1. Introduction

Please install the given Android APK into your Android emulator. Run the app. You need a secret code to start the app. What is the secret code?

Your goal is to analyze the apk file and find the valid password.

2. Solution

Install apk file on the emulator

adb install CrackMe.apk

Test the application:

Unpack the apk file:

apktool d -f -r CrackMe.apk

Analyze the source code with jadx-gui

In the MainActiviy class there is a string called secret_code

secret_code: MDE5MjgzNzQ2NTAw

Later we can see that a base64 encoding is taking place. The input will be encoded with base64 and then be compared with the base64 encoded value of secret_code.

If the input matches I’ll get the message "Congratulations! You found the secret code."
If not "Sorry incorrect, try again." will be displayed.

Let’s try to decode the secret_code value.

echo MDE5MjgzNzQ2NTAw | base64 -d

Try to enter the pin: 019283746500

Challenge solved!