ProGuard Optimizer and Obfuscator in the Android SDK

ProGuard
Optimizer and Obfuscator
in the Android SDK
Eric Lafortune
Developer of ProGuard
Technical director at Saikoa

Topics
ProGuard
– Techniques
– Results
More application protection
– Goals
– Attackers
– Types of attacks
– Protection

ProGuard
Open source
Generic
Shrinker
Optimizer
Obfuscator
For Java bytecode

ProGuard history
Java applications
Applets
2002
Midlets
2010 2012
Android apps
●
May 2002 First release
●
Sep 2010 Recommended for protecting LVL
●
Dec 2010 Part of Android SDK
●
Jan 2012 Startup Saikoa

ProGuard
Application
code
Shrink
Libraries
Android
runtime
Optimize Obfuscate
Processed
code
Android
runtime

ProGuard in Android builds
Application
Java bytecode
ProGuard
Libraries
Java bytecode
Processed
Java bytecode Dex
Dalvik bytecode
Javac
Application
Java source
Libraries
Java bytecode
XML resources
Assets Assets
Compiled
XML resourcesAapt

Why use ProGuard?
●
Application size
●
Performance
●
Remove logging, debugging, testing code
●
Protection

Application size
classes.dex size .apk size
Without
ProGuard
With
ProGuard
Reduction
ApiDemos 716 K 482 K 33 % 2.6 M 2.5 M 4 %
GoogleIO
App
3.4 M 905 K 75 % 1.9 M 906 K 53 %
ApiDemos
in Scala*
~6 M 542 K ~90 % ~8 M 2.5 M ~70 %
* [Stéphane Micheloud, http://lampwww.epfl.ch/~michelou/android/library-code-shrinking.html]

Performance: CaffeineMark
Without ProGuard
Sieve score = 6833
Loop score = 14831
Logic score = 19038
String score = 7694
Float score = 6425
Method score = 4850
Overall score = 8794
With ProGuard
Sieve score = 6666
Loop score = 15473
Logic score = 47840
String score = 7717
Float score = 6488
Method score = 5229
Overall score = 10436
Improvement: 18%
[Acer Iconia Tab A500, nVidia Tegra 2, 1.0 GHz, Android
3.2.1]

Battery life
Extreme example:
“5 x better battery life,
by removing verbose logging code
in a background service”
(but don't count on it)

Results
●
Size reduction:
– Code (classes.dex): 20 … 90%
– Application (.apk): 5 ... 70%
●
Performance improvements: 0 … 20%

How to enable ProGuard?
Ant and Eclipse: project.properties
→ only applied when building release versions
# To enable ProGuard to shrink and obfuscate
your code, uncomment this
#proguard.config=
${sdk.dir}/tools/proguard/proguard-android.txt:
proguard-project.txt
#proguard.config=
${sdk.dir}/tools/proguard/proguard-android.txt:
Tip

How to enable ProGuard?
Gradle: build.gradle
→ completely flexible
android {
buildTypes {
release {
runProguard true
proguardFile getDefaultProguardFile('proguard-android.txt')
}
}
productFlavors {
some_flavor {
proguardFile 'proguard-project.txt'
}
}
}
android {
buildTypes {
release {
runProguard true
proguardFile getDefaultProguardFile('proguard-android.txt')
}
}
productFlavors {
some_flavor {
}
}
}
New

Notes and warnings
“Closed-world assumption”
→ if debug build works fine,
then ok to ignore in proguard-project.txt:
Warning: com.dropbox.client2.DropboxAPI:
can't find referenced class org.json.simple.JSONArray
Warning: com.dropbox.client2.DropboxAPI:
can't find referenced class org.json.simple.JSONArray
-dontwarn twitter4j.internal.logging.**
-dontwarn com.dropbox.client2.**
-dontwarn twitter4j.internal.logging.**
-dontwarn com.dropbox.client2.**
Warning: twitter4j.internal.logging.Log4JLoggerFactory:
can't find referenced class org.apache.log4j.Logger
Warning: twitter4j.internal.logging.SLF4JLoggerFactory:
can't find referenced class org.slf4j.LoggerFactory
...
Warning: twitter4j.internal.logging.Log4JLoggerFactory:
can't find referenced class org.apache.log4j.Logger
Warning: twitter4j.internal.logging.SLF4JLoggerFactory:
can't find referenced class org.slf4j.LoggerFactory
...
Tip

