Google Find My Device Network implementation

[Nareshpatel]

Hello, 

I need to implement Find My Device for nRF52833. 

From Sample I could find "samples/bluetooth/fast_pair/Locator Tag" example. I tried to go through it. but I could not able to map with fast pair specification(Fast Pair  |  Google for Developers). 

I want to check in which function advertisement is prepared ?

How keys(account keys,anti-snoofing keys) are sent in encrypted ?

In the advertisement which data we are sending(like fast pair UUID, Model id, anti snoofing keys, unique mac address,salt field ) etc ?  

[Edvin]

Sorry, maybe I jumped to conclusions initally. Most people asking about this says that they don't see the fast pair popup in Android. I am not sure exactly how the keys are being used. It is based on Google/android specification, so I don't think there is much room for customization exactly there. 

I believe the advertisement data is set in NCS\nrf\subsys\bluetooth\services\fast_pair\fmdn\state.c, in e.g. fmdn_adv_data_async_update() it is set several places. At least there it calls bt_le_ext_adv_set_data().

Is it something in particular that you want to add in your advertising data?

Best regards,

Edvin

[Nareshpatel]

No problem edvin. Thank you for your support. 

I want to know do we use automatic advertisement or custom advertisement ? I need to use custom advertisement .  

If I need to change advertisement data where I implement custom advertisement in source code ? 

I also want to map Google Fast pair specification and source code.  Could you please help me on this ? 

[Edvin]

Hello,

Look at the bt_le_ext_adv_set_data(), which is called in ncs\nrf\subsys\bluetooth\services\fast_pair\fmdn\state.c.

I don't know what you want to add to your advertisements, but I would guess there are more restrictions if you are using fast pair. Is there a particular reason why you need to use that instead of normal BLE advertising? If you are using normal BLE advertising, you can put whatever you want to in your advertisement packets.

Best regards,

Edvin

[Nareshpatel]

Thank you for your reply. 

I want to add few more information like ble mac address, and some proprietary flag information so Our mobile APP team can understand it. 

Is there any restriction for fast pair frame ? Could you elaborate please Which parameters are mandatory in fast pair frame ? 

[Edvin]

The BLE/MAC address will always be visible (not on iOS, but I assume you are talking about Android, since we are discussing fast_pair). It is part of the advertising packet already.

I think it would be easier for you to modify a "normal" BLE sample first, and then you can apply it to the fast pair sample later. 

Take e.g. the peripheral_uart sample found in ncs\nrf\samples\bluetooth\peripheral_uart.

From there you can see how the advertising packet is generated near the top of main.c:

static const struct bt_data ad[] = {
	BT_DATA_BYTES(BT_DATA_FLAGS, (BT_LE_AD_GENERAL | BT_LE_AD_NO_BREDR)),
	BT_DATA(BT_DATA_NAME_COMPLETE, DEVICE_NAME, DEVICE_NAME_LEN),
};

static const struct bt_data sd[] = {
	BT_DATA_BYTES(BT_DATA_UUID128_ALL, BT_UUID_NUS_VAL),
};

Note that each of these (ad and sd) are limited to 31 bytes each, so you can't add whatever you like. But if you e.g. take away the name, that frees up a significant amount, so that you can add your stuff.

One quite flexible structure that you can use in your advertising packet is BT_DATA_MANUFACTURER_DATA. This is a field that you can use to add pretty much any data on any format that you like. The only thing is that the first two bytes needs to be your company's bluetooth ID. You can set it to 0xFFFF for testing, but for certification, you need to use your Bluetooth Company ID.

Something like this:

uint8_t my_data[20] = {0xff, 0xff, 0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef, 0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef, 0x01, 0x23};

static const struct bt_data ad[] = {
	BT_DATA_BYTES(BT_DATA_FLAGS, (BT_LE_AD_GENERAL | BT_LE_AD_NO_BREDR)),
	//BT_DATA(BT_DATA_NAME_COMPLETE, DEVICE_NAME, DEVICE_NAME_LEN),
    BT_DATA(BT_DATA_MANUFACTURER_DATA, my_data, sizeof(my_data)),
};

static const struct bt_data sd[] = {
	BT_DATA_BYTES(BT_DATA_UUID128_ALL, BT_UUID_NUS_VAL),
};

It is also possible to update the data. Just stop the advertisements, update the my_data array, and start it again.

Best regards,

Edvin

[Nareshpatel]

Thank you very much for your detailed explanation.

I want to implement Fast Pair. So I am referring Locator Tag example from Sample. I have checked Sample Advertisment of fast pair. 

It does not include MAC address. In Google's document MAC address is part of the advertisement.

Does MAC address not required for Fast Pair ?

Which fields are mandatory for fast pair frame ?

How Google identify each device uniquely ? 

[PaKa]

So the locator sample in the nRF Connect SDK includes all the mandatory features to make a FMD tag. There are restrictions on what you are allowed to do with the transmitted frames when using FMD, this as the solution should not allow user to track the devices outside the FMD ecosystem. Devices are uniquely identified on the device owner device side, not on "any device" side. This is again to protect privacy.

Adding the MAC address to the frame would break privacy hard so I would seriously doubt that will be allowed to do.

The Google FMD spec is available in the open so if you want to modify the frames then I suggest checking that before determining the solution.

Can you share why you want to do this? There may be other ways of fixing it than transmitting the MAC address.