Implementing Atbash Cipher

08 Feb, 2018
Definition: Atbash cipher is a substitution cipher with just one specific key where all the letters are reversed that is A to Z and Z to A. It was originally used to encode the Hebrew alphabets but it can be modified to encode any alphabet.
Relationship to Affine: Atbash cipher can be thought of as a special case of Affine cipher with both the keys being 25, i.e, a = 25 & b = 25
Atbash Key
Algorithm: The following key is used in the Atbash algorithm
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ZYXWVUTSRQPONMLKJIHGFEDCBA

Encryption
To encipher a message, find the letter you wish to encipher in the top row, then replace it with the letter in the bottom row.In the example below, we encipher the message ‘GEEKS FOR GEEKS’. The first letter we wish to encipher is ‘G’, which is above ‘T’, so the first ciphertext letter is ‘T’. The next letter is ‘E’, which is above ‘V’, so that comes next. The whole message is enciphered as:
GEEKS FOR GEEKS
TVVPH ULI TVVPH

Decryption
Similarly if we want to decrypt say ‘TVVPH ULI TVVPH’ we would replace the first letter ‘T’ with ‘G’. The second letter ‘V’ with ‘E’ and so forth substituting all letters including the last one ‘H’ with ‘S’. The deciphered message will be :
TVVPH ULI TVVPH
GEEKS FOR GEEKS

The Approach: Here, mapping of every element is done for the key as ‘key-value’ pairs in a dictionary and then it is used as a lookup table whenever you want to encrypt a single character.
Code:
# Python program to implement Atbash Cipher  # This script uses dictionaries to lookup various alphabetslookup_table = {'A' : 'Z', 'B' : 'Y', 'C' : 'X', 'D' : 'W', 'E' : 'V',        'F' : 'U', 'G' : 'T', 'H' : 'S', 'I' : 'R', 'J' : 'Q',        'K' : 'P', 'L' : 'O', 'M' : 'N', 'N' : 'M', 'O' : 'L',        'P' : 'K', 'Q' : 'J', 'R' : 'I', 'S' : 'H', 'T' : 'G',        'U' : 'F', 'V' : 'E', 'W' : 'D', 'X' : 'C', 'Y' : 'B', 'Z' : 'A'}  def atbash(message):    cipher = ''    for letter in message:        # checks for space        if(letter != ' '):            #adds the corresponding letter from the lookup_table            cipher += lookup_table[letter]        else:            # adds space            cipher += ' '      return cipher  # Driver function to run the programdef main():    #encrypt the given message    message = 'GEEKS FOR GEEKS'    print(atbash(message.upper()))          #decrypt the given message    message = 'TVVPH ULI TVVPH'    print(atbash(message.upper()))    # Executes the main functionif __name__ == '__main__':    main()
Output:
TVVPH ULI TVVPH
GEEKS FOR GEEKS

Advantages : Since its just an affine cipher with both a = 25 = b we don’t need to write separate functions for encryption and decryption. We can re-use the same function for both purposes.
Analysis : It has one constant key it is the easiest cipher to break and provides almost no security. Anyone can assume that it is atbash and decrypt the message by reversing the letters.
This article is contributed by Palash Nigam . If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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