# 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:

![](<../../images/image (421).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:

![](<../../images/image (495).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](<../../images/image (383).png>)

![After](<../../images/image (838).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:

![](<../../images/image (128).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:

![](<../../images/image (840).png>)

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

![](<../../images/image (629).png>)

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

![](<../../images/image (414).png>)

![](<../../images/image (424).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 **jump there if `this.o`** does not value 100000:

![](<../../images/image (1090).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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