Bank of Japan Publishes Results of CBDC PoC
The Bank of Japan (BoJ) recently concluded the second phase of its central bank digital currency (CBDC) proof of concept (PoC) project, which began in April. The results of this phase were published on Monday, and they shed light on key aspects such as the comparison between account-based and token-based CBDCs and the management of holding limits for users with multiple accounts.
Token-based CBDCs
The experiments conducted by the central bank covered a wide range of topics. Among the most intriguing findings were the advantages and disadvantages of token-based CBDCs and how to effectively impose holding limits for users with multiple CBDC balances.
Token-based CBDCs have garnered interest from various central banks, with some adopting the UTXO token model used by Bitcoin without the use of a distributed ledger. A UTXO or unspent transaction output, defines where a blockchain transaction starts and finishes. The Bank of Japan explored this model and analyzed its pros and cons.
In the initial proof of concept, both account-based and token-based CBDCs were examined, considering scenarios where the central bank managed the ledger or shared it with intermediaries like banks. In the token-based model, fixed token denominations were used, similar to physical cash in countries like India, and a centralized ledger was employed. However, in the recent phase, the central bank utilized flexible value tokens similar to UTXO and shared ledger functions with intermediaries.
The Bank of Japan favored the flexible value token model due to its ability to handle multiple requests simultaneously. However, it acknowledged that this model may require more technical resources compared to the account-based approach. Challenges may arise when implementing additional functions, such as holding limits, while maintaining optimal performance. The European Central Bank (ECB) also noted in a recent report that most payment providers are accustomed to account-based payments and would incur costs to adapt to token-based systems.
Another significant aspect explored by the BoJ was how to impose holding limits when users have multiple CBDC balances through different intermediaries. The challenge lies in determining if the overall holding limit has been breached without compromising user privacy.
Homomorphic encryption
One possible solution discussed in the report is the use of homomorphic encryption, which enables computations to be performed on encrypted data without it first needing to be decrypted. That allows for the necessary checks without intermediaries accessing the specific data being checked. Although this solution may slightly increase processing time, it could introduce a higher risk of data inconsistencies.
Alternatively, a simpler approach proposed by the central bank is to establish a per-account holding limit and a limit on the number of accounts a single user can hold, rather than imposing global limits. Ideally, users with multiple accounts would have a higher per-account holding limit compared to those with fewer accounts.
Phase 3 underway
With the next pilot phase already underway, the BoJ aims to test the end-to-end process flow and identify challenges related to integrating with external systems. Additionally, they are creating a CBDC Forum to gather input from the private sector, ensuring a collaborative approach to CBDC development.
While investigation and research into CBDCs continues, the BoJ has said that it will make a final decision on CBDC implementation by 2026.
