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"We argue that operating a bearer-based payment system to complement an account-based CBDC in order to gain offline and privacy features is not a good trade-off. Adding permanent, regular offline capabilities via the bearer-based payment instrument constantly exposes the CBDC to the severe issues inherent in offline-capable payment systems. Instead, the offline mode of operation should be restricted to scenarios where it is actually required, which mitigates the risks."

GNU Taler is a privacy-preserving payment system. Customers can stay anonymous, but merchants can not hide their income through payments with GNU Taler. This helps to avoid tax evasion and money laundering. Payments are always backed by an existing currency. Payments are made after exchanging existing money into electronic money with the help of an Exchange service, that is, a payment service provider for Taler. When making a payment, customers only need a charged wallet. A merchant can accept payments without making their customers register on the merchant's Website. Taler is based on blind signatures.

[June 2023] Digital Inverted Time-based one-time passwords (TOTP) authentication can be a practical method for buyers to prove that they made a payment to a cost-sensitive offline merchant. Using TOTP we can establish a secure channel between the payment backend and the seller operating an offline PoS. Our solution around the limitations of the CAP theorem [6] is thus to securely heal the network partition by establishing cryptographically secured communication via an untrusted buyer's device. This allows the payment system to assure consistency despite the merchant not having a connected endpoint under their control. In practice, it may suffice for sellers to only selectively check the TOTP code to still create an effective deterrent against buyers using fake payment applications. Even merchants that are generally operating online could use our design as part of an inexpensive contingency plan to continue secure operations during network outages.