## 3 Challenges and the State of the Art Progresses
### Security Concern  
Blockchain has some desirable features that would help to secure transaction data. These features include tamper resistance, distributed ledgers, cryptography secured records, and resilient to the single point failure. However, the maturity of this technology including security of the fundamental protocols and its applications is not enough, and more conditions and requirements need to be considered when blockchain is applied in business. First, we discuss and summarize the difference between blockchain security and the standard cyber security [11]. * The goal is different. The goal of the standard cyber security is to eliminate external attackers from tampering the data. The goal of blockchain security is to make data tampering infeasible by storing data copies at as many possible locations. * The infrastructure is different. The standard cyber system is usually centralized, and the security is achieved through the strict admission control and permission grant. The blockchain system is concentrated on decentralization, and availability and integrity is provided by the distributed nature of the ledger. * The environment is different. The standard cyber system is controlled by the security staff and a semi-trusted (if not fully trusted) environment is assumed. Blockchain is supposed to run on untrusted distributed devices without a central authority. Anyway, despite the fundamental difference of blockchain and the standard cyber security, blockchain still faces the security challenges that bother the standard cyber system. The work in [11] discusses these challenges in detail. First, blockchains can be overloaded by DDoS (Distributed Denial of Service) attacks. Second, since all peers in the blockchain are equivalent, any defect in an endpoint can be a defect to the whole system. Third, the application code (i.e., the smart contract in blockchain) can be the security threat. Unlike the traditional system, smart contracts can be submitted by malicious users, and therefore any malicious code in a smart contract can lead to a severe consequence. Even if the blockchain system is theoretically tamper-proof due to the cryptographic nature and the consensus protocol, there still exists creative ways to cheat. For instance, a selfish miner can trick other nodes to waste time on already-solved crypto-puzzles, and therefore subvert a blockchain even if it has less than half of the power. A malicious user can take control of an honest node's communications and trick it to accept false blocks. Another suspicious guarantee of a blockchain system is that it is not as decentralized as it intends to be. The nodes are often dominated by peers with more mining capacity, which makes Bitcoin or Ethereum a centralized system in fact. In permissioned systems, to solve the security issues, the owner may be granted more authority, which can be thought to violate the fundamental nature of blockchain. Finally, integrating with existing systems, cryptographic key material management, and providing network connection with required QoS guarantees are thought to be the major security concerns by Hyperledger [42]. The concern arising from the network quality is due to the fact that most consensus algorithms are latency sensitive and one potential DoS attack can happen by disrupting communication. [PERSON] investigated the way to enhance the security of blockchains by the formal analysis and formal verification [43]. A framework is proposed to apply formal analysis to the implementation, protocol and language layer of blockchain by using existing standards and results. [PERSON] et al. studied the blockchain access privacy in the work [4]. To ensure access privacy, anonymous communications networks, such as Tor ([[https://www.torproject.org](https://www.torproject.org)]([https://www.torproject.org](https://www.torproject.org))), have been proposed as candidate solutions. The work in [4] challenges this approach by showing that privacy can be breached on the anonymous communication networks and new approaches have to be sought.