This app emulates an NFC Forum Type 4 Tag on an Android device using Host-based Card Emulation ("HCE").
To be compliant with the specification a service is running independent on the app opened that serves an Application Identifier ("AID") with the value D2760000850101.
The app is running up to Android 13 (SDK 33, the actual one when app was created) and is tested on real Android devices with Android 9 and Android 13.
There are 3 main functionalities:
- Send a message using HCE to another device in NDEF format
- read a message using the Android's NFC reader and reading an incoming intent with NDEF data using NDEF protocol (high level API)
- read a message using the Android's NFC reader and run the complete reading sequence using IsoDep protocol (low level API)
As there are very less articles and source code for NDEF emulation via HCE available on the Internet I was very happy
to find one repository on GitHub containing a running service class, so all credits go to the original author.
I just made the class a little more "handy" to accept individual messages.
The code is from the author "TechBooster" and is available in the repository "C85-Android-4.4-Sample"; the code for the "NdefHostApduService.java" class is available here:
As the source code of the service class is under Apache License 2.0 my app got the same license.
Note: please keep in mind that the app is active since installation as it runs a HostApduService. If there are other apps on your phone providing data for NFC - AID "D2760000850101" (NDEF access) you can run into problems ("routing conflict") that needs to get resolved each time a card reader is tapped to the device and asking for an NDEF message. On your device is under the "Settings menu" an entry "Connections" - "NFC" - "Payments" (or similar) and a sub-tab "others" - there you can choose which services are active at one time.
A second note regarding the length of an emulated NDEF message: the maximum length of an NDEF message is 0x7FFF == (decimal) 32767 bytes, but I'm limiting the length to a much smaller value (see the CAPABILITY_CONTAINER_FILE):
private final static byte[] CAPABILITY_CONTAINER_FILE = new byte[] {
0x00, 0x0f, // CCLEN
0x20, // Mapping Version
0x00, 0x3b, // Maximum R-APDU data size
0x00, 0x34, // Maximum C-APDU data size
0x04, 0x06, // Tag & Length
(byte)0xe1, 0x04, // NDEF File Identifier
(byte) 0x00, (byte) 0xff, // Maximum NDEF size, do NOT extend this value
0x00, // NDEF file read access granted
(byte)0xff, // NDEF File write access denied
};
You find more detailed information in https://stackoverflow.com/a/75933443/8166854 and more information here: https://stackoverflow.com/a/75664265/8166854:
In the Type 4 Tag documentation you linked in the question at the bottom of Section 5.1.2.1 it says
The maximum size of the NDEF file is limited by the Offset and Length Le fields of ReadBinary and NDEF
Update C-APDUs (see Table 16, Table 17, Table 22, and Table 23). The maximum size of the NDEF file is
reduced to 7FFFh + FFh = 80FEh bytes.
So Android have probably made a bug fix to match the max NDEF file size defined in the spec.
Link to the documentation: http://apps4android.org/nfc-specifications/NFCForum-TS-Type-4-Tag_2.0.pdf
