hacktricks/mobile-pentesting/android-app-pentesting/google-ctf-2018-shall-we-play-a-game.md

# Google CTF 2018 - Shall We Play a Game?

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Download the APK here:

I am going to upload the APK to [https://appetize.io/](https://appetize.io) (free account) to see how the apk is behaving:

![](<../../.gitbook/assets/image (46) (1).png>)

Looks like you need to win 1000000 times to get the flag.

Following the steps from [pentesting Android](./) you can decompile the application to get the smali code and read the Java code using jadx.

Reading the java code:

![](<../../.gitbook/assets/image (47).png>)

It looks like the function that is going print the flag is **m().**

## **Smali changes**

### **Call m() the first time**

Lets make the application call m() if the variable _this.o != 1000000_ to do so, just cange the condition:

```
 if-ne v0, v9, :cond_2 
```

to:

```
 if-eq v0, v9, :cond_2 
```

![Before](<../../.gitbook/assets/image (48) (1).png>)

![After](<../../.gitbook/assets/image (49) (1).png>)

Follow the steps of [pentest Android](./) to recompile and sign the APK. Then, upload it to [https://appetize.io/](https://appetize.io) and lets see what happens:

![](<../../.gitbook/assets/image (50).png>)

Looks like the flag is written without being completely decrypted. Probably the m() function should be called 1000000 times.

**Other way** to do this is to not change the instrucction but change the compared instructions:

![](<../../.gitbook/assets/image (55).png>)

**Another way** is instead of comparing with 1000000, set the value to 1 so this.o is compared with 1:

![](<../../.gitbook/assets/image (57) (1).png>)

A forth way is to add an instruction to move to value of v9(1000000) to v0 _(this.o)_:

![](<../../.gitbook/assets/image (58) (1).png>)

![](<../../.gitbook/assets/image (52) (1).png>)

## Solution

Make the application run the loop 100000 times when you win the first time. To do so, you only need to create the **:goto\_6** loop and make the application **junp there if \_this.o**\_\*\* does not value 100000\*\*:

![](<../../.gitbook/assets/image (59).png>)

You need to do this inside a physical device as (I don't know why) this doesn't work in an emulated device.

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