Notes and warnings
Straight to the Troubleshooting page:
Warning: there were 12 unresolved references to classes or interfaces.
You may need to add missing library jars or update their versions.
If your code works fine without the missing classes, you can suppress
the warnings with '-dontwarn' options.
(http://proguard.sourceforge.net/manual/troubleshooting.html#unresolvedclass)
Warning: there were 12 unresolved references to classes or interfaces.
You may need to add missing library jars or update their versions.
If your code works fine without the missing classes, you can suppress
the warnings with '-dontwarn' options.
(http://proguard.sourceforge.net/manual/troubleshooting.html#unresolvedclass)
New

Shrinking
Also called treeshaking, minimizing, shrouding

Shrinking
●
Classes, fields, methods

Entry points
1) Activities, applications, services, fragments,...
→ provided automatically by Android build process
-keep public class * extends android.app.Activity
-keep public class * extends android.app.Application
-keep public class * extends android.app.Service
…
-keep public class * extends android.app.Activity
-keep public class * extends android.app.Application
-keep public class * extends android.app.Service
…

Entry points
2) Introspection, e.g. Guice, RoboGuice
→ must be specified in proguard-project.txt:
-keepclassmembers class * {
@javax.inject.** <fields>;
@com.google.inject.** <fields>;
@roboguice.** <fields>;
@roboguice.event.Observes <methods>;
}
-keepclassmembers class * {
@javax.inject.** <fields>;
@com.google.inject.** <fields>;
@roboguice.** <fields>;
@roboguice.event.Observes <methods>;
}
Tip

Optimization
At the bytecode instruction level:
●
Dead code elimination
●
Constant propagation
●
Method inlining
●
Class merging
●
Remove logging code
●
Peephole optimizations
●
Devirtualization
●
...

Optimization example
int answer = computeAnswer(1, 2, 3, 7);int answer = computeAnswer(1, 2, 3, 7);
int computeAnswer(int f1, int f2, int f3, int f4) {
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
return computeAnswer(f1 * f2, f3, f4, 1);
}
}
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
}
}

if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
}
}
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
}
}
do {
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
f1 = f1 * f2; f2 = f3, f3 = f4, f4 = 1;
}
} while (true);
}
do {
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
f1 = f1 * f2; f2 = f3, f3 = f4, f4 = 1;
}
} while (true);
}

if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
}
}
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
}
}
do {
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
f1 = f1 * f2; f2 = f3, f3 = f4, f4 = 1;
}
} while (true);
}
do {
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
f1 = f1 * f2; f2 = f3, f3 = f4, f4 = 1;
}
} while (true);
}
1 2 3 7

if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
}
}
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
}
}
do {
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
f1 = f1 * f2; f2 = f3, f3 = f4, f4 = 1;
}
} while (true);
}
do {
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
f1 = f1 * f2; f2 = f3, f3 = f4, f4 = 1;
}
} while (true);
} int computeAnswer() {
return 42;
}
int computeAnswer() {
return 42;
}

if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
}
}
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
}
}
do {
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
f1 = f1 * f2; f2 = f3, f3 = f4, f4 = 1;
}
} while (true);
}
do {
if (f2 == 1 && f3 == 1 && f4 == 1) {
return f1;
} else {
f1 = f1 * f2; f2 = f3, f3 = f4, f4 = 1;
}
} while (true);
} int computeAnswer() {
return 42;
}
int computeAnswer() {
return 42;
}
int answer = 42;int answer = 42;

