Encrypt only file attachments

I'm trying to extract file attachments from pdf documents, using my own
coded tool, and all works fine when file is not protected by password
protection or, if protected, is not in use the "Encrypt only file
attachments" option. With this option only the embedded file stream
objects are encrypted but it seems, in this case, Adobe use a different
encrypt algorithm or method to calculate the encrypt/decrypt key not referenced in the pdf format specifications.
Anyone can confirm this or know what can be happening?
Thanks