# ============================================

# CHAPTER 44: ENCRYPTION PROGRAM

# ============================================

# Substitution Cipher Encryption and Decryption

#

# User Story:

# As a user, I want to encrypt and decrypt messages using a substitution cipher

# so that I can securely communicate without messages being easily read by

# unintended recipients.

#

# How Substitution Cipher Works:

# 1. Create list of all possible characters (alphabet)

# 2. Create shuffled "key" - random arrangement of same characters

# 3. Encryption: Replace each character with its corresponding key character

# 4. Decryption: Reverse the process using the key

#

# Example:

# chars: a b c d e f g h...

# key: q w e r t y u i...

# "bad" encrypts to "wqr"

#

# Key Concepts:

# - String module for character sets

# - List manipulation and copying

# - Random shuffling

# - Character substitution algorithm

# - Index-based mapping

import random

import string

# =============================================

# BUILD CHARACTER SET

# =============================================

# Combine all possible characters:

# - Space character

# - Punctuation (!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~)

# - Digits (0-9)

# - ASCII letters (a-z, A-Z)

chars = "" + " " + string.punctuation + string.digits + string.ascii_letters

# Convert string to list for manipulation

# Lists are mutable, strings are not

chars = list(chars)

# =============================================

# CREATE ENCRYPTION KEY

# =============================================

# Make a copy of chars list (not a reference)

# copy() creates independent list

key = chars.copy()

# Randomly shuffle the key

# This creates the substitution cipher mapping

random.shuffle(key)

# =============================================

# DISPLAY CHARACTER MAPPINGS (For debugging)

# =============================================

# Show original characters and their encrypted equivalents

print(f"chars: {chars}")

print(f"key: {key}")

# =============================================

# ENCRYPTION PROCESS

# =============================================

# Get message from user

plain_text = input("Enter a message to encrypt: ")

# Initialize empty encrypted message

cipher_text = ""

# =============================================

# ENCRYPT EACH CHARACTER

# =============================================

# For each letter in the original message:

for letter in plain_text:

# Find position of letter in original character set

# Example: 'a' might be at index 37 in chars

index_of_letter = chars.index(letter)

# Replace with character at same position in key

# Example: chars[37] = 'a', key[37] might be 'q'

# So 'a' gets encrypted to 'q'

cipher_text += key[index_of_letter]

# =============================================

# DISPLAY RESULTS

# =============================================

print(f'original message: {plain_text}')

print(f'encrypted message: {cipher_text}')

# =============================================

# HOW TO DECRYPT (not implemented here)

# =============================================

# To decrypt, reverse the process:

# for letter in cipher_text:

# index_of_letter = key.index(letter) # Find in key

# decrypted_text += chars[index_of_letter] # Use chars at same index

# =============================================

# IMPORTANT NOTES

# =============================================

# 1. Both sender and receiver need the SAME KEY

# 2. Key must be kept secret (not shown to others)

# 3. This is a simple cipher - not cryptographically secure

# 4. For real security, use proven encryption libraries