Chapter 6 – The Caesar Cipher 71

are the integers from 0 to 25. Even if a cryptanalyst knows that the Caesar cipher was used, that
alone does not give her enough information to break the cipher. She must also know the key.

Source Code of the Caesar Cipher Program

Type in the following code into the file editor, and then save it as caesarCipher.py. Press F5 to
run the program. Note that first you will need to download the pyperclip.py module and place this
file in the same directory (that is, folder) as the caesarCipher.py file. You can download this file
from http://invpy.com/pyperclip.py

Source code for caesarCipher.py

1. 
   

   Caesar Cipher

   
   


2. 
   

   http://inventwithpython.com/hacking (BSD Licensed)

   
   


3. 
   


4. import pyperclip


5. 
   


6. 
   

   the string to be encrypted/decrypted

   
   


7. message = 'This is my secret message.'


8. 
   


9. 
   

   the encryption/decryption key

   
   


10. key = 13


11. 
    


12. 
    

    tells the program to encrypt or decrypt

    
    


13. mode = 'encrypt' # set to 'encrypt' or 'decrypt'


14. 
    


15. 
    

    every possible symbol that can be encrypted

    
    


16. LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'


17. 
    


18. 
    

    stores the encrypted/decrypted form of the message

    
    


19. translated = ''


20. 
    


21. 
    

    capitalize the string in message

    
    


22. message = message.upper()


23. 
    


24. 
    

    run the encryption/decryption code on each symbol in the message string

    
    


25. for symbol in message:


26. if symbol in LETTERS:


27. 
    

    get the encrypted (or decrypted) number for this symbol

    
    


28. num = LETTERS.find(symbol) # get the number of the symbol


29. if mode == 'encrypt':


30. num = num + key


31. elif mode == 'decrypt':


32. num = num - key


33. 
    


34. 
    

    handle the wrap-around if num is larger than the length of