BSidesSF 2019 CTF - blink(101/Mobile 50)

(1)First of all, extract the blink.apk by using 7zip.
(2)Check the extracted files. I found classes.dex.

(3)Get the dex2jar from following site.
　https://jaist.dl.sourceforge.net/project/dex2jar/dex2jar-2.0.zip

(4)Convert classes.dex to jar by using dex2jar.

   d2j-dex2jar.bat classes.dex
　->classes-dex2jar.jar is created.
(3)Extract the classes-dex2jar.jar by using 7zip.

(4)Looking for anythin under the extracted folder.

   classes-dex2jar\com\example\blink
　I found suspicious file "r2d2.class" under this folder.
(5)r2d2.class contains the Base64 text.

     It is the Base64 encoded JPEG image.

(6)Use the CyberChef to decode encoded image, and save it to file.

    https://gchq.github.io/CyberChef/

    CyberChef is Hanpa-nai.

I got the flag.
CTF{PUCKMAN}