How to initiate BLE pairing manually

[ealtan]

Hello,

I'm running nRF Connect SDK v1.9.0 on two nRF52840-DK devices, one Peripheral other Central.  

My application requires to initiate pairing, i.e. send pair request, programmatically.

For testing, I created a Peripheral device with a Service and two Characteristics.  

Via nRF Connect Desktop tool and nRF 52840 dongle, I can connect to that device, when connection establishes, it stays in "unencrypted link"  status.  

Then via Desktop tool, I click on "Pair" on the device settings configuration and it does pair with my Peripheral and gets into "encrypted link" mode. 

I want to accomplish same thing with my other nRF52840-DK device  which is in Central role.

Is there a way to initiate pair request manually ?   I couldn't see an API call for that.

All comments/hints are greatly appreciated.

E.

[Phil]

I have exactly the same issue.
I'm running nRF Connect SDK v3.3.0 on two nRF54L15-DK boards, one Peripheral and one Central.
When the Central is scanning and receives advertising from the peripheral, a button can be pressed on the Central to initiate a connection.The devices connect Ok in an unencrypted link.

I want the devices to pair and bond, or at least start the pairing process, automatically when the connection is established. 
I assumed that when the Central gets a connected complete callback it could programatically initiate the pairing process by sending a pairing request.  However, I can't find any documentation on how to do this.

Is it only possible to automatically start the pairing process from the peripheral?  
Does the peripheral have to send a security request in order to trigger the Central to send a pairing request?

We want to use pairing process "Just Works", so there in no operator involvement during the pairing process.  Our central and peripheral devices are simple and only have a button and LED, no display or keyboard.  However we want the link encrypted and the devices to be bonded.

[Vidar Berg]

Hi,

You can initiate the pairing process from the central by calling bt_conn_set_security() like in the central_uart sample here: https://github.com/nrfconnect/sdk-nrf/blob/5aa13555aa18356ea4b47766d16aa3dfbd80b978/samples/bluetooth/central_bas/src/main.c#L152. It is also important to change the security requirement on your characteristics if you want to require the link to be encrypted before any data exchange (https://academy.nordicsemi.com/courses/bluetooth-low-energy-fundamentals/lessons/lesson-5-bluetooth-le-security-fundamentals/topic/blefund-lesson-5-exercise-1/).  

[Phil]

Vidar,

Thanks for your reply.  I'll try setting the security level in the Central by calling bt_conn_set_security() to trigger the pairing process.

Is that example for SDK v3.3.0?

I found in the code for the peripheral I could not call bt_conn_set_security() from the connected() call-back.  I mean I could call it but it didn't do anything.  I had to use a kwork and call it from the kwork handler.  I presume this is because the connected() call-back is running in the BT stack, so is effectively running in an ISR.

I found that calling bt_conn_set_security() in the peripheral as soon as it connected caused pairing to start as soon as my phone connects (using nRF Connect app).   A Pairing Request window pops up on my phone.  However, it pops up twice.  It does pair and bond Ok.  This is without the phone trying to read/write any attributes.  It connects, pairs and does device discovery only.  Function bt_conn_set_security() is definitely called only once. Could the reason the pairing request pops up twice in the nRF Connect app be something to do with the phone (Android on Samsung Galaxy)?