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Sometimes it is interesting to modify the application code to access hidden information for you (maybe well obfuscated passwords or flags). Then, it could be interesting to decompile the apk, modify the code and recompile it.

**Opcodes reference:** [http://pallergabor.uw.hu/androidblog/dalvik\_opcodes.html](http://pallergabor.uw.hu/androidblog/dalvik\_opcodes.html)

# Fast Way

Using **Visual Studio Code** and the [APKLab](https://github.com/APKLab/APKLab) extension, you can **automatically decompile**, modify, **recompile**, sign & install the application without executing any command.

# Decompile the APK

Using APKTool you can access to the **smali code and resources**:

```
apktool d APP.apk
```

If **apktool** gives you any error, try[ installing the **latest version**](https://ibotpeaches.github.io/Apktool/install/)

Some **interesting files you should look are**:

* _res/values/strings.xml_ (and all xmls inside res/values/\*)
* _AndroidManifest.xml_
* Any file with extension _.sqlite_ or _.db_

If `apktool` has **problems decoding the application** take a look to [https://ibotpeaches.github.io/Apktool/documentation/#framework-files](https://ibotpeaches.github.io/Apktool/documentation/#framework-files) or try using the argument **`-r`** (Do not decode resources). Then, if the problem was in a resource and not in the source code, you won't have the problem (you won't also decompile the resources).

# Change smali code

You can **change** **instructions**, change the **value** of some variables or **add** new instructions. I change the Smali code using [**VS Code**](https://code.visualstudio.com), you then install the **smalise extension** and the editor will tell you if any **instruction is incorrect**.\
Some **examples** can be found here:

* [Smali changes examples](smali-changes.md)
* [Google CTF 2018 - Shall We Play a Game?](google-ctf-2018-shall-we-play-a-game.md)

Or you can [**check below some Smali changes explained**](smali-changes.md#modifying-smali).

# Recompile the APK

After modifying the code you can **recompile** the code using:

```bash
apktool b . #In the folder generated when you decompiled the application
```

It will **compile** the new APK **inside** the _**dist**_ folder.

If **apktool** throws an **error**, try[ installing the **latest version**](https://ibotpeaches.github.io/Apktool/install/)

## **Sing the new APK**

Then, you need to **generate a key** (you will be asked for a password and for some information that you can fill randomly):

```bash
keytool -genkey -v -keystore key.jks -keyalg RSA -keysize 2048 -validity 10000 -alias <your-alias>
```

Finally, **sign** the new APK:

```bash
jarsigner -keystore key.jks path/to/dist/* <your-alias>
```

## Optimize new application

**zipalign** is an archive alignment tool that provides important optimisation to Android application (APK) files. [More information here](https://developer.android.com/studio/command-line/zipalign).

```bash
zipalign [-f] [-v] <alignment> infile.apk outfile.apk
zipalign -v 4 infile.apk
```

## **Sign the new APK (again?)**

If you **prefer** to use \[**apksigner**]\(**[https://developer.android.com/studio/command-line/apksigner](https://developer.android.com/studio/command-line/apksigner)**)** instead of jarsigner, **you should sing the apk** after applying **the optimization with** zipaling**. BUT NOTICE THAT** YOU ONLY HAVE TO SIGN THE APPLCIATION ONCE** WITH jarsigner (before zipalign) OR WITH aspsigner(after zipaling).

```bash
apksigner sign --ks key.jks ./dist/mycompiled.apk
```

# Modifying Smali

For the following Hello World Java code:

```
public static void printHelloWorld() {
    System.out.println("Hello World")
}
```

The Smali code would be:

```
.method public static printHelloWorld()V
    .registers 2
    sget-object v0, Ljava/lang/System;->out:Ljava/io/PrintStream;
    const-string v1, "Hello World"
    invoke-virtual {v0,v1}, Ljava/io/PrintStream;->println(Ljava/lang/String;)V
    return-void
.end method
```

The Smali instruction set is available [here](https://source.android.com/devices/tech/dalvik/dalvik-bytecode#instructions).

## Light Changes

## Modify initial values of a variable inside a function

Some variables are defined at the beginning of the function using the opcode _const_, you can modify its values, or you can define new ones:

```
#Number
const v9, 0xf4240
const/4 v8, 0x1
#Strings
const-string v5, "wins"
```

## Basic Operations

```
#Math
add-int/lit8 v0, v2, 0x1 #v2 + 0x1 and save it in v0
mul-int v0,v2,0x2 #v2*0x2 and save in v0

#Move the value of one object into another
move v1,v2

#Condtions
if-ge #Greater or equals
if-le #Less or equals
if-eq #Equals

#Get/Save attributes of an object
iget v0, p0, Lcom/google/ctf/shallweplayagame/GameActivity;->o:I #Save this.o inside v0
iput v0, p0, Lcom/google/ctf/shallweplayagame/GameActivity;->o:I #Save v0 inside this.o

#goto
:goto_6 #Declare this where you want to start a loop
if-ne v0, v9, :goto_6 #If not equals, go to: :goto_6
goto :goto_6 #Always go to: :goto_6
```

## Bigger Changes

## Logging

```
#Log win: <number>
iget v5, p0, Lcom/google/ctf/shallweplayagame/GameActivity;->o:I #Get this.o inside v5
invoke-static {v5}, Ljava/lang/String;->valueOf(I)Ljava/lang/String; #Transform number to String
move-result-object v1 #Move to v1
const-string v5, "wins" #Save "win" inside v5
invoke-static {v5, v1}, Landroid/util/Log;->d(Ljava/lang/String;Ljava/lang/String;)I #Logging "Wins: <num>"
```

Recommendations:

* If you are going to use declared variables inside the function (declared v0,v1,v2...) put these lines between the _.local \<number>_ and the declarations of the variables (_const v0, 0x1_)
* If you want to put the logging code in the middle of the code of a function:
  * Add 2 to the number of declared variables: Ex: from _.locals 10_ to _.locals 12_
  * The new variables should be the next numbers of the already declared variables (in this example should be _v10_ and _v11_, remember that it starts in v0).
  * Change the code of the logging function and use _v10_ and _v11_ instead of _v5_ and _v1_.

## Toasting

Remember to add 3 to the number of _.locals_ at the begging of the function.

This code is prepared to be inserted in the **middle of a function** (**change** the number of the **variables** as necessary). It will take the **value of this.o**, **transform** it to **String** and them **make** a **toast** with its value.

```
const/4 v10, 0x1
const/4 v11, 0x1
const/4 v12, 0x1
iget v10, p0, Lcom/google/ctf/shallweplayagame/GameActivity;->o:I
invoke-static {v10}, Ljava/lang/String;->valueOf(I)Ljava/lang/String;
move-result-object v11
invoke-static {p0, v11, v12}, Landroid/widget/Toast;->makeText(Landroid/content/Context;Ljava/lang/CharSequence;I)Landroid/widget/Toast;
move-result-object v12
invoke-virtual {v12}, Landroid/widget/Toast;->show()V
```


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