Changing default key to App generated key during connection via BLE in nRF52840

Question

Hi. I am currently using an nRF52840 DK board and SDK 17.0.2. 
 My team is implementing a default key for the first connection to device. 
 During the connection, we plan to generate a private and public key in the APP side and pass the public key to the device. 
 Through this, we are trying to achieve encrypted messaging. 
 
 Is this implementation possible in the bootloader? 
 Thank you and appreciate the help.

Einar Thorsrud · Accepted Answer

Hi, 
 I am still not clear about the use case so I will make a suggestions, which may not be what you are looking fore. But it is often a good idea to use standard approaches if possible, instead of making your own, where it is easier to miss a corner case. Please let me know if any is appropriate or not, an if not perhaps explain a bit more. So for now I will ignore that you want this to happen in the bootloader, as I do not see the rational for it(?) and doing it in the application makes it closer to how this is handled in the SDK (example bootloader does not support pairing or bonding out of the box as we have never seen a use case for it). 
 If you allow anyone to do the first connection, then I suggest simply using normal connectable advertising in the application. When the first device connects you can perform pairing/bonding using LESC to have a authenticated secure connection. If you use LESC, the key is generated using Diffie–Hellman key exchange so the key remains secret. Any communication after this point will be encrypted. After a disconnect the device can advertise using a whitelist from this point so that no new device will be allowed to connect (and if an attacker spoofs the address, it will not be able to secure the link, and the link should be dropped). 
 Does the above approach fit your requirement? If it does, that would be good as it would use only standard Bluetooth concepts that are very well though out and tested and will give you very good security. 
 As you mention your custom app I wonder if it is so that you want to ensure that only users of your custom app should be allowed to communicate or use the device? If that is the case, then you could add an additional layer of security so that after securing the link using normal LESC pairing, then you exchange a key or do a challenge-response or similar so that the nRF would know that the APP is valid and other way around. If that fails, you could disconnect. I do not immediately know if or why you would want to do this, and I would not recommend it unless there is a good reason for it, but it is certainly possible. 
 Lastly you can implement all this in the bootloader, but that would increase the size and complexity of the bootloader significantly and we have no example code for it. So you should not do that unless you have a good reason for it, and I am not sure if that is the case?

Changing default key to App generated key during connection via BLE in nRF52840

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