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 withThe
the
length of the cipher
probability of
choosing
an 1.
identical
The
probability
thatlength
a
times
the
minus
The probability that
pair of letters from a
letter selected at
two randomly selected
pool
in
which
there
random
is an “A” sum/(length*(length-1));
• index=
letter in the English
are equal numbers of
alphabet letters are
the respective letters
• Then we must calculate the key length.
identical To do
so we use this equation:
• keyword= ((0.0265*length)/((0.065index)+(length*(index-0.0385))));
Friedman Example
Ciphertext:
KSMEHZBBL KSMEMPOGA JXSEJCSFL ZSY
Frequencies:
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 1 0 3 1 1 1 0 2 2 2 3 0 1 1 0 0 5 0 0 0 0 1 1 2
Length = 30 letters
((0.0265*length)/((0.065keyword=
index)+(length*(index
Key: RELATIONS
-0.0385))));
Plaintext:
TOBEORNOT TOBETHATIST HEQUEST ION
Cryptanalysis
Step One: Find the key length
Step Two: Generate possible keys
C T M
K1 K2 K3
Y R D O I B
K1 K2 K3 K1 K2 K3
S R E
K1 K2 K3
S R R R I J Y R E
K1 K2 K3 K1 K2 K3 K1 K2 K3
B Y L
K1 K2 K3
D I Y M L C
K1 K2 K3 K1 K2 K3
C Y Q
K1 K2 K3
X S R R M L Q F S
K1 K2 K3 K1 K2 K3 K1 K2 K3
D X F
K1 K2 K3
O W F K T C
K1 K2 K3 K1 K2 K3
Y J R
K1 K2 K3
R I Q Z S M X
K1 K2 K3 K1 K2 K3 K1
Cryptanalysis
Set 1:
Frequency Set 2:
Red Letters
Green
Letters
C
2
T
Y
3
R
O
2
I
S
2
Y
R
3
L
B
1
S
D
2
M
M
1
F
X
2
X
Q
1
W
K
1
J
Z
1
Frequency Set 3:
Blue
Letters
2
M
4
D
4
B
2
E
1
R
2
J
1
L
1
Y
1
C
1
Q
1
S
F
Frequency
2
1
1
2
3
1
2
1
2
2
1
2
Cryptanalysis
Most frequent letters of the English alphabet:
E, T, N, O, R, I, A, S
Ciphertext letter Possible plaintext Corresponded keyletter
word letter
Possible first letter of
the keyword
Y
E
U
Y
T
F
Y
N
L
Y
O
K
Y
R
H
Y
I
Q
Y
A
Y
Y
S
G
Cryptanalysis
Ciphertext letter
Possible plaintext
letter
Corresponded key-word
letter
Possible second letter of
the keyword
R
R
R
R
R
R
R
R
E
T
N
O
R
I
A
S
N
Y
E
D
A
J
R
Z
Cryptanalysis
Corresponded keyword letter
Possible first letter of
the keyword
U
F
L
K
H
Q
Y
G
Corresponded key-word
letter
Possible second letter of
the keyword
N
Y
E
D
A
J
R
Z
Corresponded keyword letter
Possible third letter of
the keyword
N
Y
E
D
A
J
R
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.
References
•Principles of Operating Systems: Design and
Application, Brian L. Stuart. Course Technology.
2009.
•Invitation to Cryptology, Thomas H. Barr.
Prentice Hall. 2002.