PDF encryption: how to encrypt PDF files without Acrobat or passwords

How to Unlock a Password-Protected PDF?

PDFs have two types of password security: the permissions password (or editing password), which users need to edit and print a document, and the owner password (or viewing password), which is required to open the PDF and is used as the key to encrypt it.

  How to remove a permissions password from a PDF

The permissions password is trivial to remove because of the way it was implemented – it’s only the PDF editing software that enforces the publisher’s security settings.   This allows for various removal methods, from using a free online tool to opening the document using a program like Microsoft word to convert from PDF to another file format.

  Removing a permission password with an online tool

Using an online tool is the most convenient for the majority of users.  Because these tools market themselves as a way to unlock PDF files that you have forgotten the password to, they’re entirely legal.

All you have to do is upload your file to one of the many online PDF removal tools and press a PDF convert or remove button to strip its permissions.  Here’s an example:

  Removing a permission password with the Microsoft Word PDF convert tool

You can also unlock a PDF that requires a password to open using Microsoft Word.  When you try to open a PDF in Word, it will automatically ask if you want it to perform a PDF conversion so that it can open it.

Pressing “OK” to the PDF conversion will open it in Microsoft Word, where you can edit it to your heart’s content or convert it to PDF again and modify it in a PDF editing software.  Though the PDF convert can take some time on large documents, it usually only takes a minute or two.

You can similarly enable the PDF convert option in Google Docs.  When you upload the protected document to Google Docs it will convert it from PDF to a format readable by Google’s text-editor.  When you then open the PDF you can edit it or convert it back to a PDF.

  How to Unlock a PDF protected with an owner password

The owner password or viewer password is a little more difficult to remove.  Of course, the easiest way is to simply know the password.  If a PDF requires a password to open you can simply enter the password like so:

If you don’t know the owner password, you still have some options.  You can ask somebody for it, try to guess it, or brute force, key search, or dictionary attack it with a software like Elcomsoft.  Cracking an owner password with Elcomsoft is as easy as loading the PDF file in the software and pressing “Start”.

The software offers various attack methods:

• Brute force: tries every combination of numbers and letters until one works
• Dictionary: Uses a provided list of common passwords and tries the most likely variant of each.
• Key search: The software tries all possible RC4 encryption keys until it finds the correct one. This is 100% effective and does not require a password to be cracked.  Note that Adobe depreciated the use of RC4 in 2020 as it was no longer considered secure, but some PDF encryption software may still provide this as an option. If they do, you would be unwise to select it since AES is NIST approved and much more secure.

Sophisticated tools such as this mean that, regardless of your password security level, a determined user can gain access to your document.  When you password protect a PDF with an owner password you’re only putting a time gate on the document – however long it takes the tool to guess the password or find the right RC4 key.

As we mentioned earlier, the difficulty of managing passwords means most won’t be particularly complex and therefore won’t take months or years to brute force.  In the cases where they are, however, a key search can remove the owner password in 3-4 days on a modern computer (provided the key length is 40 bits). If AES 128 or 256-bit encryption has been used then you might be in for a long wait.