Optimization: enum
int answer = getAnswer(MyEnum.GREEN);int answer = getAnswer(MyEnum.GREEN);
int getAnswer(MyEnum e) {
switch (e) {
case RED: return 1;
case GREEN: return 42;
default: return 0;
}
}
switch (e) {
case RED: return 1;
default: return 0;
}
}
enum MyEnum {
RED, GREEN, BLUE
}
enum MyEnum {
RED, GREEN, BLUE
}
New

Optimization: enum
switch (e) {
case RED: return 1;
default: return 0;
}
}
switch (e) {
case RED: return 1;
default: return 0;
}
}
enum MyEnum {
RED, GREEN, BLUE
}
enum MyEnum {
RED, GREEN, BLUE
}
switch (Internal.$SwitchMap[e.ordinal()]) {
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
class MyEnum {
static final MyEnum RED = new MyEnum(“RED”, 0);
static final MyEnum GREEN = new MyEnum(“GREEN”, 0);
static final MyEnum BLUE = new MyEnum(“BLUE”, 0);
static final MyEnum[] $VALUES =
new MyEnum[] { RED, GREEN, BLUE };
...
}
class MyEnum {
...
}
New

Optimization: enum
switch (e) {
case RED: return 1;
default: return 0;
}
}
switch (e) {
case RED: return 1;
default: return 0;
}
}
enum MyEnum {
RED, GREEN, BLUE
}
enum MyEnum {
RED, GREEN, BLUE
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
class MyEnum {
...
}
class MyEnum {
...
}
New
class Internal {
static final int[] $SwitchMap =
new int[MyEnum.values().length];
static {
$SwitchMap[MyEnum.RED.ordinal()] = 1;
$SwitchMap[MyEnum.GREEN.ordinal()] = 2;
}
}
class Internal {
static {
}
}

Optimization: enum
switch (e) {
case RED: return 1;
default: return 0;
}
}
switch (e) {
case RED: return 1;
default: return 0;
}
}
enum MyEnum {
RED, GREEN, BLUE
}
enum MyEnum {
RED, GREEN, BLUE
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
class MyEnum {
...
}
class MyEnum {
...
}
New
class Internal {
static final int[] $SwitchMap
static {
}
}
class Internal {
static {
}
}
int answer = getAnswer(2);int answer = getAnswer(2);
int getAnswer(int e) {
switch (Internal.$SwitchMap[e]) {
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
class Internal {
new int[3];
static {
$SwitchMap[1] = 1;
$SwitchMap[2] = 2;
}
}
class Internal {
new int[3];
static {
$SwitchMap[1] = 1;
$SwitchMap[2] = 2;
}
}

Optimization: enum
switch (e) {
case RED: return 1;
default: return 0;
}
}
switch (e) {
case RED: return 1;
default: return 0;
}
}
enum MyEnum {
RED, GREEN, BLUE
}
enum MyEnum {
RED, GREEN, BLUE
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
class MyEnum {
...
}
class MyEnum {
...
}
New
class Internal {
static {
}
}
class Internal {
static {
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
class Internal {
new int[3];
static {
$SwitchMap[1] = 1;
$SwitchMap[2] = 2;
}
}
class Internal {
new int[3];
static {
$SwitchMap[1] = 1;
$SwitchMap[2] = 2;
}
}
switch (e) {
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
switch (e) {
case 1: return 1;
case 2: return 42;
default: return 0;
}
}

