How secure is "Save as PDF" with encryption option in Word 2010?

Question

I'm already aware that Microsoft employs AES 128-bit advanced encryption in Word 2010 if someone wants the Word document itself password-protected. I don't. I want the PDF version of my Word document password-protected. Word 2010 has the ability to create a password-protected PDF file from the document by clicking "Options" from the "Save as" pop-up:

Others on SuperUser have pointed out that different versions of Acrobat have different implementations of password encryption, and I have no clue how Word 2010 is doing it. I just want to know if the password protection Word 2010 offers as part of its "Save as PDF" option is as secure or better than what it makes available to password-protect the Word document itself. If someone can find it in Microsoft's MSDN documentation I'd appreciate it!

Answer 1

How secure is “Save as PDF” with encryption option in Word 2010?

Plan cryptography and encryption settings for Office 2010 discusses the encryption settings for Office 2010 products, and as you already point out that includes AES 128-bit advanced encryption.

That particular link does not indicate specifically that this is used for the Save as PDF option.

However, there is a discussion on exactly this topic at What level of encryption does Word use for password protected PDFs? where the accepted answer is:

The link (Plan cryptography and encryption settings for Office 2010 ) is very helpful, but it only seems to confirm that .docx is encrypted using 128-bit AES by default.

However, after looking into the encrypted .docx and PDF documents via a code editor, they seem to carry the same 128-bit AES signature string which confirms to me that they're using the same encryption standard. So I'm going to mark this as the answer to my question.

So there is confirmation by inspection that 128-bit AES is also used for Save as PDF.

Answer 2

Since the document has to work with any copy of acrobat reader, even if the user did not have any MS Office products installed, the cipher implementation must conform to PDF standards, not MS standards.

Word2010 uses PDF standard 1.6 (just save the document, open it in foxit, and look at the file properties to confirm the spec version), so it uses a combination of AES128, PKCS#7, and DSA (for signature), with hashing in SHA384 or 512. In this case since you are password encrypting (eg it is symetric), that indicates AES128.