Step 1: Import Necessary Libraries

Let’s proceed with creating the script. We’ll need PyPDF2 for working with the PDF and itertools for generating brute-force combinations.

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import PyPDF2
import itertools
import string

Step 2: Define the Character Set and Load Dictionary

We’ll define our character set for brute-force attempts and load a dictionary of common words or phrases.

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characters = string.ascii_lowercase + string.digits
dictionary_file = "common_passwords.txt"  # A file containing common passwords or phrases, one per line

def load_dictionary(file_path):
    with open(file_path, 'r') as file:
        return [line.strip() for line in file.readlines()]

Step 3: Create the PDF Password Recovery Function

We’ll create a function that first attempts to unlock the PDF using dictionary words, and if that fails, it switches to brute-force.

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def recover_pdf_password(pdf_file, max_length):
    pdf_reader = PyPDF2.PdfReader(pdf_file)
    dictionary = load_dictionary(dictionary_file)
    
    # Attempt dictionary attack
    for word in dictionary:
        if pdf_reader.decrypt(word) == 1:
            print(f"Password found: {word}")
            return word
    
    # If dictionary attack fails, perform brute-force attack
    for length in range(1, max_length + 1):
        for guess in itertools.product(characters, repeat=length):
            password = ''.join(guess)
            if pdf_reader.decrypt(password) == 1:
                print(f"Password found: {password}")
                return password

    print("Password not found")
    return None

Step 4: Run the Script

Finally, you can run the script by specifying the PDF file’s path and the maximum length for brute-force attempts.

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pdf_file = "path/to/your/encrypted.pdf"
max_length = 6  # Adjust based on the complexity of the password
recover_pdf_password(pdf_file, max_length)

Explanation:

1. Dictionary Attack: The script first tries every word or phrase in the dictionary file. This is usually faster and more effective if the password is a common word.
2. Brute-Force Fallback: If the dictionary attack fails, the script generates all possible combinations of lowercase letters and digits up to the specified maximum length.
3. Success Check: The decrypt() method returns 1 if the correct password is found, at which point the script prints and returns the password.

Use Case:

This script is useful when you suspect that a password might be a common word but want to ensure all possibilities are covered by also attempting a brute-force attack.

Final Thoughts:

This demonstration combines the efficiency of a dictionary attack with the thoroughness of brute-force, offering a robust solution for recovering passwords from encrypted PDF files.

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Here are five related hashtags:

1. #PDFEncryption
2. #PasswordRecovery
3. #DictionaryAttack
4. #CyberSecurity
5. #DecryptPDF