy to be broken by using ciphertext only attack. Since classical ciphers are mostly used to protect information expressed in English, the statistical characteristics of English language are powerful tools to attack ancient ciphers. This is because most classical cryptosystems can not hide the statistical characteristics of plaintext messages well.
This paper introduces the basic methods of deciphering classical ciphers by means of ciphertext attack or known plaintext attack based on the statistical and cryptographic characteristics of English language.
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