Category D, time to think outside the box again. The question was straightforward :
"What is the hidden number in this file?"
The file accompanying the question was a little bit more complicated, which could be expected, given it was a level 300 challenge. Knowing that Didier Stevens, yes the evil PDF doctor, was on the THF team should have given you a strong hunch to the solution. Let's see what we got.
At first sight, this looked like an ordinary pdf, containing a message and a barcode :
What? Didier wants us to go to the nearest supermarket to get the solution ? For sure he doesn't, he's much crueler than that :-D
Using a tool like bcTester, we are able to retrieve what the barcode represents :
Uyy uspsvn ecqf zm c bhnsyt: tvwv ykuuu fhg tbvi tgfb uylgs
Allrightie ... what does this mean? It's obviously cyphertext, but what cypher did we use? And more importantly ... what was the key ?
Some of the contestants went at it and tried to find the cypher and the key themselves. They lost a lot of time :-) Looking at the PDF file you could see that there was more there than you would initially see in a regular PDF reader. Didier created a PDF file with incremental updates and as such, there was another barcode hidden in the file. This barcode revealed the golden nugget you were looking for :
Encryption Vigenere Key BRUCON
And thus you could decrypt the cyphertext, to finally find the solution:
The secreT cOde iS a NUmber: five eight ONe fOuR ZerO thRee
Thank you to all who competed in 2009. The rankings for BruCON in 2009 are here and for SANS in London are here.
We look forward to serving you a new load of exciting, mindboggling, sometimes devious, sometimes plain wicked challenges at Brucon in 2010!