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CAP Cryptographic Analysis Program

CAP Cryptographic Analysis Program. Breaking the Vigenere Cipher Help Presentation. Press Enter or click on your mouse button to continue. Breaking Vigenere. For more than 300 years cryptanalysts worked on the problem of breaking a polyalphabetic cipher (like the V-cipher)

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CAP Cryptographic Analysis Program

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  1. CAPCryptographic Analysis Program Breaking the Vigenere Cipher Help Presentation Press Enter or click on your mouse button to continue

  2. Breaking Vigenere • For more than 300 years cryptanalysts worked on the problem of breaking a polyalphabetic cipher (like the V-cipher) • In 1863, a Polish Infantry officer, Friedrich W. Kasiski, published a short book (95 pages) which changed the nature of cryptography • He had found a simple solution to the polyalphabetic ciphers • He died in 1881 without realizing that he had started a revolution in cryptography

  3. The conjuction of a repeated portion of the key with a repetition in the plaintext produces a repetition in the ciphertext. Repetitions • Kasiski’s breakthrough was the observation that: So, the size of the keyword can be determined by the nature of repeated ciphertext character strings.

  4. 6 characters 9 characters Example • Given the keyword RUN and some plaintext: R U N R U N R U N R U N R U N R U N R U N R U N R U N t o b e o r n o t t o b e t h a t i s t h e q u e s t K I O V I E E I G K I O V N U R N V J N U V K H V M G Find repeated ciphertext Count the number of characters between the repetitions NOTE: each time the key RUNR engages the repeated plaintext to be, the repeated ciphertext KIOV is produced

  5. Using CAP • CAP will perform this test automatically – select Kasiski Method under the polyalphabetic menu

  6. Next Step • once the keyword size is known it is possible to discover the keyword • create copies of the target ciphertext in columns where each column is shifted by one position in the alphabet. the low frequency letters (j,k,q,x,z) should have a combined frequency of about 2% any column in which these letters appear more frequently is rejected A column with the correct frequency is a candidate for the plaintext

  7. Using CAP • CAP provides a low frequency analysis tool:

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