  A major drawback in deploying central bank digital currencies (CDBC) is the
offline puzzle, which requires that a CBDC must keep the provision given by
cash, and, simultaneously, avoid double-spending, counterfeiting, and other
issues. The puzzle is solved by minting the coins in serials, which are stored
on a local blockchain (e.g. smartphone). The local blockchain is secured by
keys embedded in the hardware 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.
