ECDH (P-256 elliptic curve) in key exchanging phase

[Bing]

Hi,

We are planing to use N52832 to design a product using BLE secure feature. Our customer want to know whether the key exchaning procedure is using ECDH (P-256 elliptic curve) in the BLE security manager. And we are new to Security Manager working procedure. But I found there is a paragraph in Core Specification 5.0 of Bluetooth to say, during public key exchaning phase, the public-private key is generated by device its own  Elliptic Curve Diffie-Hellman (ECDH) . We want to know whether the ECDH alogrithm is implemented inside the softdevice or not. If not, where we can find the implementation of EDCH using in public-private key generating procedure in the BLE security manager? If ECDH is implemented inside the softdevice, what kind of curve parameter does it use? 

Sorry for our no understanding for the Bluetooth core specification about the security manager. If I am wrong in the above decription, please corret me.

Thanks.

[Einar Thorsrud]

Hi Wang,

The ECDH algorithm is used for LE Secure Connections, which is an enhanced security feature introduced in Bluetooth version 4.2. It is not implemented in the SoftDevice (BLE stack) itself, but rather in the SDK BLE LESC module. You can download the SDK from here and analyze the implementation.

Best regards,

Einar

[Bing]

Thanks Einar. Now I know the SDK support ECDH. But I still have doubt for how and when to use this BLE LESC module. Could you please give me a simple usage scenario.

[Einar Thorsrud]

Hi,

LE Secure Connections is a secure pairing method that replaces LE legacy parring. Both are ways to pair/bond, i.e. exchange encryption keys to be used to encrypt the Bluetooth packet exchange between two peers. LE Secure Connection is the more secure of the two and should be the preferred pairing method if high security is more important than supporting legacy devices. (You can also support both).

The SDK has a example application that demonstrate its usage, the experimental LE Secure Connections Multirole Example.

Best regards,

Einar

[Bing]

Thanks for your quickly reply. I compile the example "LE Secure Connections Multirole Example" in the directory SDKROOT\examples\ble_central_and_peripheral\experimental\ble_app_multirole_lesc, the keil mdk report the "micro_ecc_lib_nrf52.lib".  After seaching the whole SDK, I can't find the lost library file. Where can I get it? Or could you give me some web links to guide us for generating of this library file ?

[Einar Thorsrud]

You have to download the library yourself. The simplest way to do it is to go to <SDK>nRF5_SDK_15.0.0_a53641a\external\micro-ecc\ and run the build_all script. It will download and build the micro-ecc library for you.