Vigenère Cipher

Let us take a look at what is going to be discussed in this blog:

• What is Vigenere Cipher?
• Vigenere Cipher Table
• Vigenere Cipher Methods
  • Autokey Method
  • Keyword Method
  • Vigenere Cipher Calculator
  • Vigenere Cipher Code in Python
• Conclusion

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What is Vigenere Cipher?

The Vigenère Cipher, invented by the French cryptographer Blaise de Vigenère in the 16th century, is a method of encrypting and decrypting messages. It is a polyalphabetic substitution cipher, which means that it uses multiple cipher alphabets to encrypt the plaintext.

The Vigenère Cipher operates by using a keyword, typically a word or phrase, as the basis for encryption. The keyword is repeated to match the length of the plaintext message. Each letter in the keyword is then used to determine the shift value for the corresponding letter in the plaintext.

To encrypt a message, each letter of the plaintext is shifted by the corresponding letter in the keyword. This is done using a table called the Vigenère square or Vigenère tableau. The Vigenère square is a grid of alphabets, where each row represents a shift of the previous row by one position. The letter at the intersection of the keyword letter and plaintext letter in the Vigenère square gives the encrypted letter.

For example, let’s encrypt the word “HELLO” using the keyword “KEY”. First, repeat the keyword to match the length of the plaintext: “KEYKE”. Then, find the corresponding letters in the Vigenère square. The letter ‘H’ is shifted by ‘K’, resulting in ‘S’. ‘E’ is shifted by ‘E’, resulting in ‘X’. ‘L’ is shifted by ‘Y’, resulting in ‘M’. And so on. The encrypted message would be “SXMMR”.

Decryption works in a similar way. Each letter of the ciphertext is shifted back by the corresponding letter in the keyword, using the Vigenère square. By reversing the process, the original plaintext message can be obtained.

The Vigenère Cipher offers more security compared to simple substitution ciphers because it introduces variability and makes it harder to identify patterns in the encrypted message. However, it is not as strong as modern encryption methods and can be vulnerable to cryptanalysis techniques such as frequency analysis and Kasiski examination.

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Vigenere Cipher Table

It table consists of all the alphabets written 26 times in different rows. Each alphabet in every subsequent row and column is shifted cyclically to the left. This generates 26 Caesar Ciphers.

Different alphabets from different rows are used at different points in the encryption process. The alphabet chosen depends on the method used for encryption.

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Vigenere Cipher Methods

There are three main Vigenere Cipher Methods:

• Autokey Method
• Keyword Method
• Vigenere Cipher Calculator

Let’s talk about each of these in detail.

Autokey Method

The Autokey Method makes use of the Vigenere Cipher table to encrypt and decrypt the plaintext. In this method, the priming key is just one letter.

Vigenere Cipher Encoder in AutoKey Method

The first step in the autokey method is to decide on a priming key. Both sender and receiver have to agree on this key. The priming key is the single alphabet that is added to the beginning of messages to help make the key. The sender encrypts the message starting by writing the first letter of the plaintext on one line and the priming key under it. The rest of the plaintext is written as is, shifted one place to the right.

Plaintext	I	N	T	E	L	L	I	P	A	A	T
Key	R	I	N	T	E	L	L	I	P	A	A

The steps involved are as follows:

• Write the plaintext.
• Use the plaintext and the key letter to select a row and a column in the Vigenere table.
• The first letter of the plaintext is the first row and the key is the first column. For example, if the plaintext is INTELLIPAAT and the key is R, then the first row will be the one that starts with I, and the column will be the one that starts with R.
• The first letter of the ciphertext will be the letter where the first row and column intersect. In the case of our example that will be the letter Z.
• Now, this process is continued till the entire plaintext is turned into a ciphertext. For INTELLIPAAT, that will be ZVGXPWTXPAT.

Plaintext	I	N	T	E	L	L	I	P	A	A	T
Key	R	I	N	T	E	L	L	I	P	A	A
Ciphertext	Z	V	G	X	P	W	T	X	P	A	T

Vigenere Cipher Decoder in AutoKey Method

The below-mentioned steps are followed to decipher the ciphertext:

• The first letter is selected using the priming key.
• The first letter of the ciphertext is located in this row.
• The first letter of the plaintext will be the letter where the first row and column intersect.
• This process is followed until the entire ciphertext is deciphered.

