LESC unable to pair (SM_pairing_faill )

[Arepa]

Hi all,

I have tried to mimic the LESC multi role example to my own app, but I dint have luck to pair it using LESC, MITM, and perform bonding and I received a SM_pairing_faill reason 0x0b; screen capture below:

nRF connect PC

nRF connect PC failing to pair

sniffer capture

details: Peripheral running on my peripheral device nRF52 with SDK13. Central using a nRF51 using PC nRF connect firmware.

My device does not have any Input, I am trying to perform LESC with just work

sec_params.bond           = true;
sec_params.mitm           = false;
sec_params.lesc           = true;
sec_params.keypress       = false;
sec_params.io_caps        = false;
sec_params.oob            = false;
sec_params.min_key_size   = 7;
sec_params.max_key_size   = 16;
sec_params.kdist_own.enc  = 1;
sec_params.kdist_own.id   = 1;
sec_params.kdist_peer.enc = 1;
sec_params.kdist_peer.id  = 1;

I tried use nRF connect (pc version) with the multi role example and it works with no issue, and I am pretty sure it is equal on my app (same settings), but I am stuck in the SM_paring_failed.

[Hung Bui]

Hi Arepa,

Have you handled BLE_GAP_EVT_LESC_DHKEY_REQUEST event in your application ? Make sure you used the correct curve NRF_CRYPTO_BLE_ECDH_CURVE_INFO

Error 0x08 mean BLE_GAP_SEC_STATUS_DHKEY_FAILURE as it shown in the log.

Please make sure this message sequence chart is followed correctly.

[MarcoTull]

Hi Hung, Yes, it handle the BLE_GAP_EVT_LESC_DHKEY_REQUEST and it is using the NRF_CRYPTO_BLE_ECDH_CURVE_INFO

case BLE_GAP_EVT_LESC_DHKEY_REQUEST:
	static nrf_value_length_t peer_public_key_raw = { 0 };

	peer_public_key_raw.p_value = &p_ble_evt->evt.gap_evt.params.lesc_dhkey_request.p_pk_peer->pk[0];
	peer_public_key_raw.length = BLE_GAP_LESC_P256_PK_LEN;

	err_code = nrf_crypto_ecc_public_key_from_raw(NRF_CRYPTO_BLE_ECDH_CURVE_INFO,
		&peer_public_key_raw,
		&m_peer_public_key);
	APP_ERROR_CHECK(err_code);

	err_code = nrf_crypto_ecdh_shared_secret_compute(NRF_CRYPTO_BLE_ECDH_CURVE_INFO,
		&m_private_key,
		&m_peer_public_key,
		&m_dh_key);
	APP_ERROR_CHECK(err_code);

	err_code = sd_ble_gap_lesc_dhkey_reply(connectionhandle, &m_lesc_dh_key);
	APP_ERROR_CHECK(err_code);
	break;	

it looks like my peripheral is not sending the SM_pairing_rsp. My device is sending all the message sequence chart except for the sm_pairing_rsp, what event handle it?

[Hung Bui]

The sniffer indeed looks pretty strange. The slave didn't response to the pairing request and simply send pairing Confirm and Random. I suspect it could be the sniffer issue. Do you have the same problem with our example ble_app_multirole_lesc in the SDK ?

If you don't could you check and compare two sniffers trace ? One from ble_app_multirole_lesc and one from your application ?

[MarcoTull]

Hi Hung, I took a capture for the ble_app_mulirole_lesc and I am comparing it with my app it looks pretty similar the differences are on: IOcap, AuthReq and pairing is failing.

The curious is I am using the same sec_parrams and it looks like is taking some different values because of the values in the sniffer capture. the ble_app_multirole_lesc works fine for me using nrf connect version PC.

Update:

I tried performing on nrf connect pairing with only MITM protection and Bonding, no LESC. It is pairing and encrypting my data, but I don't know if it is actually enabling LESC (I did not select it on the pairing windows on nrf connect).

Another thing. Running the ble_app_multirole_lesc example I cannot write/read the characteristics before pairing, on my application, it is not happening, it should be defined on the Characteristic definition or when initializing the services.

[Hung Bui]

Hi Marco,

What is the difference in the code in your "my App" vs "my App with pairing rsp" ? Or it's just when the sniffer can and can't capture the pairing response ?

between 0x09 and 0x0D in AuthReq, the only difference is the MITM support. If you set MITM = 1 in your security params you should have 0x0D.

You can check the security level when you receive BLE_GAP_EVT_AUTH_STATUS to check if LESC is used or not.

As you can find the example worked fine, but not in yours I would suggest you to double check if there is any difference between your application and the example. I suspect it has something to do with how the DH keys is calculated.

When you set up a characteristic, and if you want the link to be encrypted before peer can read or write, you need to use one of these:

BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM

BLE_GAP_CONN_SEC_MODE_SET_ENC_WITH_MITM

BLE_GAP_CONN_SEC_MODE_SET_LESC_ENC_WITH_MITM

instead of BLE_GAP_CONN_SEC_MODE_SET_OPEN()