Comment 3 for bug 1375983

Hi Kovid,

More info about this issue just in case there were a real bug in it.

Grab http://www.hec.usace.army.mil/software/hec-hms/documentation/HEC-HMS_QuickStart_Guide_4.0.pdf.

You'll see that this pdf is also a password protected pdf, but you CAN extract its cover.

Look at the attached screenshots which show permissions for the conflictive Canon.pdf and for this document. (They're in Spanish but I suppose you can easily follow them. You can also directly test both files in your own installation of Adobe Reader XI).

In fact HEC-HMS.pdf restrictions seem STRONGER than Canon.pdf but in two areas:
+Canon.pdf needs Adobe X or later. HEC-HMS.pdf only needs 7.0 or later.
+Canon.pdf uses 256bit AES encryption, while HEC-HMS.pdf only uses 128bit.

So I can see two possible scenarios:
+Everything is working as expected. The newest flavour of the Acrobat protection actually prevents and will always prevent Calibre from getting the cover.
+Calibre pdf cover extractor is OUTDATED. I mean there's nothing in the pdf which blocks the Cover extraction but an old version of the Calibre built-in pdf renderer which cannot handle that new Adobe X procedure or 256bit AES key. A "simple" (between quotes) update of this part of the software would allow the cover extraction without further problems.

(I'm posting this update because we have clearly detected that this conflictive Canon.pdf directly crashes and reboots Kobo eReaders or even Nitro PDF Viewer: http://www.mobileread.com/forums/showthread.php?p=2953686#post2953686).