Some argue that online settlements are necessary to avoid double spending. A 2021 memo from Riksbank, Sweden’s central bank, made that assertion with little substantiation, except to say that 100% tamper-proof devices don’t exist. However, even if that were true, software countermeasures can limit the impact of a hardware hack, like holding and transaction limits and mutual authentication requirements. That’s the approach that WhisperCash has taken. Plus, BitMint has patented a hardware wallet with quantum-grade security that will erase all of the currency stored in it upon any tampering attempts..