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WebUSB Protocol

Overview

Since NFC is inaccessible on the web, applications targeted on dual interfaces (NFC and USB) may use the following protocol to communicate with the WebUSB on Chromium-based web browsers.

The communication is based on the Control Transfer. The interface with index 1 is used.

Note: you need to implement this protocol on your own USB device before communicating with it using the web version of FlutterNfcKit.

Currently, the following devices adopt this protocol:

Messages

Basically, the messages on the WebUSB interface are APDU commands. To transceive a pair of APDU commands, two phases are required:

  1. Send a command APDU
  2. Get the response APDU

Each type of message is a vendor-specific request, defined as:

bRequest Value
CMD 00h
RESP 01h
STAT 02h
PROBE FFh
  1. Probe device

The following control pipe request is used to probe whether the device supports this protocol.

bmRequestType bRequest wValue wIndex wLength Data
11000001B PROBE 0000h 1 0 N/A

The response data MUST begin with magic bytes 0x5f4e46435f494d5f (_NFC_IM_) in order to be recognized. The remaining bytes can be used as custom information provided by the device.

  1. Send command APDU

The following control pipe request is used to send a command APDU.

bmRequestType bRequest wValue wIndex wLength Data
01000001B CMD 0000h 1 length of data bytes
  1. Get execution status

The following control pipe request is used to get the status of the device.

bmRequestType bRequest wValue wIndex wLength Data
11000001B STAT 0000h 1 0 N/A

The response data is 1-byte long, 0x01 for in progress and 0x00 for finishing processing, and you can fetch the result using RESP command, and other values for invalid states.

If the command is still under processing, the response will be empty.

  1. Get response APDU

The following control pipe request is used to get the response APDU.

bmRequestType bRequest wValue wIndex wLength Data
11000001B RESP 0000h 1 0 N/A

The device will send the response no more than 1500 bytes.