BLE Man-in-the-middle protection, multiple passkeys

[pedrovfr_LNNano]

Hello all,

I am developing a BLE application with the Softdevices, the peer management module, and with MITM protection.

 I've set a bonding static passkey as shown here:

    static uint8_t static_passkey[] = STATIC_PASSKEY;
    static ble_opt_t    m_static_pin_option;
    ...

    m_static_pin_option.gap_opt.passkey.p_passkey = static_passkey;
    err_code = sd_ble_opt_set(BLE_GAP_OPT_PASSKEY, &m_static_pin_option);
    APP_ERROR_CHECK(err_code);

I want to set a general static passkey to be used for maintenance people, such as an admin bonding passkey, and other passkey for the final user ( could be randomly generated or static) .

Is there a way to implement this? Can I change the static_passkey for an array of passkeys?

thanks in advance.

[Einar Thorsrud]

Hi,

You cannot configure more than one passkey at a time, but you can call sd_ble_opt_set() with a new pass key. So you would have to keep the array in your application, and handle swapping between the keys yourself.

Note that because of the way the passkey is exchanged in BLE using static passkey is not secure, as it can be brute forced quite fast (particularly with LE Secure Connections).

[pedrovfr_LNNano]

Thanks for the reply.

Can you suggest me how to implement the handle swapping in the application?

I though setting a new passkey using the sd_ble_opt_set() after the first attempt be fail, but to do this I wish the continue on the connection and retry without the peer be informed of that. But I just found after the miss value on the passkey the SoftDevices disconnects and return BLE_GAP_EVT_AUTH_STATUS not giving me the chance to authenticate manually before it disconnects.

Any idea how to solve this case will be much appreciated!

[Einar Thorsrud]

Hi,

pedrovfr_LNNano said:

Can you suggest me how to implement the handle swapping in the application?

You just call sd_ble_opt_set() again when you want to change the key. When that is will be application dependent so that is up to you.

pedrovfr_LNNano said:

I though setting a new passkey using the sd_ble_opt_set() after the first attempt be fail, but to do this I wish the continue on the connection and retry without the peer be informed of that. But I just found after the miss value on the passkey the SoftDevices disconnects and return BLE_GAP_EVT_AUTH_STATUS not giving me the chance to authenticate manually before it disconnects.

The SDK examples and peer manager module will disconnect on failure by calling pm_handler_disconnect_on_sec_failure(). But this is really up to you, if you do not want to disconnect on failure do not call this in your peer manager event handler, and instead handle the PM_EVT_CONN_SEC_FAILED event in a different way that fits your application.

[pedrovfr_LNNano]

Hello, thanks for the reply.

Einar Thorsrud said:

The SDK examples and peer manager module will disconnect on failure by calling pm_handler_disconnect_on_sec_failure(). But this is really up to you, if you do not want to disconnect on failure do not call this in your peer manager event handler, and instead handle the PM_EVT_CONN_SEC_FAILED event in a different way that fits your application.

I have done that, but it still disconnects:

/**@brief Function for handling Peer Manager events.
 *
 * @param[in] p_evt  Peer Manager event.
 */
static void pm_evt_handler(pm_evt_t const * p_evt)
{
    ret_code_t err_code;
    pm_handler_on_pm_evt(p_evt);
    //pm_handler_disconnect_on_sec_failure(p_evt);
    pm_handler_flash_clean(p_evt);

    switch (p_evt->evt_id)
    {
        case PM_EVT_PEER_DELETE_SUCCEEDED:
            if (!delete_bonds_pending() && !delete_all_pending)
            {
                // No more peers are flagged for deletion and we are not going to delete all peers.
                advertising_start(false);
            }
            break;
        case  PM_EVT_CONN_SEC_FAILED:

            printf("FAILED BONDING");
                m_static_pin_option.gap_opt.passkey.p_passkey = ADMIN_PASSKEY;
                err_code = sd_ble_opt_set(BLE_GAP_OPT_PASSKEY, &m_static_pin_option);
                APP_ERROR_CHECK(err_code);
           
            break;

        case PM_EVT_PEERS_DELETE_SUCCEEDED:
            delete_all_pending = false;
            advertising_start(false);
            break;

        case PM_EVT_PEER_DATA_UPDATE_SUCCEEDED:
            if (     p_evt->params.peer_data_update_succeeded.flash_changed
                 && (p_evt->params.peer_data_update_succeeded.data_id == PM_PEER_DATA_ID_BONDING))
            {
                NRF_LOG_INFO("New Bond, add the peer to the whitelist if possible");
                // Note: You should check on what kind of white list policy your application should use.

                whitelist_set(PM_PEER_ID_LIST_SKIP_NO_ID_ADDR);
            }
            break;
                    /** @snippet [NFC Pairing Lib usage_1] */
          case PM_EVT_CONN_SEC_PARAMS_REQ:
          {
              // Send event to the NFC BLE pairing library as it may dynamically alternate
              // security parameters to achieve highest possible security level.
              
