- Download Bytecode Viewer.jar (decompiler and deassembler for APK application into JAVA).
- Open kali linux.
- To finish downloading Bytecode-viewer:
1, java -jar [file Bytecode download]
2, sudo apt install bytecode-viewer
- Usage: bytecode-viewer [file] to open bytecode
- Go APK file, then enter com/secondapp/example/Mainactivity


- Decompiler by Bytecode-viewer.

=> You can see properly that the answer starts from var4 which is created for new String.
=> Starting from "Success! CTFlearn{" by append function, then it is the addition of var2 and "_is_not_secure!}"
-> Structure:

**CTFlearn{var2_is_not_secure!}**

- Finally, we just analyze **var2.getText().toString()**
- Copy "b74dec4f39d35b6a2e6c48e637c8aedb" onto Google.
=> Var2 is encoded into this string by md5Hex().
- Answer: CTFlearn{Spring2019_is_not_secure!}