开头

NDK 实践-应用签名校验。

应用签名

Android 应用签名是应用打包过程的重要步骤之一，Google 要求所有的应用必须被签名才可以安装到 Android 操作系统中。

应用签名不能保证 APK 不被篡改，只是为了能够校验出 APK 是否被篡改。在系统安装过程中，如果发现 APK 被篡改，安装就会失败。

NDK 应用签名校验

为了相对安全，一些敏感操作往往会使用 Native 的方式来实现。但是别人可以通过 APK 文件获取到我们的 .so 文件，进而使用我们的 .so。

但是应用签名的证书只有我们持有，我们可以通过 Native 校验签名来判断是否是我们自己的应用，如果不是可以返回错误或直接退出应用。

动手实践

像之前一样创建一个 Native C++ 模板项目

项目准备

查看证书指纹：

新建的 Android 项目，默认的签名证书在用户根目录的 .android 目录中 ~/.android/debug.keystore

$ keytool -list -v -keystore debug.keystore
输入密钥库口令:
android

Java 获取证书指纹

public class SignatureUtil {

    public static String getSignatureStr(Context context) {
        Signature signature = getSignature(context);
        byte[] cert = signature.toByteArray();
        try {
            MessageDigest md5 = MessageDigest.getInstance("MD5");
            MessageDigest sha1 = MessageDigest.getInstance("SHA1");
            MessageDigest sha256 = MessageDigest.getInstance("SHA256");
            byte[] md5Key = md5.digest(cert);
            byte[] sha1Key = sha1.digest(cert);
            byte[] sha256Key = sha256.digest(cert);
            return String.format("MD5: %s\n\nSHA1: %s\n\nSHA-256: %s",
                    byteArrayToString(md5Key),
                    byteArrayToString(sha1Key),
                    byteArrayToString(sha256Key)
            );
        } catch (Exception e) {
            return "";
        }
    }

    public static Signature getSignature(Context argContext) {
        Signature signature = null;
        try {
            String packageName = argContext.getPackageName();
            PackageManager packageManager = argContext.getPackageManager();
            PackageInfo packageInfo = packageManager.getPackageInfo(packageName, GET_SIGNATURES);
            Signature[] signatures = packageInfo.signatures;
            signature = signatures[0];
        } catch (NameNotFoundException e) {
            e.printStackTrace();
        }
        return signature;
    }
    
    private static String byteArrayToString(byte[] array) {
        StringBuilder hexString = new StringBuilder();
        for (int i = 0; i < array.length; i++) {
            String appendString = Integer.toHexString(0xFF & array[i]).toUpperCase();
            if (appendString.length() == 1)
                hexString.append("0");
            hexString.append(appendString);
            if(i < array.length - 1)
                hexString.append(":");
        }
        return hexString.toString();
    }
}

Native 获取证书指纹

这里用到了 HASH 算法，Android NDK JNI 入门笔记-day04-NDK实现Hash算法

jbyteArray getSignatureByte(JNIEnv *env, jobject context);
void hashByteArray(HASH type, const void* data, size_t numBytes, char* resultData);
void formatSignature(char* data, char* resultData);

extern "C"
JNIEXPORT jstring JNICALL
Java_com_ihubin_ndkjni_NativeUtil_getSignature(JNIEnv *env, jclass clazz, jobject context) {
    jbyteArray cert_byteArray = getSignatureByte(env, context);
    jsize size = env->GetArrayLength(cert_byteArray);
    jbyte* jbyteArray = new jbyte[size];
    env->GetByteArrayRegion(cert_byteArray, 0, size, jbyteArray);

    char certMD5[128] = {0};
    hashByteArray(HASH_MD5, jbyteArray, size, certMD5);
    char certSHA1[128] = {0};
    hashByteArray(HASH_SHA1, jbyteArray, size, certSHA1);
    char certSHA256[128] = {0};
    hashByteArray(HASH_SHA256, jbyteArray, size, certSHA256);
    LOGD("MD5: %s", certMD5);
    LOGD("SHA1: %s", certSHA1);
    LOGD("SHA256: %s", certSHA256);

    char resultStr[1000] = {0};
    strcat(resultStr, "MD5: ");
    strcat(resultStr, certMD5);
    strcat(resultStr, "\n\nSHA1: ");
    strcat(resultStr, certSHA1);
    strcat(resultStr, "\n\nSHA256: ");
    strcat(resultStr, certSHA256);

    return env->NewStringUTF(resultStr);
}

// Native 从 Context 中获取签名
jbyteArray getSignatureByte(JNIEnv *env, jobject context) {
    // Context 的类
    jclass context_clazz = env->GetObjectClass(context);

