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Vigenére Cipher

Vigenére Cipher. Kimberly Chiffens & Maria Jannelli. Progress Report. Implementation of Vigenere encryption and decryption Applet design Implementation of Friedman attack to find key length Apply cryptanalysis algorithms. Friedman Attack Key Length.

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Vigenére Cipher

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  1. Vigenére Cipher Kimberly Chiffens & Maria Jannelli

  2. Progress Report • Implementation of Vigenere encryption and decryption • Applet design • Implementation of Friedman attack to find key length • Apply cryptanalysis algorithms

  3. Friedman Attack Key Length • Uses frequencies to count the amount of each letter in the ciphertext • We multiply the count of each letter by count minus 1 and then add up the sum. • It computes the sum of the frequencies as follows: • for(int k = 0; k < 26; k++) • sum = sum + Fcount[k]*(Fcount[k]-1); • }

  4. Friedman Attack Index of Coincidence • To find this we divide the entire sum of the frequencies with the length of the cipher times the length minus 1. • index= sum/(length*(length-1)); • Then we must calculate the key length. To do so we use this equation: • keyword= ((0.0265*length)/((0.065-index)+(length*(index-0.0385)))); The probability of choosing an identical pair of letters from a pool in which there are equal numbers of the respective letters The probability that a letter selected at random is an “A” The probability that two randomly selected letter in the English alphabet letters are identical

  5. Friedman Example Ciphertext: KSMEHZBBL KSMEMPOGA JXSEJCSFL ZSY Frequencies: Length = 30 letters Key: RELATIONS Plaintext: TOBEORNOT TOBETHATIST HEQUEST ION ((0.0265*length)/((0.065- index)+(length*(index -0.0385)))); keyword=

  6. Cryptanalysis Step One: Find the key length Step Two: Generate possible keys CTMYRDOIBSRESRRRIJYRE K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 BYLDIYMLCCYQX SRRMLQFS K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 DXFOWFKTCYJRRIQZSMX K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1

  7. Cryptanalysis

  8. Cryptanalysis Most frequent letters of the English alphabet: E, T, N, O, R, I, A, S

  9. Cryptanalysis

  10. Cryptanalysis Create all possible three-letter words by choosing first letter from the first column, second from second column and third from third column. Possible keywords: FED, FEE, FEN, LEA, KEN, KEY, HER….

  11. Cryptanalysis The answer: Deciphering the ciphertext with keyword KEY will give a plaintext: SPOON FEEDING IN THE LONG RUN TEACHES US NOTHING BUT THE SHAPE OF SPOON.

  12. References • Principles of Operating Systems: Design and Application, Brian L. Stuart. Course Technology. 2009. • Invitation to Cryptology, Thomas H. Barr. Prentice Hall. 2002.

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