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As an exercise, I wrote a program to decrypt a PDF file. The only thing that is known about the encryption password is that it is a single English word (all capital or lowercase).

The program uses a dictionary file containing over 45000 words to unlock the file. It looks like this:

dictionary.txt

AARHUS
AARON
ABABA
ABACK
ABAFT
ABANDON
ABANDONED
ABANDONING
ABANDONMENT
ABANDONS
ABASE
ABASED
ABASEMENT
ABASEMENTS
ABASES
ABASH
ABASHED
ABASHES
⋮
ZEROTH
ZEST
ZEUS
ZIEGFELD
ZIEGFELDS
ZIEGLER
ZIGGY
ZIGZAG
ZILLIONS
ZIMMERMAN
ZINC
ZION
ZIONISM
ZIONIST
ZIONISTS
ZIONS
ZODIAC
ZOE
ZOMBA
ZONAL
ZONALLY
ZONE
ZONED
ZONES
ZONING
ZOO
ZOOLOGICAL
ZOOLOGICALLY
ZOOM
ZOOMS
ZOOS
ZORN
ZOROASTER
ZOROASTRIAN
ZULU
ZULUS
ZURICH

pdf_password_breaker

"""
Brute force password breaker using a dictionary containing
English words.
"""

import sys
import PyPDF2
from pathlib import Path

def get_filename_from_user() -> Path:
    """Asks for a path from the User"""
    while True:
        filename: str = input("Enter filename in folder of script:")
        path: Path = Path(sys.path[0], filename)

        if path.is_file():
            return path.as_posix()
        print("File doesn't exist\n")


def decrypt(pdf_filename: Path, password: str) -> bool:
    """
    Try to decrypt a file. If not successful a false is returned.
    If the file passed is not encrypted also a false is passed
    """
    with open(pdf_filename, 'rb') as pdf_file:
        pdf_reader = PyPDF2.PdfFileReader(pdf_file)
        pdf_reader.decrypt(password)
        pdf_writer = PyPDF2.PdfFileWriter()

        try:
            for page_number in range(pdf_reader.numPages):
                pdf_writer.addPage(pdf_reader.getPage(page_number))
        except PyPDF2.utils.PdfReadError:
            return False

        new_name: str = pdf_filename.stem + "_decrypted.pdf"
        filename_decrypted = pdf_filename.parent / new_name

        with open(filename_decrypted, 'wb') as pdf_file_decrypted:
            pdf_writer.write(pdf_file_decrypted)
    return True


def break_encryption(pdf_filename: Path, dictionary_filename: str) -> bool:
    """Try's out words from a dictionary to break encryption"""
    with open(dictionary_filename, 'r') as dictionary_file:
        keyword: str = dictionary_file.readline().strip()

        if decrypt(pdf_filename, keyword):
            return keyword
        if decrypt(pdf_filename, keyword.lower()):
            return keyword.lower()

        while keyword:
            keyword = dictionary_file.readline().strip()

            if decrypt(pdf_filename, keyword):
                return keyword
            if decrypt(pdf_filename, keyword.lower()):
                return keyword.lower()
    return None


def pdf_password_breaker():
    """main loop"""
    filename: Path = get_filename_from_user()
    password: str = break_encryption(filename, "dictionary.txt")

    if password:
        print("File unlocked. Password was:" + password)
        return
    print("File could not be unlocked")

if __name__ == "__main__":
    pdf_password_breaker()

I tried a file with a simple password like "hello". It works but it takes a lot of time until it reaches the "hello" in the file.

I wonder if theres a way to improve the speed, or any other aspect of the code.

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14
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Looking at the documentation of PdfFileReader.decrypt It states that it return 0 if the file fails or 1 if it succeeds.

You can also use isEncrypted to check if the file was encrypted in the first place.

So you can turn the loop around. First open the pdf file see if it was encrypted at all and if so try to call decrypt with every password in your dictionary.

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  • 1
    \$\begingroup\$ And in doing that, only open the PDF file once - opening it twice for every password is a huge slow down. \$\endgroup\$ – The6P4C Jan 16 at 23:38
  • \$\begingroup\$ i tryed that approach. it got quite a bit faster. but to crack hello it still takes quite some time \$\endgroup\$ – Sandro4912 Jan 17 at 17:00

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