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

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# Google CTF 2018 - Shall We Play a Game?
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%20%28322%29.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%20%28262%29.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:
```text
if-ne v0, v9, :cond_2
```
to:
```text
if-eq v0, v9, :cond_2
```
![Before](../../.gitbook/assets/image%20%28204%29.png)
![After](../../.gitbook/assets/image%20%28329%29.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%20%28284%29.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%20%28167%29.png)
**Another way** is instead of comparing with 1000000, set the value to 1 so this.o is compared with 1:
![](../../.gitbook/assets/image%20%2811%29.png)
A forth way is to add an instruction to move to value of v9\(1000000\) to v0 _\(this.o\)_:
![](../../.gitbook/assets/image%20%28115%29.png)
![](../../.gitbook/assets/image%20%28238%29.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%20%28102%29.png)
You need to do this inside a physical device as \(I don't know why\) this doesn't work in an emulated device.