CodeBreaker2018 walkthrough, Task 2 Secrets

January 4, 2019 — 2 min read

Let's look at Task 2 before doing Task 1. It is a much quicker and easier task, and it is a good warm-up practice in reverse engineering before we dive into the much more complicated Task 1.

In this task we are asked to extract a secret key from the binaries left over by the ransomware. To solve this we need to use some reverse engineering tools.

The organizers of recommend IDAPro or Binary Ninja. IDAPro is indeed an excellent tool, but unfortunately in order to get the full functionality you have to buy the complete version which is quite pricey. I have never used the Binary Ninja, looks like a good tool, but also expensive.

To keep this accessible to everyone I have opted for the free tools Dissasembler.io and Radare

The first one is a web application, you don't need anything to install. The second one is an open source tool which is a package with many distributions. For example, on Mac you can use the excellent Homebrew package manager to install it.

brew install radare2

Radare will come in handy for Task 1. In this task we'll stick with Dissasembler.io.

Let's start.

Download libclient_crypt.so

Open Dissasembler.io in your browser
Click on the start disassembling button that looks like this.
Once the app loads, upload your libclient_crypt.so file.
After a short time your file will be disassembled and you'll see the screen like this, with the symbols pane on the left and the code pane on the right.
Scroll all the way down on the symbols pane.
Most functions names are self-explanatory, except for the one that is called func_00001930, this looks suspicious. Click on it's name to see the code.
These MOVB instructions look interesting. Seems like this function is writing a sequence of byte values to a memory location.
So, what's can we say about these bytes? We see that these values range between 0x30 and 0x59, which covers digits 0 and uppercase letters.
Let's refresh our memory about Base32 encoding here and here.
We see that the most common form of Base32 encoding uses uppercase letters A-Z and numbers 0-9.
Most likely this is what we want.
Let's convert these hex values to their ASCII representation to get the string CAYPFE6MG2DJT4EB5RIZLIAYFJAUGL3L
Let's submit it to the challenge
Voila!

Additional reading materials

If you want to find out more about the way we identified our suspect function so quickly, check out these reading materials.

Very comprehensive series of posts about linkers and shared libraries https://www.airs.com/blog/page/4?s=linkers
Linkers and loader by John R. Levin -- very comprehensive overview of linkers, loaders and shared libraries. Somewhat dated, but things haven't changed that much since 2000

Let's move on to Task 1