Modify pre shared OOB key for Legacy Connection

[Beldramma]

Hi !

I am working on SDK 15.2 and on a NRF52832.

The others devices I will interact with are on BLE 4, so I can use only Legacy Connection mode for security. 

I would like to use Out of Band mode, with a pre shared key stored on my device. The other device, a BLE Gateway, will have a file with this key stored too. 

First, I would like to know if there are examples to do that. 

Second, i would like to know if it is possible to modify the value of the key and keep its value even if we switch off the device. 

Thanks,

[Kenneth]

Hi,

It is possible to use pre-shared OOB keys yes. I made an example for nRF5 SDK v13 that show this, in principle it should be the same for more recent nRF5 SDK also. You will need to handle the BLE_GAP_EVT_AUTH_KEY_REQUEST in on_ble_evt() on both the central and peripheral device. 

\nRF5_SDK_13.0.0_04a0bfd\examples\ble_central\ble_app_hrs_c

\nRF5_SDK_13.0.0_04a0bfd\examples\ble_peripheral\ble_app_hrs

ble_app_hrs_c.zip

ble_app_hrs.zip

Here is a more updated message sequence chart:

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.s132.api.v7.2.0%2Fgroup___b_l_e___g_a_p___p_e_r_i_p_h___b_o_n_d_i_n_g___p_k___c_e_n_t_r_a_l___o_o_b___m_s_c.html

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.s132.api.v7.2.0%2Fgroup___b_l_e___g_a_p___c_e_n_t_r_a_l___b_o_n_d_i_n_g___p_k___p_e_r_i_p_h___o_o_b___m_s_c.html

Best regards,
Kenneth

[Beldramma]

Hi Kenneth, sorry for the delay, finally it was not a priority for us 2 months ago. But now I need to come back to it.

So I have read the ble_app_hrs example and have taken few pieces of program. I have added this code in my program :

//AKR
/**@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)
{
    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_PEERS_DELETE_SUCCEEDED:
        {
            advertising_start();
        }
        break;

        default:
        {
        }
        break;
    }
}

//AKR
/**@brief Function for the Peer Manager initialization.
 */
static void peer_manager_init(void)
{
    ble_gap_sec_params_t sec_param;
    ret_code_t           err_code;

    err_code = pm_init();
    APP_ERROR_CHECK(err_code);

    memset(&sec_param, 0, sizeof(ble_gap_sec_params_t));

    // Security parameters to be used for all security procedures.
    sec_param.bond           = SEC_PARAM_BOND;
    sec_param.mitm           = SEC_PARAM_MITM;
    sec_param.lesc           = SEC_PARAM_LESC;
    sec_param.keypress       = SEC_PARAM_KEYPRESS;
    sec_param.io_caps        = SEC_PARAM_IO_CAPABILITIES;
    sec_param.oob            = SEC_PARAM_OOB;
    sec_param.min_key_size   = SEC_PARAM_MIN_KEY_SIZE;
    sec_param.max_key_size   = SEC_PARAM_MAX_KEY_SIZE;
    sec_param.kdist_own.enc  = 0;
    sec_param.kdist_own.id   = 0;
    sec_param.kdist_peer.enc = 0;
    sec_param.kdist_peer.id  = 0;

    err_code = pm_sec_params_set(&sec_param);
    APP_ERROR_CHECK(err_code);

    err_code = pm_register(pm_evt_handler);
    APP_ERROR_CHECK(err_code);
}

I have called it in my main function.

Here is how I handle the BLE_GAP_EVT_AUTH_KEY_REQUEST event:

        // Upon authentication key request, reply with the preshared key in the device. //AKR
        case BLE_GAP_EVT_AUTH_KEY_REQUEST:
        {

            NRF_LOG_INFO("BLE_GAP_EVT_AUTH_KEY_REQUEST");

            err_code = sd_ble_gap_auth_key_reply(p_ble_evt->evt.gatts_evt.conn_handle,
                                                 BLE_GAP_AUTH_KEY_TYPE_OOB,
                                                 key_auth);
            APP_ERROR_CHECK(err_code);
        }
        break;

Also I have activated few stuffs in sdk_config to correctly build

#define PEER_MANAGER_ENABLED 1

#define PM_SERVICE_CHANGED_ENABLED 1

#define FDS_ENABLED 1

#define NRF_FSTORAGE_ENABLED 1

It builds correctly, but when I launch my application, I have a problem with the  ble_stack_init function:

/**@brief Function for initializing the BLE stack.
 *
 * @details Initializes the SoftDevice and the BLE event interrupt.
 */
void ble_stack_init(void)
{
    extern uint8_t __data_start__;
    ret_code_t err_code;

    err_code = nrf_sdh_enable_request();
    APP_ERROR_CHECK(err_code);

