How to decrypt encrypted PDF files
JOHN August decided to conduct an experiment. He was curious about how easy it is to crack encryption on one simple document using a fairly standard engine. So John made two files, one “simple” and one “hard”. John wondered if there were any practical ways to overcome these locks.
Didier Stevens wrote a series of articles on “How to crack encrypted PDF files.” This article provides an answer to the question posed by John Augustus. Didier explains how he deciphered the “simple” encrypted PDF files in the first part. It uses the same steps as for the “simple” PDF file to crack the “hard” PDF in the second part of the article.
The third part was published on December 28, in which he executed a brute attack on the encrypted PDF password and brute-force attack on the key (the other) encrypted PDF, both PDFs are part of the problem published by John August. Didier writes that different PDF files, encrypted with the same user password, will have different encryption keys.
You can find more information about this study in this article.