    // 得到 getPackageManager 方法的 ID
    jmethodID methodID_getPackageManager = env->GetMethodID(context_clazz, "getPackageManager", "()Landroid/content/pm/PackageManager;");

    // 获得 PackageManager 对象
    jobject packageManager = env->CallObjectMethod(context, methodID_getPackageManager);

    // 获得 PackageManager 类
    jclass packageManager_clazz=env->GetObjectClass(packageManager);

    // 得到 getPackageInfo 方法的 ID
    jmethodID methodID_getPackageInfo=env->GetMethodID(packageManager_clazz,"getPackageInfo", "(Ljava/lang/String;I)Landroid/content/pm/PackageInfo;");

    // 得到 getPackageName 方法的 ID
    jmethodID methodID_getPackageName = env->GetMethodID(context_clazz,"getPackageName", "()Ljava/lang/String;");

    // 获得当前应用的包名
    jobject application_package_obj = env->CallObjectMethod(context, methodID_getPackageName);
    jstring application_package = static_cast(application_package_obj);
    const char* package_name = env->GetStringUTFChars(application_package, 0);
    LOGD("packageName: %s", package_name);

    // 获得 PackageInfo
    jobject packageInfo = env->CallObjectMethod(packageManager, methodID_getPackageInfo, application_package, 64);
    jclass packageinfo_clazz = env->GetObjectClass(packageInfo);

    // 获取签名
    jfieldID fieldID_signatures = env->GetFieldID(packageinfo_clazz, "signatures", "[Landroid/content/pm/Signature;");
    jobjectArray signature_arr = (jobjectArray)env->GetObjectField(packageInfo, fieldID_signatures);

    // Signature 数组中取出第一个元素
    jobject signature = env->GetObjectArrayElement(signature_arr, 0);

    // 读 signature 的 ByteArray
    jclass signature_clazz = env->GetObjectClass(signature);
    jmethodID methodID_byteArray = env->GetMethodID(signature_clazz, "toByteArray", "()[B");
    jobject cert_obj = env->CallObjectMethod(signature, methodID_byteArray);
    jbyteArray cert_byteArray = static_cast(cert_obj);

    return cert_byteArray;
}

// 获得签名的 MD5 SHA1 SHA256
void hashByteArray(HASH type, const void* data, size_t numBytes, char* resultData){
    if(type == HASH_MD5) {
        MD5 md5;
        std::string md5String = md5(data, numBytes);
        int len = md5String.length()+1;
        char * tabStr = new char [md5String.length()+1];
        strcpy(tabStr, md5String.c_str());
        formatSignature(tabStr, resultData);
    } else if(type == HASH_SHA1) {
        SHA1 sha1;
        std::string sha1String = sha1(data, numBytes);
        char * tabStr = new char [sha1String.length()+1];
        strcpy(tabStr, sha1String.c_str());
        formatSignature(tabStr, resultData);
    } else if(type == HASH_SHA256) {
        SHA256 sha256;
        std::string sha256String = sha256(data, numBytes);
        char * tabStr = new char [sha256String.length()+1];
        strcpy(tabStr, sha256String.c_str());
        formatSignature(tabStr, resultData);
    }
}

// 格式化输出
void formatSignature(char* data, char* resultData) {
    int resultIndex = 0;
    int length = strlen(data);
    for(int i = 0; i < length; i++) {
        resultData[resultIndex] = static_cast(toupper(data[i]));
        if(i % 2 == 1 && i != length -1) {
            resultData[resultIndex+1] = ':';
            resultIndex+=2;
        } else {
            resultIndex++;
        }
    }
}