Why we replaced the NFC tag with a Bluetooth key

Spark started with an NFC tag. You tapped it to your phone, the phone read a token, the apps unlocked. It was pleasingly physical. It also did not work on phones without an NFC reader.

Availability was the forcing function

Plenty of budget and mid-range Android phones ship with no NFC reader at all, and on the ones that had one, the API surface looked uneven to us at the time — inconsistently exposed to apps, and restricted on some platforms. That mattered more to us than it would to a company selling elsewhere: our first market is India, where mid-range Android is the phone most people are actually holding. A focus tool that only works on some phones is not a product. Bluetooth Low Energy is on every phone that can run Spark.

A TAP

NFC Spark

A tap to unlock. Needs a phone with an NFC reader, which some budget Android phones do not have. No battery, and slightly cheaper.

HOLD ABOUT 15 S

Bluetooth Spark

Hold the phone against it for about 15 seconds. Works with any phone that runs Spark — iPhone on iOS 17 or later, Android 12 or later. Runs on a CR2032 coin cell.

What we thought we lost

NFC's virtue here was never cryptographic — a standard tag can be copied with ordinary tools, and the NFC Spark sold today is no exception. Radio looked worse: anything broadcasting a fixed value can be recorded by software in seconds. Moving to BLE with a naive static token would have been an availability upgrade and a security downgrade at the same time.

So the move came with a protocol change rather than a straight port: the static token became a challenge–response exchange, meant to restore the guarantee in software. That exchange has since been replaced by a fixed broadcast again — see the note at the top.

• Works on every phone that can run Spark, not a subset.
• No range to rely on: the Bluetooth key only unlocks with the phone held against it, so it still has to live somewhere you walk to.
• New failure mode to design around — the key now has a battery, a CR2032 coin cell.

The battery question

A flat battery should not have to mean a locked phone. That is why there are five emergency unlocks, for life, and deliberately no more: enough to handle a flat battery or a lost key, few enough that they cannot quietly become the normal way you unlock. Strict mode hides them, so under Strict mode a flat key means fresh batteries.