Google Fast Pair Service (GFPS)

The Google Fast Pair Service (Fast Pair for short) implements a Bluetooth® LE GATT Service required when Using Google Fast Pair with the nRF Connect SDK.

Service UUID

The Fast Pair service uses UUID of 0xFE2C.

Characteristics

The Fast Pair GATT characteristics are described in detail in the Fast Pair GATT Characteristics documentation. The implementation in the nRF Connect SDK follows these requirements.

Note

The Additional Data characteristic is not supported yet.

Configuration

Set the CONFIG_BT_FAST_PAIR to enable the module.

The following Kconfig options are also available for this module:

• CONFIG_BT_FAST_PAIR_STORAGE_ACCOUNT_KEY_MAX - The option configures maximum number of stored Account Keys.

• CONFIG_BT_FAST_PAIR_CRYPTO_TINYCRYPT, CONFIG_BT_FAST_PAIR_CRYPTO_MBEDTLS, and CONFIG_BT_FAST_PAIR_CRYPTO_OBERON - These options are used to select the cryptographic backend for Fast Pair. MbedTLS is used by default, whereas Tinycrypt is used by default for cases of building with TF-M as the Secure Execution Environment (CONFIG_BUILD_WITH_TFM). This is because in such case the MbedTLS API cannot be directly used by the Fast Pair service. The Oberon backend can be used to limit memory consumption.

See the Kconfig help for details.

Override of default Kconfig values

To simplify the configuration process, the GFPS modifies the default values of related Kconfig options to meet the Fast Pair requirements. The service also enables some of the functionalities using Kconfig select statement.

Bluetooth privacy

The service selects Bluetooth privacy (CONFIG_BT_PRIVACY).

During not discoverable advertising, the Resolvable Private Address (RPA) rotation must be done together with the Fast Pair payload update. Because of this, the RPA cannot be rotated by Zephyr in the background.

During discoverable advertising session, the Resolvable Private Address (RPA) rotation must not happen. Therefore, consider the following points:

• Make sure that your advertising session is shorter than the value in the CONFIG_BT_RPA_TIMEOUT option.

• Call bt_le_oob_get_local() to trigger RPA rotation and reset the RPA timeout right before the start of advertising.

Bluetooth Security Manager Protocol (SMP)

The service selects CONFIG_BT_SMP, CONFIG_BT_SMP_APP_PAIRING_ACCEPT, and CONFIG_BT_SMP_ENFORCE_MITM. The Fast Pair specification requires support for Bluetooth LE pairing and enforcing Man-in-the-Middle (MITM) protection during the Fast Pair procedure.

Firmware Revision characteristic

The Fast Pair specification requires enabling GATT Device Information Service and the Firmware Revision characteristic. For this reason, the default values of CONFIG_BT_DIS and CONFIG_BT_DIS_FW_REV, respectively, are set to enabled. The default value of CONFIG_BT_DIS_FW_REV_STR is set to CONFIG_MCUBOOT_IMAGE_VERSION if CONFIG_BOOTLOADER_MCUBOOT is enabled. Otherwise, it is set to 0.0.0+0.

MTU configuration

The Fast Pair specification suggests using ATT maximum transmission unit (MTU) value of 83 if possible. Because of this requirement, the default values of the following Kconfig options are modified by the GFPS Kconfig:

• CONFIG_BT_L2CAP_TX_MTU

• CONFIG_BT_BUF_ACL_TX_SIZE

• CONFIG_BT_BUF_ACL_RX_SIZE

• CONFIG_BT_CTLR_DATA_LENGTH_MAX

Tip

In case of nRF53 Series, this part of the configuration cannot be automatically updated for the network core and you must manually align it. The listed options must be set on the network core to default values specified by the GFPS Kconfig options.

Security re-establishment

By default, the Fast Pair service disables the automatic security re-establishment request as a peripheral (CONFIG_BT_GATT_AUTO_SEC_REQ). This is done to allow Fast Pair Seeker to control the security re-establishment.

Partition Manager

The Fast Pair provisioning data is preprogrammed to a dedicated flash memory partition. The GFPS selects CONFIG_PM_SINGLE_IMAGE to enable the Partition Manager.

