Allowing cash to die would be a grave mistake. A cashless society is a surveillance society. The recent round of protests in Hong Kong highlights exactly what we have to lose.

In Hong Kong, most people use a contactless smart card called an “Octopus card” to pay for everything from transit, to parking, and even retail purchases. It’s pretty handy: Just wave your tentacular card over the sensor and make your way to the platform.

But no one used their Octopus card to get around Hong Kong during the protests. The risk was that a government could view the central database of Octopus transactions to unmask these democratic ne’er-do-wells. Traveling downtown during the height of the protests? You could get put on a list, even if you just happened to be in the area.

So the savvy subversives turned to cash instead. Normally, the lines for the single-ticket machines that accept cash are populated only by a few confused tourists, while locals whiz through the turnstiles with their fintech wizardry.

But on protest days, the queues teemed with young activists clutching old school paper notes. As one protestor told Quartz: “We’re afraid of having our data tracked.”

Using cash to purchase single tickets meant that governments couldn’t connect activists’ activities with their Octopus accounts. It was instant anonymity. Sure, it was less convenient. And one-off physical tickets cost a little more than the Octopus equivalent. But the trade-off of avoiding persecution and jail time was well worth it.