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Comment by londons_explore | Hacker News Reader

The card itself knows its balance and is authorised to approve transactions up to a limit fully offline.

The UK already does this in some shops for low value items for NFC payments. You can tell the offline transactions because they immediately say 'approved' rather than taking a few seconds.

If it turns out the card approved something 'wrongly', for example because you had previously reported the card lost to the bank, then the bank refunds the transaction and claims the value back from the merchant. That's why many merchants have their terminals set to require online payments.

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Your card doesn't know the balance, it doesn't work like that.

Offline transactions mostly died off when the limit in the UK for contactless was raised to £100. At £20/30 (the original limits) issuers/merchants risk accept some payments not being valid (and the total limit before you had to chip and pin was fairly low top).

And worth saying, the merchant has some control on the terminal but mostly the decision of offline/online is down to the issuer and configured on the card.

Out of interest is this both debit and credit cards, or just debit?

Mostly identical. Credit cards will probably have different risk rules though - after all it's their money not yours :)

Some debit cards don't allow offline transactions, usually when the cardholder isn't allowed to be in debt.

In the olden days, you'd get a Visa Electron or Solo debit card in the UK if you were under 18 or had a poor credit history.

Visa Electron and Solo were online authorisation-only card brands (also known as "immediate authorisation").

If you didn't have enough money in your account, the transaction would be declined. Visa Electron cards didn't have embossed numbers on the front, so couldn't be used with the old-fashioned card imprinters.

Visa Electron and Solo have been discontinued now, so people with poor credit can get a Visa Debit or MasterCard debit card, but with offline authorisation disabled.

That does mean those cards can't work in some places (e.g. on aeroplanes or trains).

Credit Cards generally always support offline authorisation.

It's incredible how the banks refuse to lose here; they will screw over the merchants before taking on any liability in this matter.

When you set up the POS you can choose whether to allow offline auth or not. It makes transactions a little faster, but you’re the one who takes the hit if the card bounces later. Just comes down to whether you value speed over the extra risk.

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