Chaum designed and tried to commercialize an anonymous + offline payment system in the 90s already.
Basically he used (and invented) blind signatures to allow the bank to sign a 'coin' without knowing what they signed. The customer takes the blindly signed coins from the bank, pays at a merchant and later the merchant deposits the coins at the bank again, where the signature is checked.
In this context offline just means that the merchant can verify the authenticity of the coin without immediately needing a connection to the bank. At some point in the future, however, the merchant will have to connect to the bank to get their money.
Check out his original paper for details[1].
Offline systems have drawbacks, though. E.g the GNU Taler people made the pragmatic decision to have an online system. See chapter 1.2.1 'Offline vs Online' of Florian Dold's Phd thesis for a discussion on why[2].
Basically he used (and invented) blind signatures to allow the bank to sign a 'coin' without knowing what they signed. The customer takes the blindly signed coins from the bank, pays at a merchant and later the merchant deposits the coins at the bank again, where the signature is checked.
In this context offline just means that the merchant can verify the authenticity of the coin without immediately needing a connection to the bank. At some point in the future, however, the merchant will have to connect to the bank to get their money. Check out his original paper for details[1].
Offline systems have drawbacks, though. E.g the GNU Taler people made the pragmatic decision to have an online system. See chapter 1.2.1 'Offline vs Online' of Florian Dold's Phd thesis for a discussion on why[2].
[1]: https://chaum.com/wp-content/uploads/2022/01/Chaum-1990-Chap... [2]: https://taler.net/papers/thesis-dold-phd-2019.pdf