decrypt encrypted file

xor_decrypt.py

import sys
import os

def xor_bytes(data: bytes, key: bytes) -> bytes:
    """
    Performs a repeating-key XOR operation on the given data using the key.

    XOR decryption is the same as encryption.
    If the key is shorter than the data, it is cycled.
    """
    key_length = len(key)
    # Each byte in 'data' is XORed with the corresponding byte in 'key' (cycled)
    return bytes(data[i] ^ key[i % key_length] for i in range(len(data)))

def main():
    """
    Main entry point for the XOR decryption script.
    Handles arguments, validates files, and performs decryption.
    """
    # 1. Accepts the encrypted file and key file as command-line arguments; print usage and exit if missing.
    if len(sys.argv) < 3:
        print("Usage: python3 xor_decrypt.py <encrypted_file> <key_file>")
        sys.exit(1)

    encrypted_file_path = sys.argv[1]
    key_file_path = sys.argv[2]

    # 2. Validates both files exist; print a distinct error message for each missing file.
    if not os.path.exists(encrypted_file_path):
        print(f"Error: Encrypted file '{encrypted_file_path}' does not exist.")
        sys.exit(1)

    if not os.path.exists(key_file_path):
        print(f"Error: Key file '{key_file_path}' does not exist.")
        sys.exit(1)

    try:
        # Read the encrypted data
        with open(encrypted_file_path, 'rb') as f:
            ciphertext = f.read()

        # Read the key
        with open(key_file_path, 'rb') as f:
            key = f.read()

        if not key:
            print("Error: Key file is empty.")
            sys.exit(1)

        # 3. XOR-decrypts the file
        plaintext = xor_bytes(ciphertext, key)

        # 4. Writes the result as <name>_decrypted<ext>
        file_root, file_ext = os.path.splitext(encrypted_file_path)
        output_path = f"{file_root}_decrypted{file_ext}"

        with open(output_path, 'wb') as f:
            f.write(plaintext)

        # 5. Prints the output path to the terminal.
        print(f"Decrypted : {output_path}")

    except Exception as e:
        print(f"An error occurred: {e}")
        sys.exit(1)

if __name__ == "__main__":
    main()