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Need some information about reverse engineering and decompile (android app - jdk)

I'm new to android and I don't know much about software engineering. I wanna know about things should be done to make it harder to decompile an app. I made some research about it and I ran into this debugger JEB which seems like it can monitor some stuff while app running. Yet I don't know the advantages of using it since I don't get the critical points of an android app which is being monitored by JEB. for example:
enter image description here
appearantly the right side shows local variables. what does x / xh means for an int? or for the string value "secretKey" is the value of string seriously? if it is then all variables in my code are at risk!!!??
I think maybe one of the attempts is to make a lot of fake variables in code so that the real ones can't be seen easily. Does it make sense? And are there other leaks that I should try to prevent for example about this JEB? Does proguard have anything to do with this? As I said I donno much about soft-eng and I didn't get to understand what proguard actually does.
And another thing I've seen is https://ibotpeaches.github.io/Apktool. How to make it more mixed up for such tool to handle?
The last question, I code in java and don't think yet I have the opportunity to start native coding but for future; I've seen people say its safer to code natively. Is it by itself? or there are more libraries to keep more safety? or its because of the final apk structure?
sorry if some words look dumb. thanks in advance

You should not try to prevent reverse engineering, because doing so is largely futile. Instead, you should design your app to minimize the effect that reverse engineering would have (i.e. you should not include secret data like keys or passwords in the app).
The amount of effort that people are willing to put into reverse engineering your app depends on how important and popular it is, while obfuscation increases the effort required to reverse engineer the app. However, if your app becomes popular, no amount of obfuscation will stop people, while if it languishes in obscurity, nobody's going to bother to look at it, even if you post the source code on Github. Also, in rare cases, adding obfuscation will make people crack it just for the challenge.
Apart from that, there is the risk that obfuscation will give you a false sense of security. Anyone can devise a system that they themselves cannot break, but if you aren't an expert at reverse engineering, the measures you come up with are unlikely to stop those who are. It is best to instead assume that everything in your app is public information and design accordingly.

You can protect your code by using some powerful obfuscator tool like: Bg+Anti Decompile/Reverse Java
It has a lot of protection options:
Hide string value (helpful when you keep some sensitive info in java source, ex: "Hello world" ->ߤª )
Obfuscate with unicode characters
Hide packagename
Add fakecode to trap the decompiler tools
Check resource-string (helpful when someone try to edit the resources of your APK)
....

Is NDK an alternative to reduce reverse engineering?

I have given up trying to get Proguard to work on either Eclipse or Android Studio. The docs are arcane and whatever I try, the Proguard obfuscated apk blows. Also questions regarding Proguard don't get much attention on this forum.
Standard Android apk files are designed to almost invite reverse engineering and I'm looking for some ways to protect my code. Not all my code just some methods.
Before I started writing Android apps, I wrote apps in C and in C# and I'm familiar with both of those languages and actually prefer either of them to Java so I've recently been reading about the NDK.
Of course the NDK is about speed but I'm wondering if it also would help protect my code against reverse engineering and inspection from hackers. I understand that it is "compiled" code so I imagine that the source code would not be visible.
Am I barking up the right tree?
Thanks,
Dean

This depends on what kind of protection you are looking for. If that is a revolutionary algorithm that will allow you do some very popular task much much faster than it takes today, then your code will be reverse engineered, no matter how you obfuscate it; you need patent protection. If you want to hide some unfriendly behavior (like spying on your users), it will be discovered. So don't do it. If you want to protect some protocol of client-server communication, it may be subject to easy attack elsewhere.
But in some cases going C++ does really help; it may be easier to distribute a monolithic shared library "black box" than to deal with Java API. I don't think obfuscation can work on code that you sell as library.
When we were selling a huge communication SDK, we followed this approach, and all our Java code was simply open to customers (part was intended to be changed/customized, but some classes carried a warning in the header "Please don't modify this class, there is very high chance that the product will not work properly if you mess with it".

Native C code is harder to decompile but it can be decompiled easily using the right tools.
Especially if you only develop small parts in C and then communicate with it using JNI which is a clear interface, anybody who decompiles your app can also use it. Who needs to know how your code works if you can simply execute it?
Conclusion: native code can be harder to decompile. But usually the only difference is that you need different tools and you need more knowledge & practice to decompile. Also modifying something is usually more complex.

