NDK application Signature Check

Question

I have some security key in an application. I want to store it securly. I like to store it in a native shared library (maybe generated from some code). After that I want it to b

Answer 1

TL;DR An example can be found here.

I get a signature on native layer(C-code) another way:

• Get a path of the APK;
• Extract 'META-INF/CERT.RSA' from the APK;
• Parse 'META-INF/CERT.RSA'

The code for getting the APK path:

    static char *getPackageName() {
    const size_t BUFFER_SIZE = 256;
    char buffer[BUFFER_SIZE] = "";
    int fd = open("/proc/self/cmdline", O_RDONLY);
    if (fd > 0) {
        ssize_t r = read(fd, buffer, BUFFER_SIZE - 1);
        close(fd);
        if (r > 0) {
            return strdup(buffer);
        }
    }
    return NULL;
}

static const char *getFilenameExt(const char *filename) {
    const char *dot = strrchr(filename, '.');
    if (!dot || dot == filename) return "";
    return dot + 1;
}

char *pathHelperGetPath() {

    char *package = getPackageName();
    if (NULL == package) {
        return NULL;
    }

    FILE *fp = fopen("/proc/self/maps", "r");
    if (NULL == fp) {
        free(package);
        return NULL;
    }
    const size_t BUFFER_SIZE = 256;
    char buffer[BUFFER_SIZE] = "";
    char path[BUFFER_SIZE] = "";

    bool find = false;
    while (fgets(buffer, BUFFER_SIZE, fp)) {
        if (sscanf(buffer, "%*llx-%*llx %*s %*s %*s %*s %s", path) == 1) {
            if (strstr(path, package)) {
                char *bname = basename(path);
                NSV_LOGI("check basename[%s]", bname);
                if (strcasecmp(getFilenameExt(bname), "apk") == 0) {
                    find = true;
                    break;
                }
            }
        }
    }
    fclose(fp);
    free(package);
    if (find) {
        return strdup(path);
    }
    return NULL;
}

We can use zlib since 3 API in Android and I use minizip for convinience.

The code to extract of the META-INF/CERT.RSA below:

//return MZ_ERROR
static int32_t unzipHelperGetCertFileInfo(void *handle, mz_zip_file **file_info) {

    int32_t err = MZ_OK;

    err = mz_zip_goto_first_entry(handle);

    if (err != MZ_OK && err != MZ_END_OF_LIST) {
        NSV_LOGE("Error %d going to first entry in zip file\n", err);
        return err;
    }

    while (err == MZ_OK) {
        err = mz_zip_entry_get_info(handle, file_info);

        if (err != MZ_OK) {
            NSV_LOGE("Error %d getting entry info in zip file\n", err);
            *file_info = NULL;
            break;
        }

        if (NULL != (*file_info)->filename && strcasecmp((*file_info)->filename,"META-INF/CERT.RSA") == 0) {
            return MZ_OK;
        }

        err = mz_zip_goto_next_entry(handle);

        if (err != MZ_OK && err != MZ_END_OF_LIST) {
            *file_info = NULL;
            NSV_LOGE("Error %d going to next entry in zip file\n", err);
            return err;
        }
    }

    *file_info = NULL;

    if (err == MZ_END_OF_LIST) {
        return MZ_OK;
    }
    return err;
}

static void unzipHelperPrintFileInfo(const mz_zip_file *file_info) {
    uint32_t ratio = 0;
    struct tm tmu_date;
    const char *string_method = NULL;
    char crypt = ' ';

    usleep(500);
    NSV_LOGI("  Length  Method      Size  Attribs Ratio   Date    Time   CRC-32     Name\n");
    usleep(500);
    NSV_LOGI("  ------  -------     ----  ------- -----   ----    ----   ------     ----\n");
    ratio = 0;
    if (file_info->uncompressed_size > 0)
        ratio = (uint32_t)((file_info->compressed_size * 100) / file_info->uncompressed_size);

    // Display a '*' if the file is encrypted
    if (file_info->flag & MZ_ZIP_FLAG_ENCRYPTED)
        crypt = '*';

    switch (file_info->compression_method)
    {
        case MZ_COMPRESS_METHOD_RAW:
            string_method = "Stored";
            break;
        case MZ_COMPRESS_METHOD_DEFLATE:
            string_method = "Deflate";
            break;
        case MZ_COMPRESS_METHOD_BZIP2:
            string_method = "BZip2";
            break;
        case MZ_COMPRESS_METHOD_LZMA:
            string_method = "LZMA";
            break;
        default:
            string_method = "Unknown";
    }

