Vigenére Cipher Kimberly Chiffens & Maria Jannelli.

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Presentation transcript:

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 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); }

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

Friedman Example Ciphertext: KSMEHZBBL KSMEMPOGA JXSEJCSFL ZSY Frequencies: Length = 30 letters Key: RELATIONS Plaintext: TOBEORNOT TOBETHATIST HEQUEST ION A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 2 3 5 ((0.0265*length)/((0.065- index)+(length*(index -0.0385)))); keyword=

Cryptanalysis Step One: Find the key length Step Two: Generate possible keys C T M Y R D O I B S R E S R R R I J Y R E K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 B Y L D I Y M L C C Y Q X S R R M L Q F S K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 D X F O W F K T C Y J R R I Q Z S M X K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1 K2 K3 K1

Cryptanalysis Set 1: Red Letters Frequency Set 2: Green Letters Set 3: Blue Letters C 2 T M Y 3 R 4 D 1 O I B S E L J F X Q W K Z

Cryptanalysis Most frequent letters of the English alphabet: E, T, N, O, R, I, A, S Ciphertext letter Possible plaintext letter Corresponded key-word letter Possible first letter of the keyword Y E U T F N L O K R H I Q A S G

Cryptanalysis Ciphertext letter Possible plaintext letter Corresponded key-word letter Possible second letter of the keyword R E N T Y O D A I J S Z

Cryptanalysis Corresponded key-word letter Possible first letter of the keyword Possible second letter of the keyword Possible third letter of the keyword U N F Y L E K D H A Q J R G Z 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….

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.

Kasiski Attack The goal is to find the key length Count the gaps between repeated trigrams in the ciphertext The key is a divisor of the GCDs of the gaps

Kasiski Attack Example Ciphertext: IVE VYGARMLMY IVE KFD IVE FRL

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