Optimization: enum
switch (e) {
case RED: return 1;
default: return 0;
}
}
switch (e) {
case RED: return 1;
default: return 0;
}
}
enum MyEnum {
RED, GREEN, BLUE
}
enum MyEnum {
RED, GREEN, BLUE
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
class MyEnum {
...
}
class MyEnum {
...
}
New
class Internal {
static {
}
}
class Internal {
static {
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
class Internal {
new int[3];
static {
$SwitchMap[1] = 1;
$SwitchMap[2] = 2;
}
}
class Internal {
new int[3];
static {
$SwitchMap[1] = 1;
$SwitchMap[2] = 2;
}
}
switch (e) {
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
switch (e) {
case 1: return 1;
case 2: return 42;
default: return 0;
}
}
int answer = 42;int answer = 42;

How to enable optimization?
Ant and Eclipse: project.properties
proguard.config=
${sdk.dir}/tools/proguard/proguard-android-optimize.txt:
proguard.config=
${sdk.dir}/tools/proguard/proguard-android-optimize.txt:
Tip

How to enable optimization?
Gradle: build.gradle
android {
buildTypes {
release {
runProguard true
proguardFile getDefaultProguardFile('proguard-android-optimize.txt')
}
}
productFlavors {
some_flavor {
}
}
}
android {
buildTypes {
release {
runProguard true
proguardFile getDefaultProguardFile('proguard-android-optimize.txt')
}
}
productFlavors {
some_flavor {
}
}
}
New

Remove logging code
Specify assumptions in proguard-project.txt:
-assumenosideeffects class android.util.Log {
public static boolean isLoggable(java.lang.String, int);
public static int v(...);
public static int i(...);
public static int w(...);
public static int d(...);
public static int e(...);
public static java.lang.String getStackTraceString
(java.lang.Throwable);
}
-assumenosideeffects class android.util.Log {
public static boolean isLoggable(java.lang.String, int);
public static int v(...);
public static int i(...);
public static int w(...);
public static int d(...);
public static int e(...);
public static java.lang.String getStackTraceString
(java.lang.Throwable);
}
Tip

Obfuscation
Traditional name obfuscation:
●
Rename identifiers:
class/field/method names
●
Remove debug information:
line numbers, local variable names,...

Obfuscation example
public class MyComputationClass {
private MySettings settings;
private MyAlgorithm algorithm;
private int answer;
public int computeAnswer(int input) {
…
return answer;
}
}
private int answer;
…
return answer;
}
}

Obfuscation example
private int answer;
…
return answer;
}
}
private int answer;
…
return answer;
}
}
public class a {
private b a;
private c b;
private int c;
public int a(int a) {
…
return c;
}
}
public class a {
private b a;
private c b;
private int c;
public int a(int a) {
…
return c;
}
}

Complementary steps
Optimization Obfuscation
Irreversibly remove information

What about ART?
The new (experimental) Android RunTime
Application
Java bytecode
ProGuard
Libraries
Java bytecode
Processed
Java bytecode Dex
Dalvik bytecode
Dex2oat
Native code
Development
Device

More application protection?
public class MyVerificationClass {
public int checkSignatures() {
…
return activity
.getPackageManager()
.checkSignatures(“mypackage1”, “mypackage2”);
}
}
…
return activity
}
}

More application protection?
…
return activity
}
}
…
return activity
}
}
public class a {
public int a() {
…
return a
}
}
public class a {
public int a() {
…
return a
}
}

More application protection
●
Goals
●
Attackers
●
Types of attacks
●
Protection

What to protect?
●
Application
●
Algorithms
●
Communication
●
Assets
●
Keys
Code
Resources
Assets
Signatures

Static analysis
●
Disassemblers: dexdump, baksmali
●
Decompilers: dex2jar + jad, JD-GUI, JEB,...
●
Resources: aapt, apktool,...

Dynamic analysis
●
Debuggers: adb,...
●
Subverted runtime
●
Cracking tools

Server-side code
Client-side
application
Server-side
application
?

