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Two ESP32 boards can exchange data through visible light using one ordinary LED on each board. In the SecurePair demonstration, each LED both emits and detects light: the boards blink at one another to pair, then use the same optical link to send messages. SecurePair handles pairing and encrypted messaging; PacketLED provides the LED communication path.
How do two ESP32s communicate using one LED?
An LED is usually treated as an output, but it can also respond to incoming light and act as a simple light sensor. PacketLED uses that two-way behavior: an LED on one board sends a signal by blinking, while the other board’s LED detects the changing light. Each board has its own LED, and the LEDs face each other across a clear line of sight.
This is visible-light communication, not a wireless radio link. The underlying idea has earlier published precedent: a 2003 Mitsubishi Electric Research Laboratories report, “Very Low-Cost Sensing and Communication Using Bidirectional LEDs”, describes bidirectional LED sensing and communication. That history does not mean SecurePair uses the same implementation.
How the pairing demonstration works
- Start pairing on both ESP32 boards.
- Point the LEDs toward each other, a few centimeters apart, and watch the synchronized code: a sequence of 20 short and long blinks.
- Compare the codes shown by the two boards. Confirm on both if they match; reject the pairing if they differ.
The comparison gives the people operating the boards a chance to spot a mismatch before accepting the pairing. The project says the resulting key is saved across resets and that encrypted, authenticated messages include replay protection.
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What SecurePair does—and what it does not establish
SecurePair is the pairing and encrypted-message library; PacketLED is one way to carry its communication. The project describes its implementation as using X25519 for key agreement, HKDF-SHA256 for key derivation, AES-256-GCM for message encryption and authentication, and replay protection. These are claims in the project documentation, not independently validated findings.
The project states: “The protocol has not yet been reviewed by an independent cryptographer.” That is a significant limitation: the documentation describes a security design, but readers should not treat the demonstration as an independently audited or production-ready security system.
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LED, ESP-NOW, and LoRa transports compared
The SecurePair README lists three transport options and the following documented specifications. The figures are project-stated values, not independently measured benchmarks.
| Transport | Project-stated range | Message size | Notes |
|---|---|---|---|
| PacketLED over visible light | A few centimeters to 2 m, line of sight | 35 bytes | For pairing, the README recommends placing the LEDs a few centimeters apart; achievable distance depends on the LEDs. |
| ESP-NOW | Tens of meters | 217 bytes | Boards use a Wi-Fi channel shared by the devices. |
| LoRa | Kilometers | 216 bytes | The README lists an SX1276/78 module, but says LoRa has not yet been tried on hardware. |
These options are not interchangeable in every setup. LED communication requires alignment and line of sight; ESP-NOW uses radio and a shared Wi-Fi channel; LoRa is an optional radio transport whose hardware operation the project had not tested according to its documentation.
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What hardware and software are required?
For the LED-pairing example, the SecurePair README calls for an ESP32 board running Arduino-ESP32 core 3.x, one LED and resistor per board, a button or other yes/no input per board, and PacketLED version 1.1.0 or later. The button supplies the human confirmation step. Avoid assigning it to a boot-strapping pin: holding such a pin during reset can put the board into download mode.
The repository reports hardware testing on ESP32, ESP32-C3, and ESP32-C6 boards with Arduino-ESP32 core 3.3.12. It also identifies ESP32-S2, S3, C3, C5, C6, H2, and P4 as having the HMAC peripheral required for its encrypted-storage option. These are project-documentation statements, so check the current README and board details before choosing hardware.
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The smallest listed example pairs over light without sending messages. Other examples send messages over the LED or ESP-NOW, and one adds an SX1276/78 LoRa module. The README labels LoRa hardware testing as not yet done. The library is documented as beta version 0.5.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where the project stands
Hackaday’s Jenny List reported on the demonstration on October 5, 2026. The project README, accessed October 7, 2026, describes hardware tests of LED pairing, ESP-NOW pairing, LED and ESP-NOW messaging, pairing with a button held at startup, and encrypted storage on ESP32, ESP32-C3, and ESP32-C6 boards. It does not report independent cryptographic review, and its LoRa transport remains untested on hardware.
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For the project’s implementation details and examples, see the SecurePair repository.
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