Central can't properly encrypt to mode 1 lvl 3, pairing not supported

[Tien Tran]

Hello!

I'm trying to read data off of a commercial blood pressure monitor (Qardio arm) using an nrf51822 with the nrf51 SDK v10.0.0 as the central role (s120). I am able to connect using sd_ble_gap_connect(), and when I tried to write to a characteristic using sd_ble_gattc_write(), I get back an "Authenticated Link Required" error. I assume that this implies I need security on my connection.

So next I attempted to use sd_ble_gap_authenticate(), which resulted in "Pairing Not Supported". It should be noted that I am able to write to the blood pressure monitor using nordic's "nRF Connect for Mobile" android app (I have a google pixel running Android 7.1.2).

Following these attempts, I turned to the device manager. Below shows the parameters I've set up for the DM, which are the same parameters I tried using with sd_ble_gap_authenticate().

static void device_manager_init(bool erase_bonds) {
  uint32_t               err_code;
  dm_init_param_t        init_param = {.clear_persistent_data = erase_bonds};
  dm_application_param_t register_param;

  err_code = pstorage_init();    //must be called before initializing any modules
  APP_ERROR_CHECK(err_code);

  err_code = dm_init(&init_param); //must be called before using any dm API
  APP_ERROR_CHECK(err_code);

  memset(&register_param.sec_param, 0, sizeof (ble_gap_sec_params_t)); //ZERO out the register_param.secparam

  // Event handler to be registered with the module.
  register_param.evt_handler            = device_manager_event_handler;

  // Service or protocol context for device manager to load, store and apply on behalf of application.
  // Here set to client as application is a GATT client.
  register_param.service_type           = DM_PROTOCOL_CNTXT_GATT_CLI_ID;

  // Secuirty parameters to be used for security procedures.
  register_param.sec_param.bond         = SEC_PARAM_BOND;            // bond    = 1
  register_param.sec_param.mitm         = SEC_PARAM_MITM;            // mitm    = 1
  register_param.sec_param.io_caps      = SEC_PARAM_IO_CAPABILITIES; // io cap  = 3  (none)
  register_param.sec_param.oob          = SEC_PARAM_OOB;             // oob     = 0
  register_param.sec_param.min_key_size = SEC_PARAM_MIN_KEY_SIZE;    // min key = 7
  register_param.sec_param.max_key_size = SEC_PARAM_MAX_KEY_SIZE;    // max key = 16
  //register_param.sec_param.kdist_central.enc  = 0;
  //register_param.sec_param.kdist_central.id   = 1;
  //register_param.sec_param.kdist_central.sign = 0;
  register_param.sec_param.kdist_periph.enc   = 1;
  register_param.sec_param.kdist_periph.id    = 1;
  //register_param.sec_param.kdist_periph.sign  = 0;

  dm_register(&m_dm_app_id, &register_param);  //app_id is a global variable 
  }

Now upon DM_EVT_CONNECTION, I would attempt dm_security_setup_req(), which then triggered DM_EVT_SECURITY_SETUP_COMPLETE. I had to use setup_req() again right after this (not sure why...), which then triggered DM_EVT_LINK_SECURED. It was here that I check my security level using dm_security_status_req(), which showed me that I'm still sm:1 lv:1.

I've also tried using the sd_ble_gap_encrypt() function, which triggers BLE_GAP_EVT_CONN_SEC_UPDATE. In here, I've also found myself to still be at sm:1 lv:1.

If "pairing isn't supported", how do I create an "authenticated link" so that I may write to the bp monitor's characteristic? What is the relation of authenticate() and encrypt()? Can I use one without the other?

Also, I am still new when it comes to BLE encryption. When using the sd_ble_gap_encrypt() function, specifically regarding the "ble_gap_master_id_t" and "ble_gap_enc_info_t" structs that the function takes as inputs, how do we populate those input parameters? Where does the LTK come from?

Also for reference, after i run sd_ble_gap_authenticate(), the following data is extracted from the BLE_GAP_EVT_AUTH_STATUS:

auth_status = 0x85  //this is the Authenticated Link Required error
error_src = 1
bonded = 1
sm1_levels.... lv1 = 1, lv2 = 1, lv3 = 1
sm2_levels.... lv1 = 1, lv2 = 1, lv3 = 1
kdist_periph.... enc = 1, id = 1, sign = 1
kdist_central... enc = 0, id = 1, sign = 1

I thank you in advance for your time in helping me with this matter.

Best, Tien Tran

[Hung Bui]

I assume that the blood pressure device require lv3 bonding, meaning you need MITM.

Could you try MITM =1 and keyboard and display enabled ?