Pack the permissions (the /P entry) as unsigned long, fix #186

[Recursing]

Description
Fixes #186, by treating the /P entry as a four byte unsigned integer

How Has This Been Tested?
Tested the pdf in the issue

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have updated the README.md and other documentation, or I am sure that this is not necessary
• I have added a consice human-readable description of the change to CHANGELOG.md
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial version

[pietermarsman]

I did some research and your solution looks spot-on to me.

However, the 5 tests for this PR are failing indicating that changing this breaks other this. Could you have a look and this?

Also, make sure to add a line to the CHANGELOG.md.

[pietermarsman]

The cause of the errors is probably that self.p is negative. I've at least found one example in the tests that has a value of self.p = -12.

[Recursing]

Yeah sorry, I really didn't think self.p could be negative, will look into it

[pietermarsman]

Great! I am also trying to figure this out right now. I found a paragraph in the PDF reference that is quite helpful (right above Table 3.20):

Note: PDF integer objects are represented internally in signed twos-complement form. Since all the reserved high-order flag bits in the encryption dictionary’s P val- ue are required to be 1, the value must be specified as a negative integer. For exam- ple, assuming revision 2 of the security handler, the value -44 permits printing and copying but disallows modifying the contents and annotations.

[pietermarsman]

I guess you need to convert the signed value (e.g. -12) to an unsigned value by using the two's complement.

I cannot find anything in pdfminer.six that already implements this algorithm, so I guess you should do it yourself. There is some pseudocode on the wikipedia.

[Recursing]

You can just add 1<<32 if it's negative, and it works for all samples, but it's not the cleanest solution (should be parsed as unsigned in the first place)

Edit: I think we should just add 1<<32 and leave it like this, it seems to be coherent with the reference

I'm currently testing it on a few pdfs in my downloads folder, will push commits in an hour

[pietermarsman]

It is preferable to implement the two's complement method instead of some approximation. I know it is more fuzz than you care about, but that way we follow the PDF reference exactly. And implementing things exactly like the PDF reference saves a lot of fuzz in the future.

I just noticed that the pseudo-code on wikipedia is actually Python. You can put this function in utils.py and apply it after interpreting P here:

pdfminer.six/pdfminer/pdfdocument.py

Line 310 in b27d3d0

self.p = int_value(self.param['P'])

.

[Recursing]

It's not an approximation, it's how two's complement is defined, see the wikipedia article

[pietermarsman]

Ah, I did not get that! Than it's fine.

[pietermarsman]

@Recursing could you review it once more and see if I made some dumb mistake?

[Recursing]

Looks fine to me, really thanks for everything!

In uint_value you don't check that the number is an int (and not a float), like int_value does, and you might want to standardize the import style ( from .pdftypes import PDFException, uint_value is the only multiple import in the same line). But they seem to be both very minor things

[pietermarsman]

Thanks for all the effort!
(And explaining to me how this works, I've learned a lot!)

[harshdonga]

Hey folks, facing a similar issue, can anybody help?
Here are the logs

~/.local/lib/python3.7/site-packages/pdfminer/pdfdocument.py in __init__(self, parser, password, caching, fallback)
    742                     id_value = (b"", b"")
    743                 self.encryption = (id_value, dict_value(trailer["Encrypt"]))
--> 744                 self._initialize_password(password)
    745             if "Info" in trailer:
    746                 self.info.append(dict_value(trailer["Info"]))

~/.local/lib/python3.7/site-packages/pdfminer/pdfdocument.py in _initialize_password(self, password)
    769         if factory is None:
    770             raise PDFEncryptionError("Unknown algorithm: param=%r" % param)
--> 771         handler = factory(docid, param, password)
    772         self.decipher = handler.decrypt
    773         self.is_printable = handler.is_printable()

~/.local/lib/python3.7/site-packages/pdfminer/pdfdocument.py in __init__(self, docid, param, password)
    356         self.param = param
    357         self.password = password
--> 358         self.init()
    359         return
    360 

~/.local/lib/python3.7/site-packages/pdfminer/pdfdocument.py in init(self)
    364             error_msg = "Unsupported revision: param=%r" % self.param
    365             raise PDFEncryptionError(error_msg)
--> 366         self.init_key()
    367         return
    368 

~/.local/lib/python3.7/site-packages/pdfminer/pdfdocument.py in init_key(self)
    377 
    378     def init_key(self) -> None:
--> 379         self.key = self.authenticate(self.password)
    380         if self.key is None:
    381             raise PDFPasswordIncorrect

~/.local/lib/python3.7/site-packages/pdfminer/pdfdocument.py in authenticate(self, password)
    427     def authenticate(self, password: str) -> Optional[bytes]:
    428         password_bytes = password.encode("latin1")
--> 429         key = self.authenticate_user_password(password_bytes)
    430         if key is None:
    431             key = self.authenticate_owner_password(password_bytes)

~/.local/lib/python3.7/site-packages/pdfminer/pdfdocument.py in authenticate_user_password(self, password)
    433 
    434     def authenticate_user_password(self, password: bytes) -> Optional[bytes]:
--> 435         key = self.compute_encryption_key(password)
    436         if self.verify_encryption_key(key):
    437             return key

~/.local/lib/python3.7/site-packages/pdfminer/pdfdocument.py in compute_encryption_key(self, password)
    412         hash.update(self.o)  # 3
    413         # See https://github.com/pdfminer/pdfminer.six/issues/186
--> 414         hash.update(struct.pack("<L", self.p))  # 4
    415         hash.update(self.docid[0])  # 5
    416         if self.r >= 4:

error: 'L' format requires 0 <= number <= 4294967295

Here is the file

Python : 3.7.10
pdfminer.six: 20220524
pdfminer : 20191125