[NRF52840 - pc-ble-driver-py] - Disconnection/reconnection issue

[Christophe@]

Hello,

I am developing a Python script (Windows 10, Python 2.7.16, pc-ble-driver-py 0.12.0) to use a NRF52840 board to simulate the BLE part of an application running on a smartphone.
The smartphone connects on a device, then executes bonding with passkey, and send commands to the device.
The Python script is able to execute the same scenario.

I have an issue with the script for disconnection/reconnection. I don't have this problem with the smartphone.
Here is what is done:

With the smartphone:
Once bonding is done between the smartphone and the device, the smartphone can send commands to the device.
I enable the flight mode on the smartphone: a disconnection occurs with reason 0x13 (Remote User Terminated Connection), seen on log of the device.
Then I disable the flight mode on the smartphone. It reconnects to the device and it can send again commands to the device.

With the Python script:
After bonding, the script can send commands to the device.
I do a disconnection thanks to the function ble_gap_disconnect of ble_driver.py. On the device (log), I can see a disconnection with reason 0x13.
Then, I do a reconnection, discover services. Log on device shows that the board is connected to the device.
But when the script sends a command to the device after the reconnection, an authentication error occurs.
The authentication error is avoided with a new bonding and the same passkey.

The disconnection is different: The device keeps the bonding when the smartphone is used, but not with the board/script.

Could you please explain what should I do with pc-ble-driver-py to do the same disconnection done with the smartphone?
Is this disconnection supported with the current version of pc-ble-driver-py (0.12.0) or is there a limitation?

Thank you for your support.

Regards,
Christophe

[tesc]

Hi,

pc-ble-criver (and pc-ble-driver-py) is serialization of the SoftDevice, and the SoftDevice does not store bonding data internally. That means, in order to keep the bond on the pc side you must store that data yourself. This is what for instance the Peer Manager library in the nRF5 SDK does. See e.g. the Central Security Procedures in the SoftDevice API documentation.

Regards,
Terje

[Christophe@]

Hello Terje,

Thank you for your answer. There are a lot of workflows available in Central Security Procedures. Which one should I use to fit my needs?

Currently, I use Bonding: Passkey Entry, User Inputs on Central or OOB. Can I keep it or should I replace it with another procedure?

Regards,
Christophe

[tesc]

Hi,

You need to add procedures for reestablishing encryption, using stored keys (ediv, rand, LTK) for the bond. I.e. Encryption Establishment using stored keys. Either always after connection, or when you get a security request as in Security Request Reception.

The keys can be fetched from the SoftDevice as part of the bonding procedure, from the [in,out] parameter p_sec_keyset in sd_ble_gap_sec_params_reply(). See the API documentation for sd_ble_gap_sec_params_reply() for details.

Regards,
Terje

[Christophe@]

Hello Terje,

I succeeded to do this reconnection. I got ediv, rand and ltk from keyset using last part of code of method authenticate in ble_adapter.py  and I used encrypt method to establish encryption.

That issue is fixed. Thanks again for your support.

Regards,
Christophe

[tesc]

Hi,

I am happy to hear that you solved the issue, and wish you good luck with the rest of the project.

Regards,
Terje