Check if connection is bonded/paired

[Mike Austin (LPI)]

This is my environment:

• IDE: VSC
• SDK: NCS v2.2.0
• iPhone 14 running iOS 16.6 as Central
• nRF52-DK as Peripheral

When the Central connects to the Peripheral, I want the Peripheral to check if it has pairing/bonding info stored for that connection, and if so, set a flag.  What's the best was with NCS to check if a connection is a paired/bonded connection?

Best regards,

Mike

[Simonr]

Hi

Pairing/bonding information is stored in the flash, so it shouldn't be erased when a device goes into a hibernate/sleep mode after disconnecting unless you specifically call an "erase_bonds()" function upon disconnects. There is also a GATT Bond Management Service that we can use to manage and delete bonds from the peripheral side for example. This sample shows how to use that.

Best regards,

Simon

[Mike Austin (LPI)]

Thanks Simon.

If my reading of the BMS description is correct, then this is a service that allows the Central to request the Peripheral to remove bonding info.

The scenario I am trying to trap is when the Central (e.g. an iOS device that has previously paired and bonded to the Peripheral) is told to "Forget This Device" when a connection is not active.  Then, at some point in the future, the Central attempts to make a connection - it has no recollection it was ever paired/bonded but the Peripheral still has the pairing/bonding info stored in flash.  What then happens is the connection is refused, and unless the bonding info on the Peripheral is removed, the Central can never connect to that Peripheral again.

So, I'm trying to come up with a mechanism whereby the Peripheral detects the error situation and removes the bonding/pairing info for that Central only.

So, I'm not sure the BMS is going to do this.  Or maybe I'm not understanding the BMS fully?

Best regards,

Mike

[Susheel Nuguru]

Mike Austin (LPI) said:

So, I'm trying to come up with a mechanism whereby the Peripheral detects the error situation and removes the bonding/pairing info for that Central only.

I think you can have some logic that you can increment a counter everytime you use bt_conn_get_security to compare the security level with L2 just after connection. Clear this counter when the pairing/bonded callback goes through.

So when a new connection is made you can do something like this

if (bt_conn_get_security(cts_c.conn) >= BT_SECURITY_L2) {
	bonded_to_client = true;
	
	// too many attempts to reconnect without a successful bonding/pairing
	if(++connection.some_counter == some_value)
	{
	   delete private keys for this connection;
	   connection.some_counter=0;
	}
}

Also make sure to reset the counter when bonding goes through. Just an rough idea.