Exploring potential hurdles in the implementation of offline Central Bank Digital Currencies (CBDC) by the Bank of England.
Offline Central Bank Digital Currencies (CBDCs) are designed to address challenges in usability, transaction limits, and fraud detection, tailored for limited connectivity environments.
Usability is a key focus of offline CBDCs. Transactions can be executed without constant internet access, using methods like stored-value cards, device-to-device payments, and SMS/USSD with application-layer encryption. These solutions prioritize diverse device form factors to reach various user groups, enhancing financial inclusion in areas with limited network availability. The European Central Bank’s digital euro, for instance, provides a user experience comparable to cash, with privacy ensured by restricting transaction visibility to only payer and payee.
To mitigate risks such as double-spending, many offline CBDC designs impose transaction limits. These limits can be caps on the value or volume of offline transactions, to contain exposure during disconnection periods. The offline mode typically complements online verification for larger-value transactions, maintaining central bank oversight without undermining accessibility.
Fraud detection and security are essential components of offline CBDCs. Since offline transactions cannot be immediately verified by a trusted central ledger, cryptographic protocols are embedded within tamper-resistant user devices to prevent double-spending and unauthorized token creation. Fraud mitigation also involves unique payee identification to counter impersonation fraud, though this conflicts with privacy goals to some extent. Recovery mechanisms are considered to address risks such as lost or broken devices and aborted transactions.
However, there are still areas where work is needed. Double spending and fraud checks, as well as what happens if the secure element is compromised, are major concerns that require further exploration. A recent paper concluded that there are security, performance, and user experience challenges that need to be addressed for the offline payment functionality of a digital pound.
Devices can keep transaction records for later reconciliation, with options for full, partial, or no records. Without transaction records to reconcile with the online ledger, it's not possible to detect counterfeits and double spending. Intermediaries need to share records with each other for detection purposes when records are kept.
The trials of offline CBDCs also allowed for additional checks, including for money laundering. The trials tested a centralized system for uploading offline transaction data.
In summary, offline CBDCs tackle usability by employing multiple technical access channels and device types, manage transaction limits to contain operational risks inherent offline, and address fraud detection through cryptographic security integrated into devices and unique payee identification, all while carefully balancing privacy and operational resilience. Ongoing testing focuses on risk mitigation strategies and balancing privacy, security, and accessibility.
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