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67 lines
2.1 KiB
Markdown
67 lines
2.1 KiB
Markdown
# Google CTF 2018 - Shall We Play a Game?
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Download the APK here:
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I am going to upload the APK to [https://appetize.io/](https://appetize.io) (free account) to see how the apk is behaving:
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.png>)
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Looks like you need to win 1000000 times to get the flag.
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Following the steps from [pentesting Android](./) you can decompile the application to get the smali code and read the Java code using jadx.
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Reading the java code:
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.png>)
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It looks like the function that is going print the flag is **m().** 
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## **Smali changes**
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### **Call m() the first time**
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Lets make the application call m() if the variable _this.o != 1000000_ to do so, just cange the condition:
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```
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if-ne v0, v9, :cond_2
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```
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to:
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```
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if-eq v0, v9, :cond_2
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```
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.png>)
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.png>)
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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:
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.png>)
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Looks like the flag is written without being completely decrypted. Probably the m() function should be called 1000000 times.
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**Other way** to do this is to not change the instrucction but change the compared instructions:
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.png>)
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**Another way** is instead of comparing with 1000000, set the value to 1 so this.o is compared with 1:
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.png>)
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A forth way is to add an instruction to move to value of v9(1000000) to v0 _(this.o)_:
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.png>)
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.png>)
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## Solution
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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**:
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.png>)
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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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