Information Security: Principles and Practices Question: Find plaintext and the
ID: 3792508 • Letter: I
Question
Information Security: Principles and Practices
Question: Find plaintext and the key
A Vigenère cipher uses a sequence of "shift-by-n" simple substitutions,
where the shifts are indexed using a keyword, with "A" representing
a shift-by-0, "B" representing a shift-by-l, etc. For example, if the
keyword is "DOG," then the first letter is encrypted using a simple
substitution with a shift-by-3, the second letter is encrypted using a
shift-by-14, the third letter is encrypted using a shift-by-6, and the
pattern is repeated—the fourth letter is encrypted using a shift-by-3,
the fifth letter is encrypted using a shift-by-14, and so on. Cryptanalyze
the following ciphertext, i.e., determine the plaintext and the key. This
particular message was encrypted using a Vigenère cipher with a 3-letter
English keyword:
CTMYR DOIBS RESRR RIJYR EBYLD IYMLC CYQXS RRMLQ FSDXF
OWFKT CYJRR IQZSM X
Explanation / Answer
Step 1:
Align a given ciphertext to the 3-letters keyword as the keyword is a three letter English word (i.e., k = 3).
C_ _ Y_ _ O_ _ S_ _
Step 2:
Find frequencies of the letters that correspond to the first letter of the keyword .
Now
step 3:
Most frequent letters in the Set 1 is Y and R. Let’s start with Y.
You remember that most frequent letters of the English alphabet:
E, T, N, O, R, I, A, S
Assumption: ciphertext Y is corresponded to one of the plaintext letters E, T, N, O, R, I, A, S. Use Vigenere table to find a possible key letter for each pair:
Ciphertext letter
Possible plaintext letter
Corresponded key-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
Most frequent letter in the Set 2 is R:
Ciphertext letter
Possible plaintext letter
Corresponded key-word letter
Possible second letter of the keyword
R
E
N
R
T
Y
R
N
E
R
O
D
R
R
A
R
I
J
R
A
R
R
S
Z
Most frequent letter in the Set 3 is R
Ciphertext letter
Possible plaintext letter
Corresponded key-word letter
Possible third letter of the keyword
R
E
N
R
T
Y
R
N
E
R
O
D
R
R
A
R
I
J
R
A
R
R
S
Z
Step 4:
Create a table of the possible key-word letters by writing side by side the last columns of the tables in Step 3
Corresponded key-word letter
Possible first letter of the keyword
Corresponded key-word letter
Possible second letter of the keyword
Corresponded key-word letter
Possible third letter of the keyword
U
N
N
F
Y
Y
L
E
E
K
D
D
H
A
A
Q
J
J
Y
R
R
G
Z
Z
Step 5:
Create all possible legal three-letters English words by choosing first letter from the first column, second from second column and third from third column.
For each possible keyword you will try to decipher the ciphertext and see if the plaintext makes sense.
Possible keywords: FED, FEE, FEN, LEA, KEN, KEY, HER….
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.
Set 1: Frequency Set 2: Frequency Set 3: Frequency First letter in each set- Second leters Third Letters C 2 T 2 M 2 Y 3 R 4 D 1 O 2 I 4 B 1 S 2 Y 2 E 2 R 3 L 1 R 3 B 1 S 2 J 1 D 2 M 1 L 2 M 1 F 1 Y 1 X 2 X 1 C 2 Q 1 W 1 Q 2 K 1 J 1 S 1 Z 1 F 2