Solution - UnCrackable App for Android Level 2

 See challenge here

Use dex2jar tool to extract source code.

$~/Downloads/dex-tools-2.1-SNAPSHOT/d2j-dex2jar.sh UnCrackable-Level1.apk

Use JD-GUI to review the source code

$java -jar ~/Downloads/jd-gui-1.6.6.jar

Now we know the check is to send string into static library libfoo.so.

Unzip apk to see lib/ folder

$tree lib

lib

├── arm64-v8a

│   └── libfoo.so

├── armeabi-v7a

│   └── libfoo.so

├── x86

│   └── libfoo.so

└── x86_64

    ├── libfoo.so

If you're a lazy person like me, I use strings first to see if anything comes up.

$strings lib/x86_64/libfoo.so

"Thanks for all t" maybe our flag to capture, but didn't pass entering to the App. Looks like the ending part is missing. So, use Ghidra or IDA free or Hopper Diassembler to see the bar function

Java_sg_vantagepoint_uncrackable2_CodeCheck_bar

some ascii code strings were found at the strncpy.

The second parameter is our flag to capture

 "Thanks for all the fish"