# Caesar Cipher
# http://inventwithpython.com/hacking (BSD Licensed)

# open encrypted/decrypted file
message = open('test.txt','r+')
test = message.read()

# the encryption/decryption key
key = int(input('Key: '))

# tells the program to encrypt or decrypt
mode = input('choose: encrypt / decrypt: ')# set to 'encrypt' or 'decrypt'

# every possible symbol that can be encrypted
LETTERS = ' !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~'

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

# capitalize the string in message
#message = message.upper()

# run the encryption/decryption code on each symbol in the message string
for symbol in test:
    if symbol in LETTERS:
        # get the encrypted (or decrypted) number for this symbol
        num = LETTERS.find(symbol) # get the number of the symbol
        if mode == 'encrypt':
            num = num + key
        elif mode == 'decrypt':
            num = num - key

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

        # add encrypted/decrypted number's symbol at the end of translated
        translated = translated + LETTERS[num]

    else:
        # just add the symbol without encrypting/decrypting
        translated = translated + symbol

# save encrypted/decrypted file
message.close()
message = open('test.txt','w')
message.write(translated)
message.close()