Just implementing some methods in NDK alone will not likely help a lot. However, the resistance to reverse engineering can be improved if you also apply obfuscation and even run-time protection techniques to your NDK code. A simply web search gives a list of free tools. You could also work with commercial solution providers like Arxan. In addition, the level of obfuscation can generally be improved if you put more logic of your app in native code.

Yes if you use the NDK you can make it very difficult to reverse engineer your code and it is unlikely anybody would try.

Prevent/Make it difficult to patch Binary Assembly

I am not sure if the terminology is correct what code practices can you use to make it difficult for someone to modify the binary/assembly to bypass a check:
eg in the source code.
bool verificationResult = verify();
if (verificationResult){
allow_Something();
}else{
prevent_Something();
}
If a person looking at the disassembly version of the above code can modify the 'jump opcodes(?)' to run allow_Something even when the verification result is false.
Something similar is covered here
http://www.codeproject.com/Articles/18961/Tamper-Aware-and-Self-Healing-Code#pre0
Note I am creating the binary in C++ for it to be used via NDK on Android.

As the general consensus is so far, its impossible to prevent anyone hell-bent upon "cracking" your APK from doing so. Obfuscation techniques will only increase the complexity required to "crack" the APK once. After it gets uploaded to the myriad of the sites that offer to host APKs for free, its just a google search away from even the "noob-est" of Android noobs.
Also security through obscurity will NOT get you far.
Regarding protecting your APK from being hacked, i would recommend the following article that discusses the current state of license validation of APKs on Android. The techniques described in it should give you an idea of the common attack-vectors that you need to safeguard against.
Proguard is a good place to start obfuscating your APK.
After you manage to obtain an obfuscated APK, DO run it through the following tools and observe the de-compiled source. All these are free and open-source tools that are very popular and will surely be the first thing that any decent "cracker" will try :
1. baksmali
2. apktool
3. Dex2Jar + JD-Gui
Keep adding layers of obfuscation to your code until you are satisfied that the output of the above tools is fairly complicated to make sense. (Again do NOT under-estimate what a college-grad armed with coke, pizza and the knowledge of DVM opcodes can accomplish over a weekend).
Regarding the techniques discussed in the link you shared, i fail to see how they can be implemented to protect the .dex on Android. And if you end up implementing the verification logic in a separate .so then all the "cracker" would need to do is patch the call in your java code to the verify() function inside the .so.
UPDATE:
Additional obfuscation steps to secure the .so.
1. Do NOT follow a more or less linear path.
Adding additional jumps all over the place works by flooding the "cracker" with so many potential targets which need to be individually modified and patched and verified if the protection has been bypassed.
2. Add timing checks
This is mainly to throw off the "cracker" by making the code follow different paths during debug and actual run-time. If the time spent between two points is a lot more than usual then its a clear indication that your program is being debugged. i.e time to jump into that part of junk code that calculates the number of pianos in the world.
3. Write self modifying code
Again this thwarts static analysis. For example if your jump into the verification function does not exist in the binary but is patched everywhere as part of some init() function in the .so.
All the above techniques(and more) are described with examples in the following article on anti-debugging techniques.
A more comprehensive guide is Ultimate Anti Debugging Reference by Peter Ferrie.

Avoid using too transparent checks. Try some basic workflow obfuscating (for example XOR-ing result), this can help to defend against simple opcode replacing. But I assure you, that if someone wants (very-very) to crack you, he can do it regardless of complexity of your protection.

Dexguard is made by the same people who did Proguard, but it allows for even finer-grained options. That said, Proguard is more or less the industry standard for Android obfuscation. Though, as said above, if someone with the know-how wants to crack your app, there's no protection to be had for love or money.

The simple truth: you can't.
You can purchase utilities to obfuscate your object code but they are all trivially bypassed by any slightly motivated attacker. If your user can write to the program image (on disk or in memory) no amount of obfuscation will defend against it.
If it is extremely important, I recommend moving the important component to a device you control and provide some form of challenge-response code to access it. It won't prevent people from cracking it, but it can put up a much more significant barrier against it.

