Implementing a Simple Caesar Cipher for Data Encryption in Python

Goal : Understand the basic concepts of data encryption and implement a simple encryption algorithm.

Learning Content : Data encryption converts plaintext into ciphertext to prevent unauthorized access. Differential privacy protects individual privacy by adding noise to data.

Practice : We will implement a simple symmetric encryption method using the Caesar Cipher, a basic substitution technique that shifts each character by a fixed number of positions.

1. Install Required Libraries

No additional libraries are needed; the implementation uses pure Python.

2. Implement the Caesar Cipher Encryption Function

def caesar_cipher_encrypt(text, shift):
    """Use Caesar Cipher to encrypt text.
    Parameters:
        text (str): plaintext to encrypt
        shift (int): number of positions to shift
    Returns:
        str: ciphertext
    """
    encrypted_text = ""
    for char in text:
        if char.isalpha():
            shift_amount = shift % 26
            if char.islower():
                new_char = chr(((ord(char) - ord('a') + shift_amount) % 26) + ord('a'))
            else:
                new_char = chr(((ord(char) - ord('A') + shift_amount) % 26) + ord('A'))
        else:
            new_char = char
        encrypted_text += new_char
    return encrypted_text

# Test the encryption function
plain_text = "你好，世界！Hello, World!"
shift = 3  # shift by 3 positions
encrypted_text = caesar_cipher_encrypt(plain_text, shift)
print(f"明文: {plain_text}")
print(f"密文: {encrypted_text}")

3. Implement the Caesar Cipher Decryption Function

def caesar_cipher_decrypt(cipher_text, shift):
    """Use Caesar Cipher to decrypt text.
    Parameters:
        cipher_text (str): ciphertext to decrypt
        shift (int): number of positions to shift
    Returns:
        str: plaintext
    """
    decrypted_text = ""
    for char in cipher_text:
        if char.isalpha():
            shift_amount = shift % 26
            if char.islower():
                new_char = chr(((ord(char) - ord('a') - shift_amount) % 26) + ord('a'))
            else:
                new_char = chr(((ord(char) - ord('A') - shift_amount) % 26) + ord('A'))
        else:
            new_char = char
        decrypted_text += new_char
    return decrypted_text

# Test the decryption function
decrypted_text = caesar_cipher_decrypt(encrypted_text, shift)
print(f"密文: {encrypted_text}")
print(f"解密后的明文: {decrypted_text}")

4. Full Code Example

def caesar_cipher_encrypt(text, shift):
    """Use Caesar Cipher to encrypt text.
    Parameters:
        text (str): plaintext
        shift (int): shift amount
    Returns:
        str: ciphertext
    """
    encrypted_text = ""
    for char in text:
        if char.isalpha():
            shift_amount = shift % 26
            if char.islower():
                new_char = chr(((ord(char) - ord('a') + shift_amount) % 26) + ord('a'))
            else:
                new_char = chr(((ord(char) - ord('A') + shift_amount) % 26) + ord('A'))
        else:
            new_char = char
        encrypted_text += new_char
    return encrypted_text

def caesar_cipher_decrypt(cipher_text, shift):
    """Use Caesar Cipher to decrypt text.
    Parameters:
        cipher_text (str): ciphertext
        shift (int): shift amount
    Returns:
        str: plaintext
    """
    decrypted_text = ""
    for char in cipher_text:
        if char.isalpha():
            shift_amount = shift % 26
            if char.islower():
                new_char = chr(((ord(char) - ord('a') - shift_amount) % 26) + ord('a'))
            else:
                new_char = chr(((ord(char) - ord('A') - shift_amount) % 26) + ord('A'))
        else:
            new_char = char
        decrypted_text += new_char
    return decrypted_text

# Test encryption and decryption
plain_text = "你好，世界！Hello, World!"
shift = 3
encrypted_text = caesar_cipher_encrypt(plain_text, shift)
print(f"明文: {plain_text}")
print(f"密文: {encrypted_text}")

decrypted_text = caesar_cipher_decrypt(encrypted_text, shift)
print(f"密文: {encrypted_text}")
print(f"解密后的明文: {decrypted_text}")

5. Run the Code

Save the above script to a file named caesar_cipher.py and execute it from the command line:

python caesar_cipher.py

Sample Output

明文: 你好，世界！Hello, World!
密文: 你好，世界！Khoor, Zruog!
密文: 你好，世界！Khoor, Zruog!
解密后的明文: 你好，世界！Hello, World!

Summary : By completing this practice you have learned how to implement a basic Caesar Cipher encryption and decryption algorithm in Python, which illustrates core encryption principles. In real‑world applications more robust algorithms such as AES are used to provide stronger security.