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)/(( index)+(length*(index ))));
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 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
))));
keyword=

6. 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 K K1 K2 K3 K1 K2 K3 K1 K2 K 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 K K1 K2 K3 K1 K2 K3 K1 K2 K K1 K2 K3 K1 K2 K K1 K2 K3
D X F O W F K T C Y J R R I Q Z S M X
K1 K2 K K1 K2 K K1 K2 K3 K1 K2 K K1 K2 K3 K1 K2 K K1

7. 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

8. 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

9. 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

10. 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….

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. 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

13. Kasiski Attack Example
Ciphertext:
IVE
VYGARMLMY
IVE
KFD
IVE
FRL

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