## 1 Introduction  
Bitcoin is one of the most representative cryptocurrency, and has contributed to building a reliable and decentralized cryptocurrency environment on a P2P network without the need of central trusted authorities [1]. Blockchain is the underlying data structure technology of Bitcoin, which is a continuously growing chain of blocks each having a set of transactions that occur between participating peers. Each participant maintains a distributed ledger consisting of sequentially chained blocks, and these blocks are propagated and validated by full nodes with the proof-of-work consensus protocol (in Bitcoin) between validator nodes. When an untrusted third party participates in the service operation, blockchain suppresses any possible data manipulation through the consensus process between the peers.  
Characteristics such as immutability, irreversibility, and decentralization has attracted increasing interest in blockchain for purposes other than cryptocurrency, and numerous blockchain-based systems and applications have been proposed [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. However, the architecture of the early blockchain systems,such as Bitcoin, also had several drawbacks which made it difficult to apply to applications demanding more sophisticated operations than just 'transfer-of-ownership'. That is because most core parts of the system, like data structure and operations, are designed specifically for the purpose of cryptocurrency to avoid various network attacks (e.g., 51% majority attack).  
Despite the drawbacks, the potential of blockchain technology shown by Bitcoin has led to a number of studies to improve it as a distributed or decentralized platform applicable to diverse types of application. By applying the '_smart-contract_' concept which makes it possible to derive deterministic and immediate output under pre-defined conditions, the blockchain technology seemed effective for developing decentralized applications (_DApps_). However, a fundamental problem driven by the consensus protocol still remained; the network was far too slow to implement practical applications at a large scale. Although the Proof-of-Work consensus protocol is an excellent way to deter arbitrary attackers in an public environment with ruthless attack potential, it works as an obstacle upon the DApps, restricting it from providing general services to users.  
There have been several studies for more decisive but generous consensus protocols working on a permissioned/private network [18; 19; 20; 21], but these attempts still have the limits of scalability as the network governs the access of the nodes to demarcate from outside. That is, the scalability and performance are incompatible trade-off factors in running an application on blockchain based platforms. In addition to the limitations mentioned above, service providers encounter several difficulties when implementing actual applications running on blockchain based platforms. Service providers not only need to handle the business logic specific to their services, but also tasks of designing and maintaining blockchain systems. In Section 2, the problems that have been encountered to apply blockchain for DApps will be discussed in more detail.  
This paper presents 'Doubly Decentralized Network Blockchain' (DDNB), a platform architecture that suppresses untrusted participants' malicious behavior by separating the service and blockchain layers, and allows the service providers to conveniently implement general business logic on DApps. In DDNB, blockchain layer and the service layer are architecturally separated so that they depend only on each other's interfaces. While using the 'chaining' and 'distributed' concepts of blockchain, it provides a structure for service developers to develop DApps without the need of deeply understanding the blockchain technology.