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Abstract—Recently, the global outbreak of a blackmail virus WannaCry, makes the blockchain a hot topic. The security of blockchain is always the focus of people’s attention, and it is also the main reason why the blockchain has not been widely used all over the world. Many researches use mathematical derivation method to analyse the 51%- Attacks influence of blockchain, which is very stiff and difficult to understand. In this paper, we propose a method to simulate blockchain’s process and discover the rule between attacking method, attacking power and security of blockchain. We take 51%-Attacks as an example and use Java to simulate the running process. By adjusting the value of attacking power, we can get most states of blockchain and analyze the probability that honest state becomes attacking state. We use various forms to analyze and show the experimental result, which verify our method is correct and feasible. This method can also be implemented as a middleware software of blockchain to detect the security of blockchain.

I. INTRODUCTION - Decentralized cryptocurrencies like Bitcoin have captured the public’s interest, and have been much more successful than any prior incarnations of electronic cash [1]. Bitcoin uses a data structure known as blockchain to store the transactions. Blocks, which are batches of updates to the log, reference the parent they are extending, and thus form the structure of a chain[2]. The blockchain technology provides a decentralized, open, Byzantine faulttolerant transaction mechanism, and promises to become the infrastructure for a new generation of Internet interaction, including anonymous online payments, remittance, and transaction of digital assets [3]. Blockchain is the basic technology of bitcoin, which proves to be more valuable than the currency it supports. When we start to put this technology in use, it is very important to make sure its security. Actually, there are many kinds of attacks in blockchain such as 51%-Attack, eclipse attack, physical attack and so on. Many researches use mathematical methods to analyze the impact of attacks in order to evaluate the security of blockchain. Heilman et al[4]. use a detail mathematical method to analyze the affect of eclipse attacks. And he also use a similar method to implement, evaluate and detect a double-spending attack on bitcoin. But the attacks in blockchain have not been fully identified yet and there are still many people searching for the new attacks in blockchain. So analyzing the effect of each attack in blockchain is incomplete and there is an intense need to evaluate the security of blockchain completely. To address the aforementioned challenges, a method is proposed to analyze and evaluate the security of the blockchain.
 
We construct a model to represent the process of blockchain, in which 51%-Attack is the only attacking method, using two kinds of algorithms to simulate honest miners and attackers. By running the simulation process, we record the honest state number and attacking state number in different conditions. The result of experiments can be used to evaluate the security of blockchain. We regard the attacking states as target, and calculate the probability that each state becomes attacking state. If this probability reaches a high value, we can send a warning to all users in blockchian to defend being attacked. Meanwhile 15 2018 5th International Conference on Dependable Systems and Their Applications (DSA) 978-1-5386-9266-0/18/$31.00 ©2018 IEEE DOI 10.1109/DSA.2018.0001 we use various methods to show our experiment and analysis. According to the relevant researches, when a block connects with enough blocks, the message stored in this block can hardly be reversed and we regard this state as stable state. If this block contains illusory transactions, it is called as attacking state, or it is security state. 

In practical application, if more than six blocks are connected to a block, this block will almost be stable state and we do not need to process it any more. All these phenomenons indicate the states of blockchain are limited. If we can get most states of blockchain, the security of blockchain can be evaluated by analyzing the probability that each state of blockchain becomes attacking state. Our main contributions are summarized as follows: • A simulation model is proposed to evaluate the security of blockchain. We take the 51%-Attack for an example and implement this simulating model by Java. From this program, we get most states of blockchain including the attacking states, which validates our speculation that the states of blockchain are limited. • The relationships between attacking power and state number are revealed by some simulation experiments. The rest of this paper is organized as follows. Section 2 discusses the related work in this field. Section 3 introduces the basic concepts, and some essential parts in blockchain. 

Section 4 presents the detailed definitions of honest miner and attackers in blockchain. Section 5 proposes a novel method to solve these problems. Section 6 evaluates this approach with several experiments and analyzes the experimental result. Section 7 concludes the work in this paper and illustrates the future direction of this work. II. RELATED WORK Blockchain ensures the elimination of double spending in bitcoin which is a peer-to-peer electronic cash system. On the surface, comparing blockchain to banking systems, decentralised systems can better protect themselves against attacks, and route around damage. But there are still many attacks happening in blockchain and causing great damages to the users’ security. DDos attack is very common in blockchain system. Vasek et al[5]. present an empirical investigation into the prevalence and impact of distribute denial-of-service(DDos) attacks on operators in the bitcoin economy. They find currency exchanges, mining pools, gambling operators are much likely to be attacked than other services. And the big mining pools are much likely to be DDosed than small pools. Danny[6] introduces and analyzes the problem of bitcoin including 51%-Attack, doublespending, dust transactions and code-based attacks. These attacks all make it possible to manipulate the network for personal gain. Herrmann et al.

