The offline cash puzzle solved by a local blockchain

A major drawback in deploying central bank digital currencies (CBDC) is the offline puzzle, which requires that a CBDC must keep the liquidity provision given by cash, and, simultaneously, avoid double-spending, cloning, and other issues. The puzzle is solved by minting the coins in serial numbers, which are stored on a local blockchain inside a smartphone or EMV card. The local blockchain is strengthened by a two-stage approval architecture that mitigates attacks and enables non-repudiation handling. The coins are protected by hardware keys embedded in the microchip and can be continuously mined by the wallet to enhance security. The coins can be either minted as hot coins, which can be retrieved in case of loss, or minted as cold coins, like physical cash.