caesarCipher.py

"""Caesar Cipher, by Al Sweigart al@inventwithpython.com
The Caesar cipher is a shift cipher that uses addition and subtraction 
to encrypt and decrypt letters.
More info at: https://en.wilkipedia.org/wiki/Caesar_cipher
View this code at https://nostarch.com/big-book-small-python-projects
Tags: short, beginner, cryoitgraphy, math"""

try:
    import pyperclip # pyperclip copies text to the clipboard.
except ImportError:
    pass # If pyperclip is not installed, do nothing. it's no big deal.

# Every possible symbol that can be encrypted/decrypted:
# (!) you can add number and punctuation marks to encrypt those
# symbols as well.
SYMBOLS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

print('Caesar Cipher, by Al Sweigart al@inventwith python.com')
print('The Caesar cipher encrypts letters by shifting them over by a')
print('key number. For example, a key of 2 means the letter A is')
print('encrypted into C, the letter B encrypted into D, and so on.')
print()

# let the user emter if theu are encrypting or decrypting:
while True: #keep asking until the user enters e or d.
    print('Do you want to (e)ncrypt or (d)ecrypt?')
    response = input('> ').lower()
    if response.startswith('e'):
        mode = 'encrypt'
        break
    elif response.startswith('d'):
        mode = 'decrypt'
        break
    print('please enter the letter e or d.')

# let the user enter the key to use:
while True: # keep asking until the user enters a valid key.
    maxKey = len(SYMBOLS) - 1
    print('Please enter the key (o to {}) to use.'.format(maxKey))
    response = input('>'). upper()
    if not response.isdecimal():
        continue

    if 0 <= int(response) < len(SYMBOLS):
        key = int(response)
        break

# Let the user enter the message to encrypt/decrypt:
print('Enter the message to {}.'.format(mode))
message = input('> ')

# Caesar cipher only works on uppercase letters:
message = message.upper()

#stores the encrypted/decrypted form of the message:
translated = ''

# Encrypt/decrypt each symbol in the message:
for symbol in message:
    if symbol in SYMBOLS:
        #get the encrypted (or decrypted)number for this symbol.
        num = SYMBOLS.find(symbol) #Get the number of the symbol.
        if mode == 'encrypt':
            num = num + key
        elif mode == 'decrypt':
            num = num - key

        #Handle th wrap-around  if num is larger than the length of 
        #SYMBOLS or less than 0:
        if num >= len(SYMBOLS):
            num = num - len(SYMBOLS)
        elif num < 0:
            num = num + len(SYMBOLS)

        #Add encrypted/decrypted number's symbol to translated:
        translated = translated + SYMBOLS[num]
    else:
        #just add the symbol without encrypting/decrypting:
        translated = translated + symbol

#Display the encrypted/decrypted string to the screen:
print(translated)

try:
    pyperclip.copy(translated)
    print('Full {}ed text copied to clipboard.'.format(mode))
except:
    pass # Do nothing if pyperclip wasn't installed.