Data Sealing Java File I/O to NDK Conversion Guide

AppSealing provides Data Sealing that encrypts and protects sensitive data files. To use Data Sealing, you must follow the file naming convention below. Currently, Data Sealing only supports Native File I/O functions.

📌 Data Sealing File Naming Convention

The Data Sealing file naming convention is simple. If the file name starts with the ' asdp_ ' keyword, the file is recognized as a data sealing file. After sealing, encryption/decryption is automatically applied in the process of writing and reading this data as shown below. (res is applied only in res/raw directory)

📌 Native File I/O Support

Data Sealing currently supports only Native File I/O functions. Therefore, in an environment developed with Java/Kotlin, the file input/output part must be converted to the NDK (C/C++) environment. The following Native File I/O functions are supported:

• Supports fopen File I/O functions

fopen
fseek
ftell
fread
fwrite
fclose

• Supports AAsset functions

AAssetManager_open
AAsset_getBuffer
AAsset_getLength
AAsset_getLength64
AAsset_getRemainingLength
AAsset_getRemainingLength64
AAsset_read
AAsset_seek
AAsset_seek64
AAsset_close

📌 Basic example of converting Java to Native C/C++

Example of reading a Native File I/O file

• Java (Does not support Data Sealing)

/**
 * Read a file using FileInputStream
 *
 * @param filePath (ex) asdp_sample.txt
 * @return 
 */
public String getFileText(String filePath) {
    String dataText = "";
    FileInputStream fileStream = null;

    try {
        fileStream = new FileInputStream(filePath);

        byte[] buffer = new byte[fileStream.available()];
        int count = fileStream.read(buffer);
        if (count > 0) {
            dataText = new String(buffer, 0, count);
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        try {
            fileStream.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    return dataText;
}

• NDK (C/C++)

/**
 * Read a file using fopen
 *
 * @param env  jni function default parameter
 * @param cls  jni function default parameter
 * @param filePath (ex) asdp_sample.txt
 * @return 
 */
extern "C" JNIEXPORT jstring JNICALL
Java_com_appsealing_datasealing_test_TestNative_getFileText(JNIEnv* env,
                                                            jobject cls,
                                                            jstring filePath)
{
    const char* testPath = env->GetStringUTFChars(filePath, 0);

    FILE * fp = fopen(testPath, "rb");
    env->ReleaseStringUTFChars( filePath, testPath );
    if(fp == NULL)
    {
        LOGE("File fopen failed.");
        // File open failed
        return env->NewStringUTF("");
    }

    fseek(fp, 0, SEEK_END);
    int size = ftell(fp); // get file length using ftell

    char buffer[10240] = { 0x00, };
    int len;

    fseek(fp, 0, SEEK_SET);
    len = fread(buffer, size, 1, fp);
    LOGD("%s size: %d, len: %d\n", buffer, size, len);

    std::string dataText = buffer;

    fclose(fp);

    return env->NewStringUTF(dataText.c_str());
}

Example of reading a Native AAset file

• Java (Does not support Data Sealing)

/**
 * Read a file using Asset Manager
 *
 * @param assetName (ex) asdp_sample.txt
 * @return 
 */
public String getAssetFileText(String assetName) {
    String assetText = "";

    AssetManager assetManager = context.getAssets();
    InputStream stream = null;

    try {
        stream = assetManager.open(assetName);
        byte buffer[] = new byte[stream.available()];
        if (stream.read(buffer) > 0) {
            assetText = new String(buffer);
        }
        stream.close();
    } catch (Exception e) {
        e.printStackTrace();
    }

    return assetText;
}

• NDK (C/C++)

/**
 * Read a file using AAssetManager
 *
 * @param env  jni function default parameter
 * @param cls  jni function default parameter
 * @param assetManager (ex) java:context.getAssets() 
 * @param assetName (ex) asdp_sample.txt
 * @return 
 */
extern "C" JNIEXPORT jstring JNICALL
Java_com_appsealing_datasealing_test_TestNative_getAssetFileText(
        JNIEnv* env,
        jobject cls,
        jobject assetManager,
        jstring assetName) {

    const char* temp = env->GetStringUTFChars(assetName, 0);
    std::string asset_name = temp;
    env->ReleaseStringUTFChars( assetName, temp );

    AAssetManager* am = AAssetManager_fromJava(env, assetManager);
    AAsset* asset = AAssetManager_open(am, asset_name.c_str(), AASSET_MODE_UNKNOWN);
    if (asset == NULL)
        return env->NewStringUTF("");

    int fileSize = AAsset_getLength(asset);
    if (fileSize == 0)
        return env->NewStringUTF("");

    std::string assetText(fileSize + 1, 0x00);
    AAsset_read(asset, (void*)assetText.data(), fileSize);

    AAsset_close(asset);

    return env->NewStringUTF(assetText.c_str());
}