**1. Introduction**

About ten thousand cryptocurrencies are being traded on cryptocurrency exchanges [1], and about one hundred countries are exploring central bank digital currencies (CBDCs) in one form or another. For example, some countries are researching, some are testing, and some have already distributed CBDCs to the public [2–4]. To enhance digital financial services and improve user convenience, transfer between heterogeneous cryptocurrencies, transfer between heterogeneous CBDCs, and further transfer between cryptocurrency and CBDC should be required.

However, due to the lack of interoperability between heterogeneous blockchains, there is a problem related to the transfer between blockchain-based heterogeneous cryptocurrencies (e.g., Bitcoin [5], Ether [6], etc.) and CBDCs (e.g., US CBDC, UK CDBC, Korean CBDC, Chinese CBDC, etc.). For example, it is difficult to transfer between a Bitcoin wallet and an Ether wallet, between a US CBDC wallet and a Korean CBDC wallet, or between a Bitcoin wallet and a US CBDC wallet. Existing studies to address the lack of interoperability between heterogeneous blockchains have progressed towards centralized architectures where intermediaries handle ledger data sharing between blockchains. The sharing of ledger data that records the transaction history of cryptocurrencies and CBDCs is an essential operation for the interoperability between heterogeneous blockchains.

**Citation:** Park, K.; Youm, H.-Y. Proposal of Decentralized P2P Service Model for Transfer between Blockchain-Based Heterogeneous Cryptocurrencies and CBDCs. *Big Data Cogn. Comput.* **2022**, *6*, 159. https://doi.org/10.3390/ bdcc6040159

Academic Editors: Peter R.J. Trim, Yang-Im Lee and Min Chen

Received: 7 November 2022 Accepted: 15 December 2022 Published: 19 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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This paper proposes a decentralized peer-to-peer (P2P) service model for transferring between blockchain-based heterogeneous cryptocurrencies and CBDCs. The proposed service model provides a solution for transferring between blockchain-based heterogeneous cryptocurrencies and CBDCs without centralized intermediaries, such as cryptocurrency exchanges, banks, transfer service providers, and so on.

The contribution of this paper is as follows: to the best of our knowledge, there has been no previous study on a decentralized P2P service model for transferring between blockchain-based heterogeneous cryptocurrencies and CBDCs, and the proposed service model, based on an interoperable architecture that shares ledger data without intermediaries between heterogeneous blockchains, provides a solution for transferring between blockchain-based heterogeneous cryptocurrencies and CBDCs. The proposed decentralized P2P service model improves user convenience and ledger data security compared to the existing centralized service model.

This paper is organized into the following sections. Section 1 introduces cryptocurrency market trends and CBDC-related activities. Section 2 proposes an interoperable architecture to share ledger data without intermediaries between heterogeneous blockchains. Section 3 describes related studies including a problem with the transfer between blockchainbased heterogeneous cryptocurrencies and CBDCs. Section 4 proposes a decentralized P2P transfer service model to solve the problem identified in Section 3. Section 5 identifies security threats to the proposed service model and specifies security requirements to counter those security threats. Section 6 discusses the results and concludes the paper.

#### **2. Interoperable Architecture between Heterogeneous Blockchains**

This section proposes an interoperable architecture to share ledger data without intermediaries between heterogeneous blockchains. Interoperability between heterogeneous blockchains should be required to transfer between blockchain-based heterogeneous cryptocurrencies and CBDCs.

The proposed interoperable architecture is based on the proposed service model in Section 4 for sharing ledger data between heterogeneous blockchains. The proposed interoperable architecture is a decentralized architecture without intermediaries, whereas existing interoperable architectures, such as the inter-blockchain communication (IBC) protocol and the heterogeneous multi-chain framework described in Section 3.2, are centralized architectures with intermediaries.

In Figure 1, the blockchain-based interoperable management system (BIMS) maintains the registered information of blockchains and distributes common operations (COPs) to the contact nodes running on the blockchains. The BIMS does not store and maintain the ledger data from blockchain-1 and blockchain-2. Blockchain-1 and blockchain-2 can directly share ledger structure and ledger data through contact node-1 and contact node-2. The registered information of the blockchains includes the names of the blockchains, names of the consensus algorithms, names of the cryptocurrencies, IP addresses of the contact nodes, and more. The contact nodes running on the heterogeneous blockchains share data between the heterogeneous blockchains by common operations.

**Figure 1.** The interoperable architecture between heterogeneous blockchains.

The common operations are described as follows:


In Figure 2, the ledger data sharing process based on the interoperable architecture between heterogeneous blockchains is described as follows:


10. Contact node-1 transforms the ledger data of blockchain-2 by COP-6, and then contact node-1 stores the transformed ledger data to blockchain-1 by COP-7. Contact node-1 removes the transformed ledger data and the ledger data of blockchain-2 by COP-8.

**Figure 2.** The ledger data sharing process based on the interoperable architecture between heterogeneous blockchains.
