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Check .apk-signature in C/native Code

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抹茶落季
抹茶落季 2020-12-14 05:01

I have developed an Android application which also contains a native part written in C (which does not depend on the app).

The application itself is useless if the s

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  • 青春惊慌失措
    2020-12-14 05:56

    As I have been asked to publish some code how I'm now checking the CRC-code of my Java-application from within C, here are some snippets.

    I cannot post a complete working solution as it's spread over multiple lines due to performance-reasons, but I hope this is most complete and working:

    In your MyApplication.java:

    public class MyApplication extends Application {
    private static Context context;

    public static Context getAppContext() {
        return MyApplication.context;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        MyApplication.context = getApplicationContext();
    }
}

    Android.mk:

    LOCAL_CFLAGS += -O3 -DDEBUG_MODE=0 -DCLASSES_CRC=2331492378

    Inside your C-code:

    #define LOG_TAG "Your Log Tag"

#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)
#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR, LOG_TAG, __VA_ARGS__)

#if DEBUG_MODE
#define LOGDH(...) LOGD(__VA_ARGS__)
#define LOGIH(...) LOGI(__VA_ARGS__)
#define LOGEH(...) LOGE(__VA_ARGS__)
#else
#define LOGDH(...) //
#define LOGIH(...) //
#define LOGEH(...) //
#endif

int isSecure = -1;

jclass MyApplication;
jclass Context;
jclass ApplicationInfo;


jclass ZipFile;
jclass ZipEntry;
jclass CheckedInputStream;
jclass Adler32;
jclass Checksum;

jmethodID MyApplication_getAppContextMethodId;
jmethodID Context_getApplicationInfoMethodId;

jmethodID ZipFile_ConstructorMethodId;
jmethodID CheckedInputStream_ConstructorMethodId;
jmethodID Adler32_ConstructorMethodId;

jmethodID ZipFile_getEntryMethodId;
jmethodID ZipFile_getInputStreamMethodId;
jmethodID CheckedInputStream_readMethodId;
jmethodID CheckedInputStream_getChecksumMethodId;
jmethodID Checksum_getValueMethodId;

jfieldID ApplicationInfo_flagsFieldId;
jfieldID ApplicationInfo_FLAG_DEBUGGABLEFieldId;
jfieldID ApplicationInfo_sourceDirFieldId;


static long getClassesCRC(JNIEnv *env) {
    jobject appContextInstance = (*env)->CallStaticObjectMethod(env,
            MyApplication, MyApplication_getAppContextMethodId);
    if (!appContextInstance) {
        LOGEH("Unable to get instance of AppContext");
        return false;
    }

    jobject applicationInfoInstance = (*env)->CallObjectMethod(env,
            appContextInstance, Context_getApplicationInfoMethodId);
    if (!appContextInstance) {
        LOGEH("Unable to get instance of ApplicationInfo");
        return false;
    }

    jobject zipFileInstance = (*env)->NewObject(env, ZipFile,
            ZipFile_ConstructorMethodId,
            (*env)->GetObjectField(env, applicationInfoInstance,
                    ApplicationInfo_sourceDirFieldId));
    if (!zipFileInstance) {
        LOGEH("Unable to get instance of ZipFile");
        return -1;
    }

    jstring classesDexString = (*env)->NewStringUTF(env, "classes.dex");
    jobject zipEntryInstance = (*env)->CallObjectMethod(env, zipFileInstance,
            ZipFile_getEntryMethodId, classesDexString);
    if (!zipFileInstance) {
        LOGEH("Unable to get instance of ZipEntry");
        return -1;
    }
    (*env)->DeleteLocalRef(env, classesDexString);

    jobject adler32Instance = (*env)->NewObject(env, Adler32,
            Adler32_ConstructorMethodId);
    if (!adler32Instance) {
        LOGEH("Unable to get instance of Adler32");
        return -1;
    }

    jobject inputStreamInstance = (*env)->CallObjectMethod(env, zipFileInstance,
            ZipFile_getInputStreamMethodId, zipEntryInstance);
    if (!inputStreamInstance) {
        LOGEH("Unable to get instance of InputStream");
        return -1;
    }

    jobject checkedInputStreamInstance = (*env)->NewObject(env,
            CheckedInputStream, CheckedInputStream_ConstructorMethodId,
            inputStreamInstance, adler32Instance);
    if (!checkedInputStreamInstance) {
        LOGEH("Unable to get instance of CheckedInputStream");
        return -1;
    }

    int bufferSize = 128;
    jbyteArray bufferBytes = (*env)->NewByteArray(env, bufferSize);

    while ((*env)->CallIntMethod(env, checkedInputStreamInstance,
            CheckedInputStream_readMethodId, bufferBytes) > 0) {
    }
    (*env)->DeleteLocalRef(env, bufferBytes);