How to go from cracked APK to java code? 1-click tool cracked my app

So I published my android app, I proguarded it, applied LVL and even changed some of the LVL code but offcourse, I found it cracked somewhere using a tool called Lucky Patcher. I am not going to ask how can I protect against 1 click tools like that, as I guess there is no single answer ( unless you have an idea and can point me toward).
I need to ask you to help figure out how my code was cracked. I understand that this tool takes APK file and removes licensing. Now given that, how can I take this APK file and reverse engineer it back to Java files to see how the tool cracked my code (so I fix it)
Please help me
Thanks

After Proguard, there's no way to decompile your code into humanly-readable Java.
While it makes the reverse engineering process more difficult, it's not impossible for a clever reverser to figure out the hidden algorithm.
As for tools,
Use android-apktool to decompile to smali and extract all the encoded xml resources.
Use dex2jar to translate Dalvik into jar
and finally jd-gui to see the resulting reversed java code

There's a lot of info here on how to go from a DEX file back to Java source. Also, have you looked at this blog post which addresses many of the ways to protect your source?

piracy is a big issue , and i don't think that any platform or OS can be fully protected from it .
however , google already made some tutorials regarding protection against it , for example:
http://www.google.com/events/io/2011/sessions/evading-pirates-and-stopping-vampires-using-license-verification-library-in-app-billing-and-app-engine.html
also:
http://android-developers.blogspot.co.il/2010/09/securing-android-lvl-applications.html
i think that you can also put some sophisticated obstacles using C instead of java.
also , as google suggests, consider using a different approach : make the core features free , and make the rest purchaseable via in-app billing . you can also add ads and a feature to remove them by in-app billing as well .

I was thinking about this and it seems like if you really wanted to secure your application from hackers there is really only 1 way to do it. You can implement all kinds of fancy methods of insuring your application is licensed and paid for as described in the google article but all it takes is a good hacker to decompile your application and find where the code is and then comment it out or change a function to always return true.
Instead, implement some portion of your application that is required for use in jni/ndk, and check for validation in that code. It doesn't have to be extremely complicated code but you can't just put something like a function (eg. checkValidity) as a user could easily comment the java call that calls into the ndk. Instead you should make some call to your ndk to actually do something that is non-trivial for your application to run -- something the user can't just comment out or switch out with a defined function that does the same thing. From within the ndk code do the verification of your application's integrity/licensing and if it fails kill the application or whatever you need to do.
In order to bypass this the hacker would need to re-implement the ndk code or reverse engineer it.. Which should be much more complicated and not worth while.
This obviously isn't a simple solution and still won't guarantee your application never gets hacked, but it should be much harder to break than the other methods..

I personally think that Obfuscation {Proguard, Dexguard} and native {.so} are pretty effective way to go if used properly.
It definitely deters less experienced 'players' and definitely complicates the life of even experienced 'players'
Don't simply copy/paste the Google android example codes....

Copy-protection traps

Just about to release a free version of my app and I'm looking towards the free-mium model to give extra options to users. However, I'm definitely worried about it being pirated too quickly for me to make any anything.
Does anybody have some quality copy-protection techniques for Android? And I'm not talking about the pos LVL that is provided. I'm looking for some sneaky traps to detect if my code has been tampered with. Any ideas welcome; gotta make it hard enough on them that its just not worth it.

It will only get pirated if it's popular, so you have a long way to go :). Generally, obfuscate your code, don't use the LVL as is since there are tools that disable it automatically. Not sure what your idea of a 'sneaky trap' is, but watch this for some ideas on how to protect your app.
Those are mentioned in the video, but:
use ProGuard to obfuscate your code
to detect if your code has been changed, you can check the CRC of classes.dex or check if the APK has been signed with your certificate (if someone changes your code, they'll have to resign it). However the antilvl tool effectively disables the APIs you would use to check for tampering. So you need to do it in native code if you want it to be effective.
don't do your checks on startup, but later on so they are harder to detect.
if possible, have a server side component to your license/tampering checking. Think about how your app should behave if there is no network connection. Cache server responses? For how long? Deny access right away? Allow access always?, etc.

Protection can always be broken, all you can do is make it harder to break.
Something you might do would be writing some essential part(s) of your code in C and call it via the NDK. Then you can do some testing in there, because decompiling the C part will be much harder than decompiling bytecode.

You can use ProGuard in eclipse to obfuscate your code. It optimizes, and obfuscates your code by removing unused code and renaming classes, fields, and methods with semantically obscure names. The result is a smaller sized .apk file that is more difficult to reverse engineer. That way your code will be more tamper proof.
Reference : developer.android