Settings

The GFPS uses Zephyr’s Settings to store Account Keys. Because of this, the GFPS selects CONFIG_SETTINGS.

Implementation details

The implementation uses BT_GATT_SERVICE_DEFINE to statically define and register the Fast Pair GATT service. The Fast Pair service automatically handles all of the requests received from the Fast Pair Seeker. No application input is required to handle the requests.

Bluetooth authentication

The Bluetooth pairing is handled using a set of Bluetooth authentication callbacks (bt_conn_auth_cb). The pairing flow and the set of Bluetooth authentication callbacks in use depend on whether the connected peer follows the Fast Pair pairing flow:

• If the peer follows the Fast Pair pairing flow, the Fast Pair service calls bt_conn_auth_cb_overlay() to automatically overlay the Bluetooth authentication callbacks. The function is called while handling the Key-based Pairing request. Overlying callbacks allow the GFPS to take over Bluetooth authentication during Fast Pair Procedure and perform all of the required operations without interacting with the application.

• If the peer does not follow the Fast Pair pairing flow, normal Bluetooth LE pairing and global Bluetooth authentication callbacks are used.

API documentation

Header file: include/bluetooth/services/fast_pair.h

Source files: subsys/bluetooth/services/fast_pair

group bt_fast_pair

Fast Pair API.

Enums

enum bt_fast_pair_adv_mode

Fast Pair advertising mode. Used to generate advertising packet.

Values:

enumerator BT_FAST_PAIR_ADV_MODE_DISCOVERABLE

Fast Pair discoverable advertising.

enumerator BT_FAST_PAIR_ADV_MODE_NOT_DISCOVERABLE_SHOW_UI_IND

Fast Pair not discoverable advertising, show UI indication.

enumerator BT_FAST_PAIR_ADV_MODE_NOT_DISCOVERABLE_HIDE_UI_IND

Fast Pair not discoverable advertising, hide UI indication.

enumerator BT_FAST_PAIR_ADV_MODE_COUNT

Number of Fast Pair advertising modes.

Functions

size_t bt_fast_pair_adv_data_size(enum bt_fast_pair_adv_mode fp_adv_mode)

Get Fast Pair advertising data buffer size.

Parameters

• fp_adv_mode – [in] Fast Pair advertising mode.

Returns

Fast Pair advertising data buffer size in bytes if the operation was successful. Otherwise zero is returned.

int bt_fast_pair_adv_data_fill(struct bt_data *adv_data, uint8_t *buf, size_t buf_size, enum bt_fast_pair_adv_mode fp_adv_mode)

Fill Bluetooth advertising packet with Fast Pair advertising data.

Provided buffer will be used in bt_data structure. The data must be valid while the structure is in use.

The buffer size must be at least bt_fast_pair_adv_data_size. Caller shall also make sure that Account Key write from a connected Fast Pair Seeker would not preempt generating Fast Pair not discoverable advertising data. To achieve this, this function and bt_fast_pair_adv_data_size should be called from context with cooperative priority.

Parameters

• adv_data – [out] Pointer to the Bluetooth advertising data structure to be filled.

• buf – [out] Pointer to the buffer used to store Fast Pair advertising data.

• buf_size – [in] Size of the buffer used to store Fast Pair advertising data.

• fp_adv_mode – [in] Fast Pair advertising mode. According to Fast Pair specification, when no Account Key has been saved on the device both Fast Pair not discoverable advertising modes result in the same advertising data.

Returns

0 if the operation was successful. Otherwise, a (negative) error code is returned.

void bt_fast_pair_set_pairing_mode(bool pairing_mode)

Enable or disable Fast Pair pairing mode.

Pairing mode must be enabled if discoverable Fast Pair advertising is used. Pairing mode must be disabled if not discoverable Fast Pair advertising is used.

It is user responsibility to make sure that proper pairing mode is used before advertising is started. Fast Pair pairing mode is enabled by default.

Parameters

• pairing_mode – [in] Boolean indicating if device is in pairing mode.