## 6 Conclusions and Future Work  
Blockchain cloud provides users with a cost-effective service through on-demand resource sharing, but the widely adopted virtualization technique in the cloud also introduces additional security risks for blockchain. ACDs emerge as a solution that constantly changes the attack surface exploitable for attackers. However, there is still a lack of standard methods to quantitatively evaluate the overall cost and benefit of adopting ACDs in combination. In this work, we address the effectiveness of ACDs in solving the security problem brought by resource sharing in the blockchain cloud. This paper proposes SPM, a layered model to evaluate the effectiveness of an ACD network adopting technologies such as NVP on each node and VM migration. To the best of our knowledge, this work is the first to analyze the security of a network that combines multiple ACDs using a hierarchical combination of methods involving Martingale theory along with SRN, Poisson process, and Markov chain. We distinguish three kinds of attacks aiming at a specific target within three layers, namely single-step attacks (on the bottom layer), repetitive attacks against a node before the migration (middle layer), and attacks after the migration (on the top layer). First, the SRN-based model captures the attack's fine-grained information, the output of which is used later as an input to the second layer of analysis. Then, the Poisson process is used to describe the process of repetitive attacks on a single node. These two layers evaluate the defensive ability of each ACD node. Over different migration periods, the random walk of an attacker along the attack chain is described by a Markov chain, which is set up to calculate the steady-state probability of compromising the target. Moreover, the Markov chain is studied to build a Martingale sequence to calculate the time of compromising the target and the time of repairing the network after it being entirely compromised. Finally, based on the simulations, we discuss how to choose applicable and optimum configurations from the network designers' standpoint. On the other hand, the synergy of combining multiple ACD mechanisms is demonstrated. The findings in this paper include (1) a hierarchical structure loosely coupled with three-layer models, which improves the flexibility and scalability; (2) the thorough assessment included various metrics describing the effectiveness of ACDs adopted at different network layers, which improves the evaluation scope; (3) an integrated evaluation analyzed the complete attacking process in an ACD network integrated various ACD technology. For our future works, we plan to further research the defensive process to revise our assumptions that can help generalize our approach. In addition, we will adopt the proposed evaluation method in a more elaborate and practical case study, such as defensive experiments in a real cloud environment. By doing so, we aim at verifying the proposed model with extensive evaluations through real-world experiments rather than theoretical simulations. Besides, we then plan to evaluate and compare different ACD technologies, such as MTD and CMD and summarize the main positive and negative aspects of each approach. We believe that this combination of evaluation methods gives expectations toward building stronger defensive mechanisms in the actual systems. [PERSON] contributed to concept conceptualization, formal analysis, investigation, methodology, preparing figures, writing--original draft. [PERSON] contributed to the investigation, validation, visualization, writing--review & editing. [PERSON] contributed to funding acquisition, project administration, resources, supervision, writing--review & editing. [PERSON] contributed to validation, visualization, writing--review & editing. [PERSON]-[PERSON] contributed to supervision, validation, writing--review & editing. All authors have read and agreed to the published version of the manuscript.