   //           APP_ERROR_CHECK(nfc_ble_pair_on_pm_params_req(p_evt));
          } break;
          /** @snippet [NFC Pairing Lib usage_1] */

            
        default:
            break;
    }
}

 

Here the debug terminal during a bonding attempt:

<debug> app: pm_whitelist_get returns 0 addr in whitelist and 0 irk whitelist
<info> app: Fast advertising.
<debug> nrf_sdh_ble: BLE event: 0x10.
<info> app: Connected
<debug> nrf_ble_gq: Registering connection handle: 0x0000
<debug> nrf_sdh_ble: BLE event: 0x12.
<debug> nrf_sdh_ble: BLE event: 0x12.
<debug> nrf_sdh_ble: BLE event: 0x13.
<info> peer_manager_handler: Connection security procedure started: role: Peripheral, conn_handle: 0, procedure: Bonding
<debug> nrf_sdh_ble: BLE event: 0x15.
<info> app: Passkey: 123456
<debug> nrf_sdh_ble: BLE event: 0x12.
FAILED BONDING<debug> nrf_sdh_ble: BLE event: 0x19.
<info> peer_manager_handler: Connection security failed: role: Peripheral, conn_handle: 0x0, procedure: Bonding, error: 132
<debug> nrf_sdh_ble: BLE event: 0x11.
<info> app: Disconnected

I am receiving the BLE event BLE_GAP_EVT_DISCONNECTED in the ble event handler:

/**@brief Function for handling BLE events.
 *
 * @param[in]   p_ble_evt   Bluetooth stack event.
 * @param[in]   p_context   Unused.
 */
static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context)
{
    ret_code_t err_code = NRF_SUCCESS;
    pm_handler_secure_on_connection(p_ble_evt);
    /////PEDROMOD
    pm_handler_secure_on_error(p_ble_evt);
    ////////////
    switch (p_ble_evt->header.evt_id)
    {
        case BLE_GAP_EVT_CONNECTED:
            NRF_LOG_INFO("Connected");
            err_code = bsp_indication_set(BSP_INDICATE_CONNECTED);
            APP_ERROR_CHECK(err_code);
            m_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;
            err_code = nrf_ble_bms_set_conn_handle(&m_bms, m_conn_handle);
            APP_ERROR_CHECK(err_code);
            err_code = nrf_ble_qwr_conn_handle_assign(&m_qwr, m_conn_handle);
            APP_ERROR_CHECK(err_code);
            err_code = nrf_ble_cgms_conn_handle_assign(&m_cgms, m_conn_handle);
            APP_ERROR_CHECK(err_code);
            break;

        case BLE_GAP_EVT_DISCONNECTED:
            NRF_LOG_INFO("Disconnected");
            if (delete_bonds_pending())
            {
                // Advertising is started by PM_EVT_PEERS_DELETE_SUCCEEDED or PM_EVT_PEERS_DELETE_SUCCEEDED event.
                delete_disconnected_bonds();
            }
            else
            {
                advertising_start(false);
            }
            m_conn_handle = BLE_CONN_HANDLE_INVALID;
            //cgm_stop = true;
            break;

        case BLE_GAP_EVT_PHY_UPDATE_REQUEST:
        {
            NRF_LOG_DEBUG("PHY update request.");
            ble_gap_phys_t const phys =
            {
                .rx_phys = BLE_GAP_PHY_AUTO,
                .tx_phys = BLE_GAP_PHY_AUTO,
            };
            err_code = sd_ble_gap_phy_update(p_ble_evt->evt.gap_evt.conn_handle, &phys);
            APP_ERROR_CHECK(err_code);
        } break;

        case BLE_GATTC_EVT_TIMEOUT:
            // Disconnect on GATT Client timeout event.
            NRF_LOG_DEBUG("GATT Client Timeout.");
            err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gattc_evt.conn_handle,
                                             BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
            APP_ERROR_CHECK(err_code);
            break;

        case BLE_GATTS_EVT_TIMEOUT:
            // Disconnect on GATT Server timeout event.
            NRF_LOG_DEBUG("GATT Server Timeout.");
            err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gatts_evt.conn_handle,
                                             BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
            APP_ERROR_CHECK(err_code);
            break;
        case BLE_GAP_EVT_PASSKEY_DISPLAY:
        {
            char passkey[PASSKEY_LENGTH + 1];
            memcpy(passkey, p_ble_evt->evt.gap_evt.params.passkey_display.passkey, PASSKEY_LENGTH);
            passkey[PASSKEY_LENGTH] = 0;

            NRF_LOG_INFO("Passkey: %s", nrf_log_push(passkey));
        } break;
        case BLE_GAP_EVT_AUTH_KEY_REQUEST :
        {

            printf("KEY REQUEST");
        } break;
        default:
            // No implementation needed.
            break;
    }
}

Still not sure how the soft devices is handling the passkey, but once it doesn't match the connection is ended. Any ideias how to workaround this?

[Einar Thorsrud]

Hi,

From the log we only see that you get BLE event 0x11, which is BLE_GAP_EVT_DISCONNECTED, but not why. Can you print the disconnect reason? It is not unlikely that the peer disconnects when the pairing fails. If so, you need to modify the behavior of the peer in order to prevent the disconnect (if possible). What is the peer device her?