How to unlock a “secured” (read-protected) PDF in Python?

问题

In Python I'm using pdfminer to read the text from a pdf with the code below this message. I now get an error message saying:

File "/usr/local/lib/python2.7/dist-packages/pdfminer/pdfpage.py", line 124, in get_pages
    raise PDFTextExtractionNotAllowed('Text extraction is not allowed: %r' % fp)
PDFTextExtractionNotAllowed: Text extraction is not allowed: <cStringIO.StringO object at 0x7f79137a1
ab0>

When I open this pdf with Acrobat Pro it turns out it is secured (or "read protected"). From this link however, I read that there's a multitude of services which can disable this read-protection easily (for example pdfunlock.com. When diving into the source of pdfminer, I see that the error above is generated on these lines.

if check_extractable and not doc.is_extractable:
    raise PDFTextExtractionNotAllowed('Text extraction is not allowed: %r' % fp)

Since there's a multitude of services which can disable this read-protection within a second, I presume it is really easy to do. It seems that .is_extractable is a simple attribute of the doc, but I don't think it is as simple as changing .is_extractable to True..

Does anybody know how I can disable the read protection on a pdf using Python? All tips are welcome!

================================================

Below you will find the code with which I currently extract the text from non-read protected.

def getTextFromPDF(rawFile):
    resourceManager = PDFResourceManager(caching=True)
    outfp = StringIO()
    device = TextConverter(resourceManager, outfp, codec='utf-8', laparams=LAParams(), imagewriter=None)
    interpreter = PDFPageInterpreter(resourceManager, device)

    fileData = StringIO()
    fileData.write(rawFile)
    for page in PDFPage.get_pages(fileData, set(), maxpages=0, caching=True, check_extractable=True):
        interpreter.process_page(page)
    fileData.close()
    device.close()

    result = outfp.getvalue()

    outfp.close()
    return result

回答1:

As far as I know, in most cases the full content of the PDF is actually encrypted, using the password as the encryption key, and so simply setting .is_extractable to True isn't going to help you.

Per this thread:

Does a library exist to remove passwords from PDFs programmatically?

I would recommend removing the read-protection with a command-line tool such as qpdf (easily installable, e.g. on Ubuntu use apt-get install qpdf if you don't have it already):

qpdf --password=PASSWORD --decrypt SECURED.pdf UNSECURED.pdf

Then open the unlocked file with pdfminer and do your stuff.

For a pure-Python solution, you can try using PyPDF2 and its .decrypt() method, but it doesn't work with all types of encryption, so really, you're better off just using qpdf - see:

https://github.com/mstamy2/PyPDF2/issues/53

回答2:

I had some issues trying to get qpdf to behave in my program. I found a useful library, pikepdf, that is based on qpdf and automatically converts pdfs to be extractable.

The code to use this is pretty straightforward:

import pikepdf

pdf = pikepdf.open('unextractable.pdf')
pdf.save('extractable.pdf')

回答3:

In my case there was no password, but simply setting check_extractable=False circumvented the PDFTextExtractionNotAllowed exception for a problematic file (that opened fine in other viewers).

回答4:

The 'check_extractable=True' argument is by design. Some PDFs explicitly disallow to extract text, and PDFMiner follows the directive. You can override it (giving check_extractable=False), but do it at your own risk.

来源：https://stackoverflow.com/questions/28192977/how-to-unlock-a-secured-read-protected-pdf-in-python

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