Android ProGuard: Most Aggressive Optimizations

According to the comment from the optimization file, the optimizations introduce certain risks and if used, the app must be tested thoroughly. According to my experience, it is necessary to disable code/simplification/advanced, because it caused final local variables which were initialized outside a lambda being NULL inside the lambda. It was very difficult to debug and find. Therefore my optimization settings are as follows:

-optimizations !code/simplification/cast,!code/simplification/advanced,!field/*,!class/merging/*,!method/removal/parameter,!method/propagation/parameter

Note that code/simplification/arithmetic must be also disabled if you target Android 2.0 and lower (which is very unlikely). Besides that, I also had to disable method/removal/parameter and method/propagation/parameter, because these implicitly enable code/simplification/advanced (see ProGuard manual for more info).

Remember that the best ProGuard configuration - is a configuration with a minimum of exceptions. Under the exceptions I understand:

 -keepclassmembers class * extends android.content.Context {
    public void *(android.view.View);
    public void *(android.view.MenuItem);
 }

Let's walk through proguard-android-optimize.txt and look on optimisation/obfuscation options.

For detailed description of ProGuard options I use this

-optimizations !code/simplification/arithmetic,!code/simplification/cast,!field/*,!class/merging/* This - list of possible optimisation, ! mean negate, so this optimization is not used

-optimizationpasses 5 Specifies the number of optimization passes to be performed. By default, a single pass is performed. Multiple passes may result in further improvements. If no improvements are found after an optimization pass, the optimization is ended. Only applicable when optimizing.
Usage: OK, and looks like that default 5 passes is enough

-allowaccessmodification Specifies that the access modifiers of classes and class members may be broadened during processing. This can improve the results of the optimization step.
Usage: OK, yes looks like improve optimization

-dontpreverify When targeting Android, preverifing is not necessary, so dontpreverify turn it off to reduce the processing time a bit. But this option is not impact on unbreakability of code.
Usage: OK, just to little bit reduse processing time

-dontusemixedcaseclassnames Specifies not to generate mixed-case class names while obfuscating. By default, obfuscated class names can contain a mix of upper-case characters and lower-case characters. This creates perfectly acceptable and usable jars.
Usage: QUESTIONABLE, I can't find exact reason why this option added, but looks like change class name from abcdef to AbCdEf doesn't make code unbreakable

-dontskipnonpubliclibraryclasses Specifies not to ignore non-public library classes. As of version 4.5, this is the default setting.
Usage: OK, very useful

The following options aren't include to proguard-android-optimize.txt:

-mergeinterfacesaggressively Specifies that interfaces may be merged, even if their implementing classes don't implement all interface methods... setting this option can reduce the performance of the processed code on some JVMs
Usage: BAD, look dangerous for Android, don't included into config, sumular of prohibition of class/merging/ in optimizations

-overloadaggressively Specifies to apply aggressive overloading while obfuscating. Multiple fields and methods can then get the same names, as long as their arguments and return types are different, as required by Java bytecode (not just their arguments, as required by the Java language)
Usage: BAD, Google's Dalvik VM can't handle overloaded static fields.

So I know only one more useful for obfuscation and non dangerous option:
-repackageclasses ''

-repackageclasses '' Specifies to repackage all class files that are renamed, by moving them into the single given package. Without argument or with an empty string (''), the package is removed completely. This option overrides the -flattenpackagehierarchy option.
Usage: OK, Used by Google, so look like we at least have found the option which we can add to our config

Also please notice about decoding stack traces. ProGuard also removes the filename and line numbers from the stacktrace. This makes finding errors very complicated. You can keep the line numbers by adding the following code to your config:

-renamesourcefileattribute SourceFile 
-keepattributes SourceFile,LineNumberTable

This will keep the line numbers but replace the filename in the stacktrace with "SourceFile".

Also do not forget that the ProGuard looks vulnerable because it does not encrypt the string resources, so consider using DexGuard or encrypt important strings (like tokens, urls) themselves.