categories: code

This is the title of a great talk at the last chaos communication congress in berlin (27c3).

When writing my own pdf parser for a homework assignment that I put way too much ambition into, I encountered all of what is mentioned in that talk and was also able to realize how bad the situation really is. I just deleted three paragraphs of this post where I started to rant about how frickin bad the pdf format is. How unworkable it is and how literally impossible to perfectly implement. But instead, just watch the video of the talk and make sure to remember that it is even worse than Julia Wolf is able to make clear in 1h she was given.

So after I stopped myself from spreading another wave of my pdf hate over the internets lets look at the issue at hand:

I wanted to sign up online for a new contract with my bank. One of the requirements was, that I entered a number code that was supposedly only accessible to me once I printed out a pdf document. You heard me right - the pdf document only contained a gray box where the number was supposed to be and only upon printing it, it should reveal itself. I still have no clue how this is supposed to work, but assume it is some weird javascript (yes pdf can contain javascript) or proprietary forms extension. Or maybe even flash (yes, the acrobat reader contains an implementation of flash). Or it might just be native bytecode (yes, you can put native, platform specific bytecode into a pdf that the reader will then execute for you - isnt it great?). Needless to say that no pdf renderer I had at hand (I tried poppler based programs and mupdf) was able to give me the number - even when trying to print it where the magic was supposed to happen. So when I was already down to setting up a qemu instance to install windows xp so that I could install acrobat reader to finally open the document and print it to another pdf so that I could see that number, I thought again and wrote some additional code to my pdf parser that allowed me to investigate that pdf more thoroughly. And indeed, just by chance, I spotted a number in the annotation area of the document which looked just like the six digit number I needed. Tried it and voila it worked.

This is the snippet I uncompressed from the pdf to (just by chance) find the number I was looking for. The 000000 piece was actually containing the number I needed.

6 0 obj
/DA (/Arial 14 Tf 0 g)
/Rect [ 243.249 176.784 382.489 210.297 ]
/FT /Tx
/MK <<
/BG [ 0.75 0.75 0.75 ]
/Q 1
/P 4 0 R
/AP <<
/N 7 0 R
/V (000000)
/T (Angebotskennnummer)
/Subtype /Widget
/Type /Annot
/F 36
/Ff 1

So let me say: WTF? My bank not only requires me to resort to one specific pdf implementation (namely the acrobat reader by adobe) but also requires me to pay to a US based company first to have an operating system that reader software works on? Or am I really supposed to go through the raw pdf source by hand?? Bleh...

Also, dont ask for my code - it's super dirty and unreadable. Instead look at the mupdf project. It supplies a renderer which is massively superior to poppler in terms of speed (even suitable for embedded devices) and comes with a program called pdfclean which does the same thing my program did so that I was able to get the number I needed.

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