caprinux

TLDR

We are provided with an APK file, which can be statically decompiled via tools like JADX or JEB.

From the APK file, we can obtain the native library that is used to validate the 20 character passcode (between 0x0 - 0xF)

We can reverse the passcode checking function to brute force many possible passcode combinations.

We can use each of this combination to decrypt the ciphertext, one will finally work to give us the flag.

caprinux

GeekCTF Writeups

Pwn

Memo0

Upon decompiling the program, we see that the bulk of the code is in the login() function.

We can see that our input is encoded, then compared