    mz_zip_time_t_to_tm(file_info->modified_date, &tmu_date);
    usleep(500);
    NSV_LOGI(" %7"PRIu64"  %6s%c %7"PRIu64" %8"PRIx32" %3"PRIu32"%%  %2.2"PRIu32"-%2.2"PRIu32\
               "-%2.2"PRIu32"  %2.2"PRIu32":%2.2"PRIu32"  %8.8"PRIx32"   %s\n",
             file_info->uncompressed_size, string_method, crypt,
             file_info->compressed_size, file_info->external_fa, ratio,
             (uint32_t)tmu_date.tm_mon + 1, (uint32_t)tmu_date.tm_mday,
             (uint32_t)tmu_date.tm_year % 100,
             (uint32_t)tmu_date.tm_hour, (uint32_t)tmu_date.tm_min,
             file_info->crc, file_info->filename);

}

unsigned char *unzipHelperGetCertificateDetails(const char *fullApkPath, size_t *len) {

    unsigned char *result = NULL;
    int32_t err = 0;
    int32_t read_file = 0;

    void *handle = NULL;
    void *file_stream = NULL;
    void *split_stream = NULL;
    void *buf_stream = NULL;
    char *password = NULL;

    int64_t disk_size = 0;
    int16_t mode = MZ_OPEN_MODE_READ;
    int32_t err_close = 0;

    if (mz_os_file_exists(fullApkPath) != MZ_OK) {
        NSV_LOGE("file %s doesn't exit\n", fullApkPath);

    }
    mz_stream_os_create(&file_stream);
    mz_stream_buffered_create(&buf_stream);
    mz_stream_split_create(&split_stream);

    mz_stream_set_base(split_stream, file_stream);

    mz_stream_split_set_prop_int64(split_stream, MZ_STREAM_PROP_DISK_SIZE, disk_size);

    err = mz_stream_open(split_stream, fullApkPath, mode);
    mz_zip_file *file_info = NULL;
    if (err != MZ_OK) {
        NSV_LOGE("Error opening file %s\n", fullApkPath);
    } else {
        handle = mz_zip_open(split_stream, mode);

        if (handle == NULL) {
            NSV_LOGE("Error opening zip %s\n", fullApkPath);
            err = MZ_FORMAT_ERROR;
        } else {
            err = unzipHelperGetCertFileInfo(handle, &file_info);
            if (err == MZ_OK && NULL != file_info) {
                unzipHelperPrintFileInfo(file_info);
                //unzip
                err = mz_zip_entry_read_open(handle, 0, password);
                if (err != MZ_OK) {
                    NSV_LOGW("Error %d opening entry in zip file\n", err);
                } else {
                    result = calloc(file_info->uncompressed_size, sizeof(unsigned char));
                    if (NULL != result) {
                        read_file = mz_zip_entry_read(handle, result,
                                                      (uint32_t) (file_info->uncompressed_size));
                        if (read_file < 0) {
                            free(result);
                            result = NULL;
                            err = read_file;
                            NSV_LOGW("Error %d reading entry in zip file\n", err);
                        } else {
                            NSV_LOGI("read %d from zip file\n", read_file);
                            *len = (size_t) read_file;
                        }
                    }
                }
            }
        }

        err_close = mz_zip_close(handle);

        if (err_close != MZ_OK) {
            NSV_LOGE("Error in closing %s (%d)\n", fullApkPath, err_close);
            err = err_close;
        }

        mz_stream_close(split_stream);

    }
    mz_stream_split_delete(&split_stream);
    mz_stream_buffered_delete(&buf_stream);
    mz_stream_os_delete(&file_stream);

    return result;
} 

For parsing META-INF/CERT.RSA I use parts of the code from one public repository. It's too big to be posted on StackOverflow, so the full source code of the working example can be found here.

upd:

Here is an example how we can get MD5 from a signature(using mbed TLS):

    size_t len_in = 0;
    size_t len_out = 0;
    content = unzipHelperGetCertificateDetails(path, &len_in);
    LOGI("unzipHelperGetCertificateDetails finishes\n");
    if (!content) {
        return;
    }
    LOGI("pkcs7HelperGetSignature starts\n");
    unsigned char *res = pkcs7HelperGetSignature(content, len_in, &len_out);
    LOGI("pkcs7HelperGetSignature finishes, len_out:[%zu]\n", len_out);
    if (NULL == res) {
        return;
    }
    LOGI("calculating md5\n");
    unsigned char md5sum[16] = {""};
    mbedtls_md5((unsigned const char *) res, len_out, md5sum);
    char md5string[33];
    for (int i = 0; i < 16; ++i) {
        sprintf(&md5string[i * 2], "%02x", (unsigned int) md5sum[i]);
    }
    LOGI("md5:[%s]\n", md5string);