Abstract It has been mentioned by Shannon that a Vigenère autokey ciphertext can be transformed into a Vigenère ciphertext, provided the length of the priming key is known. We revisit this transformation and argue that combining it with the Friedman test, in fact, allows revealing of the length of the priming key. Thus, Shannon’s transformation together with the Friedman test allow transforming a Vigenère autokey ciphertext into a Vigenère ciphertext without any prior information. This allows an adversary to solve a Vigenère autokey ciphertext as if it was a Vigenère ciphertext. In addition, we show that the same method can be used to solve a modification of the Vigenère autokey cipher used in the Russian Civil War in 1917–1922.

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关于破解 Vigenère 自动密钥密码的说明

摘要 Shannon 已经提到，只要知道启动密钥的长度，就可以将 Vigenère 自动密钥密文转换为 Vigenère 密文。我们重新审视了这种转换，并认为将其与弗里德曼检验相结合，实际上可以揭示启动键的长度。因此，香农变换与弗里德曼检验一起允许在没有任何先验信息的情况下将 Vigenère 自动密钥密文转换为 Vigenère 密文。这允许攻击者像解析 Vigenère 密文一样解析 Vigenère 自动密钥密文。此外，我们展示了相同的方法可用于解决 1917-1922 年俄罗斯内战中使用的 Vigenère 自动密钥密码的修改。