Trousse social

The second (and the last) android challenge !

Same start that the other one, we open the .apk in jadx.

WOW ! That’s a lot of directories ! When can see the com folder, in it we can found the MainActivity of the application, but the code hasn’t decompiled very well, as you can see :

Let’s get back in the context → It’s 4am, I’m tired, i don’t want to reverse this. So i tried to get a .jar from the .apk .

Getting a .jar

I used dex2jar : like this

1

$ sh dex2jar-2.0/d2j-dex2jar.sh trousse_social.apk

I opened it in jd-gui , looked like this :

As you can see, it’s much better. Our input str1 is xored with “IMTHEBOSS”, then each char (his charCode ) is multplied by 2 and 5 is added , finaly each char compared with an array of charCodes.

I made a little python script to reverse this

And we got the flag : MCTF{FuCk_Tw1tt3R}

Conclusion

Also a nice challenge, he gets really easier when converter to a jar and passed through jd-gui

Hope you like this little writeup !