String encryption
Be creative!
String KEY = “Secret key”;String KEY = “Secret key”;
String KEY = new String(Base64.decode(“U2VjcmV0IGtleQo=”));String KEY = new String(Base64.decode(“U2VjcmV0IGtleQo=”));

Reflection
java.lang.reflect
System.out.println(“Hello world!”);System.out.println(“Hello world!”);
Class clazz =
Class.forName(“java.io.PrintStream”);
Method method =
clazz.getMethod(“println”,
new Class[] { String.class });
method.invoke(nul, new Object[] { “Hello world!” });
Class clazz =
Class.forName(“java.io.PrintStream”);
Method method =
clazz.getMethod(“println”,
new Class[] { String.class });
method.invoke(nul, new Object[] { “Hello world!” });

Dynamic class loading
http://android-developers.blogspot.be/2011/07/custom-class-loading-in-dalvik.html
classes.dex
extra.dex
classes.dex

libutil.so
libutil.so
Native code
Java Native Interface
classes.dex
libutil.so
classes.dex

Data encryption
●
Standard cryptography APIs
●
SQLCipher
●
android.drm
Cipher cipher =
Cipher.getInstance(“AES/CFB/NoPadding”);
cipher.init(Cipher.ENCRYPT_MODE,
key, initializationVector);
byte[] decrypted = cipher.doFinal(encrypted);
Cipher cipher =
Cipher.getInstance(“AES/CFB/NoPadding”);
cipher.init(Cipher.ENCRYPT_MODE,
key, initializationVector);
byte[] decrypted = cipher.doFinal(encrypted);

White-box cryptography
Research: http://www.whiteboxcrypto.com/
Real examples: DES, AES
final int KEY = 42;
int decryptValue(int encryptedValue) {
return encryptedValue * KEY;
}
final int KEY = 42;
return encryptedValue * KEY;
}
encryptedValue += 5;
encryptedValue *= 21;
encryptedValue -= 105;
encryptedValue += encryptedValue;
return encryptedValue;
}
encryptedValue += 5;
encryptedValue *= 21;
encryptedValue -= 105;
encryptedValue += encryptedValue;
return encryptedValue;
}

Application checks
●
Check application certificate:
●
Check debug flag:
boolean debug =
(activity.getApplicationInfo().flags &
ApplicationInfo.FLAG_DEBUGGABLE) != 0;
boolean debug =
(activity.getApplicationInfo().flags &
ApplicationInfo.FLAG_DEBUGGABLE) != 0;
byte[] certificateData = activity
.getPackageInfo(activity.getPackageName(),
PackageManager.GET_SIGNATURES)
.signatures[0]
.toByteArray();
byte[] certificateData = activity
.getPackageInfo(activity.getPackageName(),
PackageManager.GET_SIGNATURES)
.signatures[0]
.toByteArray();

Environment checks
●
Emulator detection
●
Root detection

Conclusion
Nothing is unbreakable, but you can raise the bar:
●
ProGuard
●
String encryption
●
Reflection
●
Dynamic class loading
●
Native code
●
Data encryption
●
Application checks
●
Environment checks
●
… DexGuard

DexGuard
●
Specialized for Android
●
Code and data
●
Layers of protection
●
Recommended techniques
●
Low-level techniques
●
Transparent
Tip
Code
Resources
Assets
Signatures

ProGuard - DexGuard
Open source
Generic
Shrinker
Optimizer
Obfuscator
For Java bytecode
Closed source
Specialized
Shrinker
Optimizer
Obfuscator
Protector
For Android
Compatible

ProGuard guide – Android SDK
developer.android.comdeveloper.android.com

ProGuard website
proguard.sourceforge.netproguard.sourceforge.net

ProGuard manual
proguard.sourceforge.netproguard.sourceforge.net
Tip

Saikoa website
www.saikoa.comwww.saikoa.com

Questions?
Open source
Shrinking
Optimization
Obfuscation
Java bytecode
ProGuard
Saikoa
DexGuard
Dalvik bytecode
Protection

ProGuard Optimizer and Obfuscator in the Android SDK

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