BLE Secure connection with LE Secure Connection

[Hamza_HK]

Hi, 

I am working on implementing the security aspect of our BLE application using nRF52832 MCU with SDK 17. 

The goal is to establish a secure connection between peripheral and phone, on the phone side I'm using nrf connect. I have looked into different implementations regarding the secure connection establishment on BLE and come to conclusion that LE Secure Connection is most suitable for our application. 

The peer manager settings are as follows: 

// Security parameters to be used for all security procedures.
    sec_param.bond           = SEC_PARAM_BOND; //True
    sec_param.mitm           = SEC_PARAM_MITM; //True 
    sec_param.lesc           = SEC_PARAM_LESC; //True 
    sec_param.keypress       = SEC_PARAM_KEYPRESS; //False
    sec_param.io_caps        = BLE_GAP_IO_CAPS_DISPLAY_ONLY; 
    sec_param.oob            = SEC_PARAM_OOB; //False
    sec_param.min_key_size   = SEC_PARAM_MIN_KEY_SIZE; //7
    sec_param.max_key_size   = SEC_PARAM_MAX_KEY_SIZE; //16
    sec_param.kdist_own.enc  = 1;
    sec_param.kdist_own.id   = 1;
    sec_param.kdist_peer.enc = 1;
    sec_param.kdist_peer.id  = 1;

We are using static passkey that is expected from the phone input. During idle state in the main code we access the nrf_ble_lesc_request_handler().

The implementation chosen is based on this following connection establishment process from infocenter: 
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Currently our application works such that we are able to enter the passkey on the phone. But I am not sure what happens afterwards. Are the authentication steps listed in LESC Pairing all happening accordingly? 

Our goal is to establish secure connection through e.g. ECDH, where we have a shared secret which the devices calculate and get the symmetric key. Which keys are used by default and where can we access them?

nrf_ble_lesc.c file has the following: 

__ALIGN(4) static ble_gap_lesc_p256_pk_t m_lesc_public_key;                             /**< LESC ECC Public Key. */
__ALIGN(4) static ble_gap_lesc_dhkey_t   m_lesc_dh_key;                                 /**< LESC ECC DH Key. */
....
....
uint8_t  * p_shared_secret    = m_lesc_dh_key.key;

Where is m_lesc_dh_key.key value set? 

Thank you for all the help! 

Best regards,
Hamza

[Simonr]

Hi

Setting these defines in the peer manager settings, that should make connections go through this pairing process. In order to know if all the authentication steps are done we would need a sniffer trace of the pairing process. Please check out the nRFSniffer to see how to capture a trace using an nRF52 DK as a sniffer device for instance.

The keys are generated by the nrf_ble_lesc_keypair_generate() 

Best regards,

Simon

[Hamza_HK]

Hi Simon! 

By default, the keys generated in nrf_ble_lesc_keypair_generate, are those random keys? How is that possible if both devices has to have the same shared secret to compute correct LTK? 

We want to simply have a pre-shared key on the our devices such that the phone application has to have the correct key (shared secret) for it to acquire the symmetric key, based on ECDH protocol.

Regards,
Hamza