As part of the Bank of England’s central bank digital currency (CBDC) design phase, it explored an offline CBDC for its digital pound. At this stage it was only interested in technology issues, so it tested solutions provided by Thales, Secretarium, IDEMIA Secure Transactions, Quali-Sign and Consult Hyperion. It concluded the solutions were technically capable of delivering final payments, but found challenges relating to usability and the prevention and detection of counterfeits and double spending.
Offline usability challenges
The first challenge was that the offline and online CBDC balances are kept separate in the wallet, which users might find odd given they don’t care about the technical ramifications. Sometimes wifi outages can catch a user off guard. But if they haven’t already moved money into the offline balance, they won’t be able to use the offline functionality unless someone else pays them offline.
Offline payments tend to use secure elements either on a smartphone, a special SIM or smart cards. Given they have limited storage capacity, this caps the number of transactions that are possible before reconnecting to the network. One of the solutions tested was particularly limited on this front.
Imposing transaction limits in order to address potential risks has the side effect of impacting usability. And they are often not practical. For example, any kind of time limits are challenging because smart cards don’t have clocks. On smartphones the time on the clock might be changed. An alternative is to limit the number of transactions, if the secure element is compromised the transaction count could be manipulated. Although in that case, the CBDC has a problem anyway, because the private keys are also likely to be vulnerable.
Preventing and detecting fraud
The primary line of defense against counterfeiting and double spending is the cryptographic keys used within the secure element of the device. However, if somehow these are compromised, there’s a need to detect this has happened.
After executing a transaction offline, when the device is within wifi range it subsequently performs a reconciliation with the online ledger to highlight fraud or double spending. However, this is after the fact, so it doesn’t prevent double spending.
Devices can keep transaction records for later reconciliation. The possibilities are to keep full transaction records, partial records or no records, which renders the transactions anonymous. The Bank of England observed that without transaction records to reconcile with the online ledger, it’s not possible to detect counterfeits and double spending at all. And even when records are kept, the intermediaries need to share the records with each other for detection purposes. Various privacy preserving technologies were tested to safeguard personal information.
Additionally, the trials tested having a centralized system for uploading offline transaction data, using confidential computing to protect personal data. That allowed additional checks, including for money laundering.
The paper concluded that the trials “demonstrated that it might be technically feasible to implement an offline payment functionality for a digital pound but there are security, performance, and user experience challenges which need to be explored further.”
Hence, two major areas where work is needed is for double spending and fraud checks, and what happens if the secure element is compromised. While secure elements are widely used for payments, they are usually combined with simultaneous online checks.
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