Example:
Key: deceptivedeceptivedeceptive
Plaintext: wearediscoveredsaveyourself
Ciphertext: ZICVTWQNGRZGVTWAVZHCQYGLMGJ
KEY 3 4 2 4
15 19 8 21 4 3
PT
22 4 0 17 4
CT
25 8 2 21 19 22 16 13 6 17 25 6 21
15
8
19
4 3
21
0
18
0
19 22
4
3
4 2 4
4
3
8 18 2 14 21 4 17
2
4
8
15 19
21 4
4
21 4 24 14 20 17 18
11 5
2
6
6
21 25 7
16 24
11 12
9
Another Example:
Key: LEMONLEMONLE
Plaintext: ATTACKATDAWN
Ciphertext: LXFOPVEFRNHR
Key 11
4
12
14
13
11
4
12
14
13
11
4
PT
0
19
19
0
2
10
0
19
3
0
22
13
CT
11
23
5
14
15
21
4
5
17
13
7
17
Narrative Report: The Polyalphabetic Cipher
Introduction:
In the realm of cryptography, the Polyalphabetic Cipher holds a significant place. Its development and
usage have intrigued cryptographers and codebreakers alike, challenging them with its complexity and
resilience. The Vigenère cipher is probably the best-known example of a polyalphabetic cipher. Vigenère
Cipher, a cryptographic technique dating back to the 16th century, is a fascinating encryption method that
has played a significant role in the history of cryptography. This narrative report aims to delve into the
concept, historical background, encryption process, decryption process, and significance of the Vigenère
Cipher.
Historical Background:
The Vigenère Cipher was first introduced by Giovan Battista Bellaso in 1553 in his book “La cifra del Sig.
Giovan Battista Bellaso.” However, it was later refined and popularized by the French cryptographer
Blaise de Vigenère, who added several important elements to the cipher. Due to his contributions, the
cipher became known as the Vigenère Cipher.
Encryption Process:
The Vigenère Cipher is a polyalphabetic substitution cipher, meaning that it utilizes multiple alphabets
for encryption. Unlike simpler ciphers like the Caesar Cipher, the Vigenère Cipher employs a keyword or
passphrase to determine the shift value for each letter of the plaintext.
To encrypt a message using the Vigenère Cipher, the sender selects a keyword or passphrase. The length
of the keyword is crucial, as it determines the number of alphabets to be used for encryption. Each letter
of the keyword corresponds to a shift value, typically determined by its position in the alphabet. The
message is then divided into blocks, and each letter is shifted according to the corresponding value in the
keyword. If the message is longer than the keyword, the keyword is repeated until it matches the length
of the message. The shifted letters are combined to form the ciphertext.
Decryption Process:
Decrypting a message encrypted with the Vigenère Cipher requires knowledge of the correct keyword or
passphrase. Using the same keyword and applying the reverse shift operation, the original plaintext can
be retrieved from the ciphertext. Each letter of the ciphertext is shifted back by the corresponding value
in the keyword, and the decrypted letters are combined to reveal the original message.
Cryptanalysis and Breaking the Cipher:
Initially, the Vigenère Cipher was considered unbreakable due to its polyalphabetic nature, which makes
simple frequency analysis ineffective. However, as cryptanalysis techniques advanced, methods were
developed to break the cipher. One notable breakthrough was achieved by Charles Babbage and
Friedrich Kasiski in the 19th century. They discovered that by analyzing repetitions in the ciphertext, the
length of the keyword could be determined. Once the keyword length was known, further analysis and
frequency distribution of the text allowed for the decryption of the message.
Significance and Legacy:
The Vigenère Cipher holds historical significance as one of the earliest and most famous polyalphabetic
substitution ciphers. It demonstrated the importance of utilizing multiple alphabets for encryption,
adding a layer of complexity and security to the cipher. The Vigenère Cipher’s popularity and historical
impact can be attributed to its resistance against simple frequency analysis methods, which were
common in that era.
While the Vigenère Cipher has been surpassed in terms of security by modern encryption algorithms, its
legacy remains significant. The cipher’s existence and exploration paved the way for the development of
more advanced cryptographic techniques. It inspired further research and improvements in encryption
algorithms, leading to the creation of more secure methods of communication and data protection.
Conclusion:
The Vigenère Cipher, introduced by Blaise de Vigenère and refined from Bellaso’s work, is a
polyalphabetic substitution cipher that employs a keyword or passphrase for encryption and decryption.
Despite being susceptible to cryptanalysis techniques developed in later years, the Vigenère Cipher’s
historical significance and impact on the evolution of cryptography cannot be understated. Its
exploration provides valuable insights into the development of encryption methods and serves as a
reminder of the ever-evolving nature of information security.
Narrative example:
Certainly! Here’s an example of the Vigenère cipher in action:
Suppose we have the plaintext message: “HELLO” and the key: “KEY”. We will encrypt the plaintext using
the Vigenère cipher.
Step 1: Converting the plaintext and key to numeric values.
Plaintext: H -> 7, E -> 4, L -> 11, L -> 11, O -> 14
Key: K -> 10, E -> 4, Y -> 24
Step 2: Repetition of the key to match the length of the plaintext.
Plaintext: 7, 4, 11, 11, 14
Key: 10, 4, 24, 10, 4
Step 3: Adding the plaintext and key values modulo 26.
Ciphertext: (7 + 10) % 26 = 17 ®, (4 + 4) % 26 = 8 (I), (11 + 24) % 26 = 9 (J), (11 + 10) % 26 = 21 (V), (14 + 4)
% 26 = 18 (S)
The resulting ciphertext is “RIJVS.”
To decrypt the ciphertext, you would follow the same steps in reverse.
Mathematic formula:
Encryption process:
Ci = (Pi + Ki mod m) mod 26
Deception process:
Pi = (Ci – Ki mod m) mod 26