[7] evaluate the potential of double spending attacks on bitcoin and analyze the bitcoin system, in particular the double spend protection procedure, and identify a weakness in certain usage scenarios. As many attacks being found, people expect to analyze the features in order to solve or defend them. Decker et al.[8] analyze how bitcoin uses a multi-hop broadcast to propagate transactions and blocks through the network to update the ledger replicas. All these researches on blockchain are very important and useful to improve its security. With the development of blockchain, there are many applications based on it. Watanabe et al.[9] propose a new mechanism to secure a blockchain applied to contract management such as digital rights management. Digital contracts can decrease users’ fees and make it impossible for anyone to change or deny the content. Based on the imbalances caused by asymmetric information and opaque supply chains, Badzar el at.[10] aim to contribute to the research field of logistics and supply chain management by exploring the potential of blockchain technology within logistics. They use blockchain to increase supply chain transparency for both suppliers and consumers, which will improve the security and normalization of shopping online. Wilkinson et al.[11] create an open source software project seeking to prove conceptually that cloud storage applications can be made more decentralized, more secure, and efficient. Even some people use blockchain as a software connector. Xu et al.[12] propose a new method that new forms of distributed software architectures can find agreements on their shared states without trusting a central integration point or any particular participating components. Using blockchain technology in software engineering, it will make software developing process more standard and improve the success rate of developing software. According to Alibaba’s outlook about ten top technologies in the future.

A. Blockchain and Mining Blockchain represents a novel application of cryptography and information technology to ageold problems of financial record-keeping, and they may lead to far reaching changes in corporate governance[13]. It can be used to record the transactions in bitcoin. Each block in blockchian consists of a unique ID, the ID of former block, transactions, time stamp and so on, which are parts of the protocol. A valid block contains a solution to an encryption puzzle and the bitcoin address which is to be credited with a reward for solving the encryption puzzle[14]. The most important and famous process of blockchain is mining. Miners are always collecting the transactions which are created in a period of time. Then the miners will use the Hash256 algorithm to process these transactions and get a string, which must start with some zero and the number of zero must be more than the threshold value. If not, the miners have to repeat the mining process. The blockchain system has set the threshold value to represent the difficulty of the problem. 

And this value is dynamically adjusted every ten minutes. The larger the threshold value is, the more operations have to be done. When the string fits for the requirement, the miners will be rewarded a certain number of bitcoin that did not exist before. This process is similar to the bank to issue money. After processing the block successfully, the miners will publish it over the network and other miners will validate whether these transactions and the mining string are correct. If two blocks have the same former block, the blockchain will create a fork. After that, miners can add a new block to any branch. In order to keep unanimous, the blockchain protocol stipulates that only transactions in the longest chain will be accepted and others will be ignored. Miners would better mine the new block after the longest chain. With this protocol, the blockchain is consistent and every miner will have the same structure. Only in a very short interval, the blockchain will be different, which is the main reason why attacks can occur in the blockchain. In real conditions, the average mining interval is ten minutes. Therefore accidental bifurcation is rare, and occurs on average once about every 60 blocks[8]. Attackers use this property to reverse the transactions. In order to solve these challenges, the Nakamoto consensus method is proposed, which is a mathematical method to solve the problem of trust. 

B. Nakamoto Consensus The most important feature of blockchain is a set of protocols, which are called Nakamoto consensus. It can solve the problem of trust without a center organization. In theory, there is not a robust method to ensure consensus, any participant could, for example, spend the same bitcoins multiple times (known as the double-spend problem), or claim it had more currency than it really did[15]. The Byzantine fault tolerant protocol can be used to solve this problem, which is similar to the voting process. In blockchain, every node can read all transactions and collect new transactions which have more fee or other features. After mining a new block successfully and broadcast the new block to other nodes, other nodes will validate this block and decide whether connect this new block to the branch or not. Everyone can join blockchain and express their opinion. Hence if the voting process is so easy for participants, all transactions including illusory transactions also can be recorded in the blockchain, which makes the blockchain losing credible feature and becoming unsafe. Proof-of-work algorithm is proposed to solve this challenge, which is like academic certificate. Proofof-work requires expensive computing hardware and particular machine. If a miner finishes proof-ofwork, it represents this miner may not be an attacker, or an attacker has to spend more time attacking the blockchain. The correctness of blockchain, so long as over a half of the network’s power are honest, will be believable.

IV. PROBLEM - The security of blockchain is always the focus of our attention, which will be affected by many kinds of factors such as the types of attacks, state of network,progress in science and so on. In this section, We propose the formal representation of 51%-Attack, honest miner, attacker and the protocol of blockchain. A. 51%-Attack Representation One of the most famous attacking method in blockchain is 51%-Attack[17], which is hypothesized that a group of miners control more than 50% of the network’s mining hashrate, or computing power. The attackers would be able to prevent new transactions from gaining confirmations, halting them between merchants and clients. Attackers can complete proof-of-work quicker than honest miners. As a result, their transactions will be connected to the longest chain. The more mining hashrate they control, the faster attacks happen in blockchain. 51%-Attack can be used to reverse transactions and spend same coins many times when attackers control more than 50% network’s mining hashrate. Satoshi Nakamoto[18] proposed the framework of bitcoin and calculated the attacking probability on different computing power that attackers controlled. The speed of honest chain and attacking chain is characterized as a binomial random walk. 

The rate that an attack can catch up with the honest chain is presented as follows[8]: qz = 1 p ? q (q/p) z p>q (1) p represents the probability that honest nodes find the new block first, q is the probability that attackers mine the new block faster than honest miners and qz is the probability attackers catch up honest nodes from z blocks behind. In probability aspect, if attacking power is larger than the honest power, the attackers are certain to catch up with the honest chain successfully. Actually, the probability of attacking is effected by many factors such as the change of the network’s mining power, the difficulty of proof-ofwork and so on. In this paper, we simulate the whole process of blockchain and count most states of blockchain. Then we can calculate the security probability of blockchain by analyzing every state. In order to know more details about the process of blockchain, we use formulas to represent the protocol of blockchain from the aspects of honest miner and attacker.

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