    // Configure the BLE stack using the default settings.
    // Fetch the start address of the application RAM.
    uint32_t app_ram_base = 0;

    err_code = nrf_sdh_ble_default_cfg_set(APP_BLE_CONN_CFG_TAG, &app_ram_base);
    APP_ERROR_CHECK(err_code);

    // Enable BLE stack.
    err_code = nrf_sdh_ble_enable(&app_ram_base);

    // Check if we do memory allocation correctly.
    if (err_code == NRF_SUCCESS)
    {
        // Verify that value provided in linked script (LD file) matches the SD calculations.
        if (app_ram_base == (uint32_t) &__data_start__)
        {
            //NRF_LOG_INFO("Soft Device enabled, __data_start__ is set correctly.");
            printf("Soft Device enabled, __data_start__ is set correctly.\n");
        }
        else
        {
            //NRF_LOG_INFO("Soft Device enabled, __data_start__ is set incorrectly (should be = 0x%08X instead of 0x%08X).", app_ram_base, (uint32_t) &__data_start__);
            printf("Soft Device enabled, __data_start__ is set incorrectly (should be = 0x%08X instead of 0x%08X).\n", app_ram_base, (uint32_t) &__data_start__);
            APP_ERROR_CHECK(NRF_ERROR_FORBIDDEN);
        }
    }
    else if (err_code == NRF_ERROR_NO_MEM)
    {
        // Not enough memory for the Soft Device (value provided is too low).
        //NRF_LOG_INFO("Soft Device failed to enabled, __data_start__ is set incorrectly (should be = 0x%08X instead of 0x%08X).", app_ram_base, (uint32_t) &__data_start__);
    }
    APP_ERROR_CHECK(err_code);

    // Register a handler for BLE events.
    NRF_SDH_BLE_OBSERVER(m_ble_observer, APP_BLE_OBSERVER_PRIO, ble_evt_handler, NULL);
}

app_ram_base and __data_start are different, app_ram_base = 0x2000 38C0 and __data_start  = 0x200038D4.

However, I have declared RAM_START = 0x2000 38C0. Do you know why I have this error and how i can handle it ?

Thanks !

[Kenneth]

Assuming you are using SES:

The actual ram required by the softdevice depends on the number of links and other configuration, so you typically need to update this during development.

[Beldramma]

Hi Kenneth

Yeah I am on SES, and I have done it few times before when I added services. But it is not the same problem here, indeed usually in these cases I need to increase the ram allocated for the app. So the message printed will be " __data_start__ is set incorrectly (should be = a instead of b)" with a > b.

But in my case it is : __data_start__ is set incorrectly (should be = 0x200038C0 instead of 0x200038D4), with a < b. Plus, I am already initialized at this value :

I have tried to increase the ram_start, but I had the same results.

[Beldramma]

Also I have a question about the message sequence chart for the peripheral :

I see that I have to handle to events, BLE_GAP_EVT_SEC_PARAMS_REQUEST, and BLE_GAP_EVT_AUTH_KEY_REQUEST.

For the BLE_GAP_EVT_SEC_PARAMS_REQUEST event, the chart says that I have to call the sd_ble_gap_sec_params_reply function, and for this function it is said in the documentation that the last parameter is [in/out], but I don't know how to fill it. It is not indicated in the documentation, can you help me about that ?

[Kenneth]

If you include peer manager, then it will handle most of these events already (e.g. BLE_GAP_EVT_SEC_PARAMS_REQUEST):
https://infocenter.nordicsemi.com/topic/sdk_nrf5_v17.0.2/lib_peer_manager.html

The input/output passkey, display, oob are really the only exceptions I can think of. 

[Beldramma]

Ok, so for now I am trying to use the ble_app_hrs with Legacy connection and OOB. I have filled the security parameters as it is told in the peer-manager documentation. I have handled the BLE_GAP_EVT_AUTH_KEY_REQUEST by adding the call of the sd_ble_gap_auth_key_reply function.I have also modified the battery service to put the SEC_MITM security level for the read access.

I have also modified the battery service to put the SEC_MITM security level for the read access.

To connect to my device, I use for now my phone with the nrfConnect app.

After doing that, I am not sure what i have to wait for. I can still connect to my device without any key asked. have checked, I don't enter in the event handler for the BLE_GAP_EVT_AUTH_KEY_REQUEST.

Wha I have to do to enter in this event so I can send my OOB key ?

When I connect to my device, I have this message from the peer_manager_handler : 

<info> peer_manager_handler: Connection secured: role: Peripheral, conn_handle: 0, procedure: Encryption

Is the security only applied when we want to read/write the attributes ?

Then I have tried to read the Battery value, but nothing is happening., I cannot read the value and nothing is printed on the debug terminal so I have no information.

Do you know what I have to do ?