"We found this suspicious application on a burner phone. The user claims there is a hidden flag inside, but the 'Unlock' button doesn't seem to work. Find the flag."
Files provided:
If you are determined to download an APK from an archive site while searching for "apk4f 2021," follow these rules:
We have two ways to solve this:
We can calculate the flag ourselves using Java or Python since we have the logic. apk4f 2021
Calculating the Java hashCode:
In Java, the hashCode() of a String is calculated as:
$$s[0] \times 31^n-1 + s[1] \times 31^n-2 + ... + s[n-1]$$
We can quickly calculate this using a Python snippet:
s = "SuperSecretKey2021" # Java's String hashCode algorithm h = 0 for c in s: h = (31 * h + ord(c)) & 0xFFFFFFFF # Handle 32-bit signed integer overflow if h >= 0x80000000: h -= 0x100000000
print(f"CTFh_android_reversing")
Alternatively, running String.valueOf("SuperSecretKey2021".hashCode()) in any Java REPL yields the hash.
Let's assume the hash calculated is -123456789 (example).
The Flag:
CTF-123456789_android_reversing
(Note: The actual hash value for "SuperSecretKey2021" would be calculated during the CTF execution. In many CTFs, the flag is simply hardcoded in a string resource or a native .so library. If the flag was in a native library, the approach would shift to using Ghidra or IDA Pro to analyze the C/C++ code.) "We found this suspicious application on a burner phone
The logic is straightforward:
However, a responsible review must highlight why using APK4F was—and remains—highly risky. These factors significantly outweigh the convenience:
If you are looking for a file named apk4f_2021.apk or similar: