Resolvable Private Address Generation Cycle

[Genyu]

The Central device is using Resolvable Private Addresses.

Even though private_addr_cycle_s is set to BLE_GAP_DEFAULT_PRIVATE_ADDR_CYCLE_INTERVAL_S (15 minutes),

When observed with a BLE sniffer, the Resolvable Private Address changes with each BLE connection.

I thought the address would change according to the set cycle regardless of BLE connections. Is that correct?

We're using a fixed IRK without bonding. Could that be causing this?

We are developing in the following environment:

• SDK: nRF5 SDK 17.1.0
• SoftDevice: S140 v7.2.0

[Amanda Hsieh]

Hi, 

I thought the address would change according to the set cycle regardless of BLE connections. Is that correct?

Correct. See this post. 

Regards,
Amanda H.

[Genyu]

thank you for your reply.

In terms of specifications, I understand that Resolvable Private Address (RPA) is updated at a set interval.

I am calling the following function with a fixed IRK (m_irk).

ble_gap_privacy_params_t privacy_params;
memset(&privacy_params, 0, sizeof(privacy_params));
privacy_params.privacy_mode         = BLE_GAP_PRIVACY_MODE_DEVICE_PRIVACY;
privacy_params.private_addr_cycle_s = BLE_GAP_DEFAULT_PRIVATE_ADDR_CYCLE_INTERVAL_S;
privacy_params.private_addr_type    = BLE_GAP_ADDR_TYPE_RANDOM_PRIVATE_RESOLVABLE;
privacy_params.p_device_irk         = &m_irk;

err_code = sd_ble_gap_privacy_set(&privacy_params);
APP_ERROR_CHECK(err_code);

Do you know why PRA is updated so often?

[Genyu]

Hi Amanda,

Thank you for the prompt confirmation and response.

> But the thing is that the scanner/initiator regenerates a new address each time softdevice starts a new scanning.

Can this behavior be considered a bug in the Softdevice?
The initiator continuing to use the same address for 15 minutes is the correct behavior, right?

Thank you.

[Amanda Hsieh]

The scanner/advertiser regenerates a new address when it starts a new scanning/advertisement.

Only if the scanning/advertisement is continuously running, the address will be changed every 15 minutes or as configured. 

[Genyu]

Amanda Hsieh 

Hi, Amanda.
Thank you for your response.

> The scanner/advertiser regenerates a new address when it starts a new scanning/advertisement.

Would it be appropriate to consider this behavior to correspond to the following description in the Bluetooth SIG Specification?

> If the Link Layer is reset, a new private address shall be generated and the timer started with any value in the allowed range.

Thank you.

[Amanda Hsieh]

Genyu said:

Would it be appropriate to consider this behavior to correspond to the following description in the Bluetooth SIG Specification?

From the developer:

Resolvable Private Address (RPA) is more used to protect an advertising device from being tracked by malicious peers. In the Bluetooth SIG Specification, "If the Link Layer is reset, a new resolvable private address shall be generated and the timer started with any value in the allowed range." Here "Link layer reset" means HCI reset, softdevice regenerates a new address not only on HCI reset and "when the timer expired", but also when starting a new advertising/scanning. 

However, the spec does not prevent us from generating a new address more often. Therefore, the softdevice follows the privacy requirement in the spec.

-Amanda H.

[Genyu]

Amanda Hsieh 

Hi, Amanda

> However, the spec does not prevent us from generating a new address more often. Therefore, the softdevice follows the privacy requirement in the spec.

Thank you, my question has been cleared up!