Challenge: Santa has a secret switch that blows his sleigh if anyone steals them. It’s nor near the halfway across the globe, but the timer is coming to an end. Will you be able to save Xmas?

First challenge that had a file attached: 2019_12_03.zip inside RunRunRudolph45.exe

Just by looking at it you could see it’s a .NET exe. I used https://github.com/icsharpcode/ILSpy to decompile it. The nice thing about this tool is that it also decompiles baml files back to xaml.

Poking a bit around the code I quickly found the core of the challenge.

The SuperSecretKey was in the mainwindow.xaml and it was, guess, “SuperSecretKey”!

In any case, in the code you can see, that the check does not modify your input, but rather decrypts the cyphertext. Now either statically reverse engineer the decryption code, or just bang together all nedded parts of the code to print out the decrypted text. Since I wasn’t working on my dev PC and did not have either VSCode or VS installed, I used https://dotnetfiddle.net/ and here is the snip: https://dotnetfiddle.net/mGB7SU

Flag: xmas{Santas_gOt_tO_m4ke_1t_T0_town}

What did I learn: There are a lot of online tools!