text_encryption.py

# Caesar Cipher Implementation in Python


# Define the function to encrypt/decrypt text
def caesar_cipher(text, key_value, mode='encrypt'):
   
    """Encrypts or decrypts text using the Caesar Cipher. Caesar Cipher shifts each letter by a fixed number (key_value)."""

    result = ''     
    
    # Reverse the shift for decryption
    if mode == 'decrypt':
        key_value = -key_value  
        # Ensure the shift is within 1-25 range (1-25 means in alphabetincal order.)
    

    for char in text:
        # Check if the character is a letter
        if char.isalpha():
            # Determine the starting character (A or a)
            start = ord('A') if char.isupper() else ord('a')
            # ord function gets the ASCII value of the character.

            # 1. Convert letter to a number (0-25)
            # 2. Add the shift value
            # 3. Use the modulus operator (%) to wrap around the alphabet (A-Z)
            # 4. Convert the number back to a letter

            new_char_code = (ord(char) - start + key_value) % 26 + start   
            # % 26 ensures that if we go past 'Z' or 'z', we wrap around to the beginning of the alphabet.
            
            result += chr(new_char_code)
            
        else:
            # Keep non-alphabetic characters (like spaces, numbers, punctuation) as they are
            result += char
    
    return result
 
# Example usage:

'''(plaintext means user input text which we want to encrypt/decrypt),(ciphertext means encrypted text), 
   (encryption_key means the number by which we want to shift the letters)'''


# 1. Define the plaintext and the key (shift value)
plaintext = input("Enter the text:\t\n")
encryption_key = int(input("Enter the key:\t\n"))  # The key must be a number

print(f"Original Text: {plaintext}")

# 2. Encryption
ciphertext = caesar_cipher(plaintext, encryption_key, mode='encrypt')
print(f"Encrypted Text: {ciphertext}")

# 3. Decryption
decrypted_text = caesar_cipher(ciphertext, encryption_key, mode='decrypt')
print(f"Decrypted Text: {decrypted_text}")

# In simple terms, This works on an simple logic of shifting letters by a fixed number in the alphabet.By using ASCII values we can easily manipulate the characters.