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Comment by mvelbaum | Hacker News Reader

Hmm, I actually have an old password protected PDF (pass is prolly around ~10 chars, letters+digits) whose password I forgot.

Are there legit services offering brute-force cracking? How long would it take, and how much it would it cost?

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View the source of the pdf, find the trailer dictionary (ctrl-f "/Encrypt"), and note the object number of the encryption dictionary. An example trailer below, here the encryption dictionary happens to be object ID 94,0

    % Trailer dictionary
    trailer
    <<
        /Size 95         % number of objects in the file
        /Root 93 0 R     % the page tree is object ID (93,0)
        /Encrypt 94 0 R  % the encryption dict is object ID (94,0)
        /ID [<1cf5...>]  % an arbitrary file identifier
    >>

Use the object id to find the encryption dictionary:

    % Encryption dictionary
    94 0 obj
    <<
        /Filter /Standard   % use the standard security handler
        /V 1                % algorithm 1
        /R 2                % revision 2
        /U (xxx...xxx)      % hashed user password (32 bytes)
        /O (xxx...xxx)      % hashed owner password (32 bytes)
        /P 65472            % flags specifying the allowed operations
    >>
    endobj

Generally, if /V and /R are both 4 or less, you can find tools to crack it yourself on a normal PC.

More info on the values of /V and /R here: https://qpdf.readthedocs.io/en/stable/encryption.html

This is what I get:

  20 0 obj
  <<
      /R 4
      /O (...)
      /U (...)
      /P -1548
      /Length 128
      /V 4
      /EncryptMetadata true
      /Filter /Standard
      /StmF /StdCF
      /StrF /StdCF
      /CF <<
      /StdCF <<
      /AuthEvent /DocOpen
      /CFM /AESV2
  >>>>>>
  endobj

I tried a bunch of tools, including https://www.elcomsoft.com/apdfpr.html?src=prog_apdfprp to get it to quickly remove the password, but nothing worked :/

Ah, that's the exception to my "generally" comment. That "/CFM /AESV2" means AES-128, instead of the much weaker RC4...which would be if you saw "/CFM /V2".

So you're left with brute force or dictionary attacks for your 10 char A-Za-z0-9 password.

thanks! :)

You may not need to crack the password. https://smallpdf.com/unlock-pdf for example. Here is a PDF password cracker: https://www.lostmypass.com/file-types/pdf/

I'm very wary downloading and running software on my computer in general, but I'd be even moreso with software from some unknown company providing a hacking tool.

Definitely better to do this in a VM, but then your performance would take a hit

I tried the free tools already -- none seemed to work :(

Is it purely lowercase? Someone gave an example of how to do this with 10 characters a-z 0-9 a few years ago: https://nicholaslyz.com/blog/2021/07/23/cracking-pdf-hashes-...

Also, is the password completely random? If it's not, you'd have a much easier time using a large dictionary rather than every possible 10-character word.

As in the article, it looks like on a consumer GPU you'd get on the order of 10MH/s, or 10 million hashes per second. I need someone to check my math on this, but 36**10/10_000_000 GPU-seconds (36 characters, optimistically assuming [a-z0-9] lowercase) is about 4000 GPU-days, so out of reach. The A100 attempt from the article only made it about 15x faster, so you'd still need to wait a few months at best. (This is with the assumption you know it's pure lowercase.)

Can someone confirm my math on this?

This question, albeit for a longer password at 20 chars, was discussed at [0].

[0] https://security.stackexchange.com/questions/61346/how-long-...

10 chars doesn't seem impossible to me (just a feeling). Don't about such services.

Keep in mind 10 character spaces should be represented by the selection pool size (A-Z = 26, a-z = 26, 0-9 = 10, so 26+26+10=62). You now can say 62^10 = total possible guesses available. Certainly you can start to make intelligent decisions on guessing properties and priorities to reduce your time.

Also I didn't discuss entropy here in order to represent it more basically for the parent comment.

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