    jobject checksumInstance = (*env)->CallObjectMethod(env,
            checkedInputStreamInstance, CheckedInputStream_getChecksumMethodId);
    if (!checksumInstance) {
        LOGEH("Unable to get instance of CheckSum");
        return -1;
    }

    return (*env)->CallLongMethod(env, checksumInstance,
            Checksum_getValueMethodId);
}

static bool isDebuggable(JNIEnv *env) {
    jobject appContextInstance = (*env)->CallStaticObjectMethod(env,
            MyApplication, Application_getAppContextMethodId);
    if (!appContextInstance) {
        LOGEH("Unable to get instance of AppContext");
        return false;
    }

    jobject applicationInfoInstance = (*env)->CallObjectMethod(env,
            appContextInstance, Context_getApplicationInfoMethodId);
    if (!appContextInstance) {
        LOGEH("Unable to get instance of ApplicationInfo");
        return false;
    }

    int FLAG_DEBUGGABLE = (*env)->GetStaticIntField(env, ApplicationInfo,
            ApplicationInfo_FLAG_DEBUGGABLEFieldId);
    int flags = (*env)->GetIntField(env, applicationInfoInstance,
            ApplicationInfo_flagsFieldId);

    return (0 != (flags &= FLAG_DEBUGGABLE));
}

static bool isSecureEnvironment(JNIEnv *env) {

    //isSecure = true; // TODO remove this

    if (isSecure == -1) {
        isSecure = true;

        if (isDebuggable(env)) {
            // someone used the app in debug-mode
#if DEBUG_MODE != 1
            // TODO report

#endif
            LOGEH("App IS DEBUGGABLE!");
            isSecure = false;
        } else {

            // check CRC
            long classesCRC = getClassesCRC(env);
            if (classesCRC != (long) CLASSES_CRC) {
#if DEBUG_MODE != 1
                // TODO report
#endif
                LOGEH("CRC-CHECK FAILED: %lu", classesCRC);

                isSecure = false;
            }
        }
    }
    return isSecure;
}


static bool initJavaClasses(JNIEnv * env) {
    jclass local = (*env)->FindClass(env, "eu/my/MyApplication");
    MyApplication = (*env)->NewGlobalRef(env, local);
    if (!MyApplication) {
    LOGEH("Unable to find the MyApplication class");
    return false;
    }
    local = (*env)->FindClass(env, "android/content/Context");
    Context = (*env)->NewGlobalRef(env, local);
    (*env)->DeleteLocalRef(env, local);
    if (!Context) {
    LOGEH("Unable to find the Context class");
    return false;
    }
    local = (*env)->FindClass(env, "android/content/pm/ApplicationInfo");
    ApplicationInfo = (*env)->NewGlobalRef(env, local);
    (*env)->DeleteLocalRef(env, local);
    if (!ApplicationInfo) {
    LOGEH("Unable to find the ApplicationInfo class");
    return false;
    }

    local = (*env)->FindClass(env, "java/util/zip/ZipFile");
    ZipFile = (*env)->NewGlobalRef(env, local);
    (*env)->DeleteLocalRef(env, local);
    if (!ZipFile) {
    LOGEH("Unable to find the ZipFile class");
    return false;
    }

    local = (*env)->FindClass(env, "java/util/zip/ZipEntry");
    ZipEntry = (*env)->NewGlobalRef(env, local);
    (*env)->DeleteLocalRef(env, local);
    if (!ZipEntry) {
    LOGEH("Unable to find the ZipEntry class");
    return false;
    }

    local = (*env)->FindClass(env, "java/util/zip/CheckedInputStream");
    CheckedInputStream = (*env)->NewGlobalRef(env, local);
    (*env)->DeleteLocalRef(env, local);
    if (!CheckedInputStream) {
    LOGEH("Unable to find the CheckedInputStream class");
    return false;
    }

    local = (*env)->FindClass(env, "java/util/zip/Adler32");
    Adler32 = (*env)->NewGlobalRef(env, local);
    (*env)->DeleteLocalRef(env, local);
    if (!Adler32) {
    LOGEH("Unable to find the Adler32 class");
    return false;
    }

    local = (*env)->FindClass(env, "java/util/zip/Checksum");
    Checksum = (*env)->NewGlobalRef(env, local);
    (*env)->DeleteLocalRef(env, local);
    if (!Checksum) {
    LOGEH("Unable to find the Checksum class");
    return false;
    }

    return true;
}

static bool initJavaMethods(JNIEnv * env) {
    MyApplication_getAppContextMethodId = (*env)->GetStaticMethodID(env,
    MyApplication, "getAppContext", "()Landroid/content/Context;");
    if (!MyApplication_getAppContextMethodId) {
    LOGEH("Unable to find the getAppContext method");
    return false;
    }
    Context_getApplicationInfoMethodId = (*env)->GetMethodID(env, Context,
    "getApplicationInfo", "()Landroid/content/pm/ApplicationInfo;");
    if (!Context_getApplicationInfoMethodId) {
    LOGEH("Unable to find the getApplicationInfo method");
    return false;
    }

