String filePath = "";
        SecurityHelper.getInstance()
                .fileMD5Async(filePath)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Subscriber<String>() {
                    @Override
                    public void onCompleted() {

                    }

                    @Override
                    public void onError(Throwable e) {

                    }

                    @Override
                    public void onNext(String s) {
                        // TODO
                        unsubscribe();
                    }
                });