66.13 PDF Password Protect


An individual pdf file can be password encrypted to avoid trivial access. Two passwords are available, a user and owner (aka master or permissions) password. Here we set them both to be the same. The key length for encryption is also specified, with possible lengths being 40, 128, and 256. We choose 256 which is the PDF 2.0 encryption method and is supported by Acrobat X. The -- stops processing command line options. Two filenames are expected, the input pdf and the output pdf.

qpdf --encrypt <password> <password> 256 -- mydoc.pdf mydoc_enc.pdf

For more information read the docs or review the built-in help:

qpdf --help=encryption

A pdf can have two passwords. The user password is used to encrypt the document and is generally difficult to crack. The owner password is simply used to block printing and editing, for example, and depends on the software to respect it.

There are ways to decrypt without the password but they are not trivial. A brute force attack would try every possible password which is quite time-consuming and computationally expensive for long and complex passwords. A dictionary attack tries to reduce compute time using a list of commonly used passwords. Older pdf encryption methods (such as RC4) have vulnerabilities that can be exploited. AES is more secure. And there are tools that can attempt to crack passwords.

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