Secret Key Generation Based off of Physical Layer

[Kebab]

I'm working on a project to get two devices to agree on a secret key based off of physical layer properties.

My solution is for devices A and B to make 64 RSSI measurements based off of the channel to the other device and then generate a key using these measurements. Measurements may be spread out but B always checks RSSI rapidly after A to allow channel reciprocity and matching keys to be generated. I am not fixed on what radio technology to use (BLE, Zigbee etc).

I have simulations working for this but I am a complete newbie to actually implementing it. I have the SDK, segger embedded studio, command line, nrf connect and termite installed and have managed to get basic examples like the uart example working.

I have knowledge in C but am not sure how to go about editing the examples or creating my own code. How can I get two nRF52840 DK's to make such measurements?

Thanks

[Vidar Berg]

Hi,

You will likely get better security if you use the built-in Random number generator and a standardized key exchange method such as "Diffie-hellman" for creating the shared secret.  The ECDH Example provided in SDK 15.2.0 should be a good starting point to get an understanding of the nRF crypto API. Also, we have BLE examples that support LESC pairing (diffie helman key exchange) for securing the link. ble_app_hrs is one of the examples that supports it.   

[Kebab]

HI Vidar,

Thanks for your reply however the purpose of the project is to develop and use the method I mentioned in the original post. I cannot use any already standardized methods to generate the key.

[Vidar Berg]

Ok, maybe you can start with the UART central and peripheral example, then add RSSI sampling and an implementation of your own key generation. You may find this blog post useful (link) although it covers encryption with static keys. 

[Kebab]

Ok thanks. I have seen a similar problem here but even doing this is a bit beyond me. I just don't know where to begin with adding the features I am looking for. For instance, where does the line "BLE_GAP_EVT_CONNECTED" come from or is there a source where this terminology is explained?

Sorry about all the basic questions but I appreciate the help.