    ZipFile_ConstructorMethodId = (*env)->GetMethodID(env, ZipFile, "<init>",
    "(Ljava/lang/String;)V");
    if (!ZipFile_ConstructorMethodId) {
    LOGEH("Unable to find the constructor method");
    return false;
    }

    CheckedInputStream_ConstructorMethodId = (*env)->GetMethodID(env,
    CheckedInputStream, "<init>",
    "(Ljava/io/InputStream;Ljava/util/zip/Checksum;)V");
    if (!CheckedInputStream_ConstructorMethodId) {
    LOGEH("Unable to find the constructor method");
    return false;
    }

    Adler32_ConstructorMethodId = (*env)->GetMethodID(env, Adler32, "<init>",
    "()V");
    if (!Adler32_ConstructorMethodId) {
    LOGEH("Unable to find the constructor method");
    return false;
    }

    ZipFile_getEntryMethodId = (*env)->GetMethodID(env, ZipFile, "getEntry",
    "(Ljava/lang/String;)Ljava/util/zip/ZipEntry;");
    if (!ZipFile_getEntryMethodId) {
    LOGEH("Unable to find the getEntry method");
    return false;
    }

    ZipFile_getInputStreamMethodId = (*env)->GetMethodID(env, ZipFile,
    "getInputStream", "(Ljava/util/zip/ZipEntry;)Ljava/io/InputStream;");
    if (!ZipFile_getInputStreamMethodId) {
    LOGEH("Unable to find the getInputStream method");
    return false;
    }

    CheckedInputStream_readMethodId = (*env)->GetMethodID(env, CheckedInputStream,
    "read", "([B)I");
    if (!CheckedInputStream_readMethodId) {
    LOGEH("Unable to find the read method");
    return false;
    }

    CheckedInputStream_getChecksumMethodId = (*env)->GetMethodID(env,
    CheckedInputStream, "getChecksum", "()Ljava/util/zip/Checksum;");
    if (!CheckedInputStream_getChecksumMethodId) {
    LOGEH("Unable to find the getChecksum method");
    return false;
    }

    Checksum_getValueMethodId = (*env)->GetMethodID(env, Checksum, "getValue",
    "()J");
    if (!Checksum_getValueMethodId) {
    LOGEH("Unable to find the getValue method");
    return false;
    }

    return true;
}

static bool initJavaFields(JNIEnv * env) {
    ApplicationInfo_flagsFieldId = (*env)->GetFieldID(env, ApplicationInfo, "flags",
    "I");

    if (!ApplicationInfo_flagsFieldId) {
    LOGEH("Unable to find the flags field");
    return false;
    }

    ApplicationInfo_FLAG_DEBUGGABLEFieldId = (*env)->GetStaticFieldID(env,
    ApplicationInfo, "FLAG_DEBUGGABLE", "I");
    if (!ApplicationInfo_FLAG_DEBUGGABLEFieldId) {
    LOGEH("Unable to get static field FLAG_DEBUGGABLE");
    return false;
    }

    ApplicationInfo_sourceDirFieldId = (*env)->GetFieldID(env, ApplicationInfo,
    "sourceDir", "Ljava/lang/String;");
    if (!ApplicationInfo_sourceDirFieldId) {
    LOGEH("Unable to get static field sourceDir");
    return false;
    }

    return true;
}


jint JNI_OnLoad(JavaVM* vm, void* reserved) {
    (void) reserved; // Suppress the warning.
    JNIEnv * env;

    if ((*vm)->GetEnv(vm, (void **) &env, JNI_VERSION_1_6) != JNI_OK) {
    return -1;
    }

    if (!initJavaClasses(env)) {
    return -1;
    }

    if (!initJavaMethods(env)) {
    return -1;
    }

    if (!initJavaFields(env)) {
    return -1;
    }

    return JNI_VERSION_1_6;
}

    Don't forget to add the methods of the MyApplication into the Proguard-settings to prevent their remove!

    Usage:

    1. Create your APK with -DDEBUG_MODE=1
    2. start the APK on your Android
    3. read the value inside "CRC-check failed:xxx"
    4. take that value and insert it into CLASSES_CRC=xxxx
    5. switch to -DDEBUG_MODE=0
    6. rebuild your APK
    7. check if your app is running well
    8. publish

    This method is a little more complicated as it requires 2 builds to get a valid APK. But as the CRC is checked inside C and not just the dummy-signature of the APK-file is taken, this check is nearly bullet-proof.

    In my situation, I'm for exampling not even doing the setup of the license-mechanisms in case the signature is invalid.

    Additionally, as this method only uses numbers and not chars, it's fully compiled.

    Hope this helps someone!

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