> Cryptocurrency could also solve this problem. Offline, without double spending risk? Absolutely not, or at least not without a lot of extra headaches. For t

> Cryptocurrency could also solve this problem. Offline, without double spending risk? Absolutely not, or at least not without a lot of extra headaches. For that, you'll need at least some trusted hardware (generally an antithesis to trustless crypto schemes) and/or a clever incentive system (e.g. with senders staking a multiple of their balance as collateral, and never being quite sure if receivers are really offline, or only pretending to be, ready to claim their stake once they get publicly verifiable proof of double spending).