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Hub AI
Kasiski examination AI simulator
(@Kasiski examination_simulator)
Hub AI
Kasiski examination AI simulator
(@Kasiski examination_simulator)
Kasiski examination
In cryptanalysis, Kasiski examination (also known as Kasiski's test or Kasiski's method) is a method of attacking polyalphabetic substitution ciphers, such as the Vigenère cipher. It was first published by Friedrich Kasiski in 1863, but seems to have been independently discovered by Charles Babbage as early as 1846.
In polyalphabetic substitution ciphers where the substitution alphabets are chosen by the use of a keyword, the Kasiski examination allows a cryptanalyst to deduce the length of the keyword. Once the length of the keyword is discovered, the cryptanalyst lines up the ciphertext in n columns, where n is the length of the keyword. Then each column can be treated as the ciphertext of a monoalphabetic substitution cipher. As such, each column can be attacked with frequency analysis. Similarly, where a rotor stream cipher machine has been used, this method may allow the deduction of the length of individual rotors.
The Kasiski examination involves looking for strings of characters that are repeated in the ciphertext. The strings should be three characters long or more for the examination to be successful. Then, the distances between consecutive occurrences of the strings are likely to be multiples of the length of the keyword. Thus finding more repeated strings narrows down the possible lengths of the keyword, since we can take the greatest common divisor of all the distances.
The reason this test works is that if a repeated string occurs in the plaintext, and the distance between corresponding characters is a multiple of the keyword length, the keyword letters will line up in the same way with both occurrences of the string. For example, consider the plaintext:
The word "the" is a repeated string, appearing multiple times. If we line up the plaintext with a 5-character keyword "beads" :
The word "the" is sometimes mapped to "bea", sometimes to "sbe" and other times to "ead". However, it is mapped to "sbe" twice, and in a long enough text, it would likely be mapped multiple times to each of these possibilities. Kasiski observed that the distance between such repeated appearances must be a multiple of the encryption period.
In this example, the period is 5, and the distance between the two occurrences of "sbe" is 30, which is 6 times the period. Therefore, the greatest common divisor of the distances between repeated sequences will reveal the key length or a multiple of it.
The difficulty of using the Kasiski examination lies in finding repeated strings. This is a very hard task to perform manually, but computers can make it much easier. However, care is still required, since some repeated strings may just be coincidence, so that some of the repeat distances are misleading. The cryptanalyst has to rule out the coincidences to find the correct length. Then, of course, the monoalphabetic ciphertexts that result must be cryptanalyzed.
Kasiski examination
In cryptanalysis, Kasiski examination (also known as Kasiski's test or Kasiski's method) is a method of attacking polyalphabetic substitution ciphers, such as the Vigenère cipher. It was first published by Friedrich Kasiski in 1863, but seems to have been independently discovered by Charles Babbage as early as 1846.
In polyalphabetic substitution ciphers where the substitution alphabets are chosen by the use of a keyword, the Kasiski examination allows a cryptanalyst to deduce the length of the keyword. Once the length of the keyword is discovered, the cryptanalyst lines up the ciphertext in n columns, where n is the length of the keyword. Then each column can be treated as the ciphertext of a monoalphabetic substitution cipher. As such, each column can be attacked with frequency analysis. Similarly, where a rotor stream cipher machine has been used, this method may allow the deduction of the length of individual rotors.
The Kasiski examination involves looking for strings of characters that are repeated in the ciphertext. The strings should be three characters long or more for the examination to be successful. Then, the distances between consecutive occurrences of the strings are likely to be multiples of the length of the keyword. Thus finding more repeated strings narrows down the possible lengths of the keyword, since we can take the greatest common divisor of all the distances.
The reason this test works is that if a repeated string occurs in the plaintext, and the distance between corresponding characters is a multiple of the keyword length, the keyword letters will line up in the same way with both occurrences of the string. For example, consider the plaintext:
The word "the" is a repeated string, appearing multiple times. If we line up the plaintext with a 5-character keyword "beads" :
The word "the" is sometimes mapped to "bea", sometimes to "sbe" and other times to "ead". However, it is mapped to "sbe" twice, and in a long enough text, it would likely be mapped multiple times to each of these possibilities. Kasiski observed that the distance between such repeated appearances must be a multiple of the encryption period.
In this example, the period is 5, and the distance between the two occurrences of "sbe" is 30, which is 6 times the period. Therefore, the greatest common divisor of the distances between repeated sequences will reveal the key length or a multiple of it.
The difficulty of using the Kasiski examination lies in finding repeated strings. This is a very hard task to perform manually, but computers can make it much easier. However, care is still required, since some repeated strings may just be coincidence, so that some of the repeat distances are misleading. The cryptanalyst has to rule out the coincidences to find the correct length. Then, of course, the monoalphabetic ciphertexts that result must be cryptanalyzed.