Ilam CTF has been hold on 23rd Nov 2018. Unfortunately I’ve planned other things for 22-23 Nov 2018 and because of the delay in holding the CTF, I couldn’t attend this CTF.
However, I could download the Android Reverse question for future analysis. And the flag is here:
But the WriteUP!
Start to Android Reverse WriteUp
I’ve installed it on an emulator and the application crashed.
I’ve analyzed the crash logs and found an arithmetic exception at MainActivity:
After that I’ve found that the key was hidden in some functions, in which they are playing with an initial String (aWxhbV9jdGZfM).
Then I just copied the code of functions to my Java editor and started to play with them.
The result was something like this:
As you see, the results from f3 start to change in characters. I mean, they all looks like base64 till f3 but then, they change to something odd.
I’ve checked the functions. All the functions change a part of input string and add a new part to it. I though that it was an encrypted string but I did not found any cryptographic function call or even dynamic call inside the code. Then the Key should be Base64 Encoded!
As a simple test, I just put the parts together and decoded them.
Voila. The flag was in there:
There was just a point. They have added an extra part to the base64 encoded text which I removed it. You can find the extra part at Yoyo.yo function which ends with an arithmetic exception.
Therefore, the flag is:
The original APK is attached here for who may interest.