Vernam Cipher in Cryptography

Vernam Cipher is a method of encrypting alphabetic text. It is one of the Substitution techniques for converting plain text into cipher text. In this mechanism we assign a number to each character of the Plain-Text, like (a = 0, b = 1, c = 2, … z = 25). 

Method to take key: In the Vernam cipher algorithm, we take a key to encrypt the plain text whose length should be equal to the length of the plain text. 

Encryption Algorithm:

1. Assign a number to each character of the plain-text and the key according to alphabetical order. 
2. Bitwise XOR both the number (Corresponding plain-text character number and Key character number). 
3. Subtract the number from 26 if the resulting number is greater than or equal to 26, if it isn’t then leave it.

Example 1:

Plain-Text: O A K
Key: S O N

O ==> 14 = 0 1 1 1 0
S ==> 18 = 1 0 0 1 0
Bitwise XOR Result: 1 1 1 0 0 = 28

Since the resulting number is greater than 26, subtract 26 from it.  Then convert the Cipher-Text character number to the Cipher-Text character.

28 - 26 = 2 ==> C
CIPHER-TEXT: C

Similarly, do the same for the other corresponding characters,

PT: O  A  K
NO: 14 00 10

KEY: S  O  N
NO:  18 14 13

New Cipher-Text is after getting the corresponding character from the resulting number. 

CT-NO: 02 14 07
CT:    C  O  F

Example 2:

Plain-Text: RAMSWARUPK
Key: RANCHOBABA 

Now according to our encryption algorithm, we assign a number to each character of our plain text and key.

PT:   R  A  M   S   W   A  R   U   P   K
NO:   17 0  12  18  22  0  17  20  15  10

KEY:  R   A  N   C  H  O   B  A  B  A  
NO:   17  0  13  2  7  14  1  0  1  0 

Now Bitwise XOR the number of Plain-Text and Key and after doing the XOR operation and subtraction operation (if required), we will get the corresponding Cipher-Text character number. 

CT-NO: 0  0  1  16  17  14  16  20  14  10 

Since there are no numbers that are greater than or equal to 26 we do not have to subtract 26 from any of them.

New Cipher-Text is after getting the corresponding character from the number. 

CIPHER-TEXT: A  A  B  Q  R  O  Q  U  O  K 

Note: For the Decryption apply the just reverse process of encryption.

For the implementation please refer to this article: Implementation of Vernam Cipher or One Time Pad Algorithm

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