Key	R	I	N	T	E	L	L	I	P	A	A
Ciphertext	Z	V	G	X	P	W	T	X	P	A	T
Plaintext	I	N	T	E	L	L	I	P	A	A	T

Security

The autokey method for Vigenere Cipher is not very secure. We can only use 26 keys, which makes breaking the code fairly easy. Anyone who wants to break the code needs to only try each alphabet as the priming key before they make a sensible message out of it. This can be done manually in a short time.

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Keyword Method

The Keyword Method also uses the Vigenere Cipher table to encrypt and decrypt the plaintext. In this method, the key is usually more than one letter.

Vigenere Cipher Encoder in Keyword Method

The Vigenere Cipher encoder is similar to the autokey method. The difference is that instead of using just one priming key, the Vigenere Cipher encoder uses a keyword that is more than one letter. So, there are endless possibilities of a possible keyword. The steps involved are as follows:

• The sender will write the keyword repeatedly under the plaintext to form the key.
• This will then be encoded, just like the autokey method, until an entire ciphertext is formed.

For this example, let us use the phrase ALL IS WELL and the keyword as CAKE.

Plaintext	A	L	L	I	S	W	E	L	L
Key	C	A	K	E	C	A	K	E	C
Ciphertext	C	L	V	M	U	W	P	P	N

Vigenere Cipher Decoder in Keyword Method

To decode this, the following steps can be followed:

• Start by writing the keyword repeatedly till the length of the ciphertext.
• Write the ciphertext under it and decode it just as is done in the autokey method.

Key	C	A	K	E	C	A	K	E	C
Ciphertext	C	L	V	M	U	W	P	P	N
Plaintext	A	L	L	I	S	W	E	L	L

Security

This method is much more secure than the autokey method, but that does not mean it is not vulnerable. The cipher is more secure with longer keywords.

Vigenere Cipher Calculator

If the Vigenere table is not available or accessible, for whatever reason, then encryption and decryption processes can be carried out using the Vigenere algebraic formula. For this method to work, the alphabets have to be converted into numbers (0-25).

The encryption formula is:

Ei = (Pi + Ki) mod 26

The decryption formula is:

Di = (Ei – Ki) mod 26

Where E is the encipher, D is the decipher, P is the plaintext, and K is the key.

If the value of D_i is ever negative, 26 is added to that value.

For example, let us take our plaintext to be CYBER SECURITY.

So,

The key can be BEST.

Encryption: Ei = (Pi + Ki) mod 26

Decryption: Di = (Ei – Ki) mod 26

Remember, if the value of D_i is ever negative, 26 is added to that value.

Vigenere Cipher Code in Python

Let us take a look at what the Vigenere Cipher calculator will look like as Python code.

#Python code to implement
# Vigenere Cipher
# This function generates the
# key in a cyclic manner until
# it's length isn't equal to
# the length of original text
def generateKey(string, key):
key = list(key)
if len(string) == len(key):
  return(key)
else:
  for i in range(len(string) -
                                                                            	len(key)):
  key.append(key[i % len(key)])
 return("" . join(key))
            	
# This function returns the
# encrypted text generated
# with the help of the key
def cipherText(string, key):
 cipher_text = []
  for i in range(len(string)):
   x = (ord(string[i]) +
ord(key[i])) % 26
  x += ord('A')
   cipher_text.append(chr(x))
 return("" . join(cipher_text))
            	
# This function decrypts the
# encrypted text and returns
# the original text
def originalText(cipher_text, key):
orig_text = []
for i in range(len(cipher_text)):
  x = (ord(cipher_text[i]) -
  ord(key[i]) + 26) % 26
  x += ord('A')
 orig_text.append(chr(x))
return("" . join(orig_text))    	
# Driver code
if __name__ == "__main__":
 string = "CYBERSECURITY"
 keyword = "BEST"
 key = generateKey(string, keyword)
 cipher_text = cipherText(string,key)
 print("Ciphertext :", cipher_text)
 print("Original/Decrypted Text :",
 originalText(cipher_text, key))

Output

Ciphertext: DCTXSWWVVVAMZ
Original or Decrypted Text: CYBERSECURITY

Conclusion

Vigenere Cipher is a simple polyalphabetic substitution method that goes from a simple to advanced method. The Vigenere Square or Table is an important tool used in this Cipher. You can use this cipher in three different ways as per your needs. All the three methods involve different steps. The autokey method is the least secure method. Even though the keyword method has its vulnerabilities, it is more secure than the autokey method. The Python Code method is relatively the most secure method.

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