TL;DR: This post has a lot of details. Skip to the Summary if you know the challenge and are here just for the solution.
Door icon made by Freepik from www.flaticon.com is licensed by CC 3.0 BY. Between Christmas and New Year’s I attended the 35th Chaos Communication Congress (CCC), 35C3, in Leipzig, Germany, together with Malmö based Xil hackerspace. It was my third congress (in a row).
Since 2012 there has been a Capture The Flag (CTF) competition at congress.