#!/usr/bin/python3
# -*- coding: utf-8 -*-

# Caesar Cipher
# https://www.nostarch.com/crackingcodes (BSD Licensed)

# The string to be encrypted/decrypted:
message: str = "All human beings have three lives: public, private, and secret. Gabriel Garcia Marquez"

# The encryption/decryption key:
key: int = 13

# Whether the program encrypts or decrypts:
# Set to either 'encrypt' or 'decrypt'.
mode: str = "encrypt"

# Every possible symbol that can be encrypted:
SYMBOLS: str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890 !?."

# Stores the encrypted/decrypted form of the message:
translated: str = ""

for symbol in message:
    # Note: Only symbols in the `SYMBOLS` string can be encrypted/decrypted.
    if symbol in SYMBOLS:
        symbolIndex = SYMBOLS.find(symbol)

        # Perform encryption/decryption:
        if mode == "encrypt":
            translatedIndex = symbolIndex + key
        elif mode == "decrypt":
            translatedIndex = symbolIndex - key

        # Handle wrap-around, if needed: What is this computing?
        if translatedIndex >= len(SYMBOLS):
            translatedIndex = translatedIndex - len(SYMBOLS)
        elif translatedIndex < 0:
            translatedIndex = translatedIndex + len(SYMBOLS)

        translated = translated + SYMBOLS[translatedIndex]
    else:
        # Append the symbol without encrypting/decrypting:
        translated = translated + symbol

# Output the translated string:
print(translated)