3 ms·

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

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?

mikequinlan 3y ago
You may not need to crack the password. https://smallpdf.com/unlock-pdf https://smallpdf.com/unlock-pdf for example. Here is a PDF password cracker: https://www.lostmypass.com/file-types/pdf/ https://www.lostmypass.com/file-types/pdf/
- mvelbaum 3y ago
  I tried the free tools already -- none seemed to work :(
- pcthrowaway 3y ago
  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
Etheryte 3y ago
This question, albeit for a longer password at 20 chars, was discussed at [0]. [0] https://security.stackexchange.com/questions/61346/how-long-would-it-take-to-bruteforce-an-aes-128-protected-pdf-knowing-the-key-is https://security.stackexchange.com/questions/61346/how-long-...
ycuser2 3y ago
10 chars doesn't seem impossible to me (just a feeling). Don't about such services.
- kingforaday 3y ago
  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.
tyingq 3y ago
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 https://qpdf.readthedocs.io/en/stable/encryption.html
- mvelbaum 3y ago
  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 https://www.elcomsoft.com/apdfpr.html?src=prog_apdfprp to get it to quickly remove the password, but nothing worked :/
  - tyingq 3y ago
    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.
    - mvelbaum 3y ago
      thanks! :)
btdmaster 3y ago
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-with-hashcat/ 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?

mikequinlan 3y ago

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

mvelbaum 3y ago
I tried the free tools already -- none seemed to work :(
pcthrowaway 3y ago
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

Etheryte 3y ago

This question, albeit for a longer password at 20 chars, was discussed at [0]. [0] https://security.stackexchange.com/questions/61346/how-long-would-it-take-to-bruteforce-an-aes-128-protected-pdf-knowing-the-key-is https://security.stackexchange.com/questions/61346/how-long-...

ycuser2 3y ago

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

kingforaday 3y ago
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.

tyingq 3y ago

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 https://qpdf.readthedocs.io/en/stable/encryption.html

mvelbaum 3y ago
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 https://www.elcomsoft.com/apdfpr.html?src=prog_apdfprp to get it to quickly remove the password, but nothing worked :/
- tyingq 3y ago
  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.
  - mvelbaum 3y ago
    thanks! :)

btdmaster 3y ago

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-with-hashcat/ 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?