nRF52832 BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM failed

[Max Liao]

Hi,

I got a problem that when I try to use BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM for characteristic.

As I follow the tutorial to create the custom service, when I changed "BLE_GAP_CONN_SEC_MODE_SET_OPEN" to "BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM" then the application run failed and show fatal error.

Is there any point that I miss it ? Or how to create the characteristic with encryption correctly ?

I used SDK v15 BTY.

Thanks.

[Einar Thorsrud]

Hi,

Can you be more specific to what error you get? The simplest way to see where it fails and with what error code is to enable/define DEBUG and logging. Then you should see which file and line number the error is caught, and what the error code was.

[Max Liao]

Hi Einar Thorsrud

Sorry, the error code is 7.

I follow the custom service to build my own service, but in this tutorial using the OPEN characteristic.

But I want to use the encryption character, so follow the final step in Step6 in tutorial to set "BLE_GAP_CONN_SEC_MODE_SET_OPEN", and change it to "BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM"  in main.c, then I got the error.

[Einar Thorsrud]

Hi,

I do not see any reference to the CCCD in your code snippets. It is used for the client to enable or disable notification on the characteristic, so you must add it when you enable notification (set char_md.char_props.notify = 1;). This is covered in the Custom Service Tutorial.

Regarding the second question regarding encryption, this depend on the behavior of the phone. For example, iOS will only automatically increase security (perform bonding) after it has attempted to read or write to a characteristic that required a higher security level. In this case, you should be able to trigger a pairing from nRF Connect manually. Also, the nRF can send a security request to the phone to initiate pairing at an earlier stage.

[Max Liao]

Dear Einar Thorsrud

I do enable the notify to char_md struct (in line 22 in below code), I found some thing wrong with previous code, but after I modified it still have error.

static uint32_t btp_info_trek_stat_char_add(btp_info_t * p_info, const btp_info_init_t * p_info_init) {
    ble_uuid_t          ble_uuid;
    ble_gatts_char_md_t char_md;
    ble_gatts_attr_md_t attr_md;
    ble_gatts_attr_md_t cccd_md;
    ble_gatts_attr_t    attr_char_value;

    // The ble_gatts_char_md_t structure uses bit fields. So we reset the memory to zero.
    memset(&char_md, 0, sizeof(char_md));
    memset(&attr_md, 0, sizeof(attr_md));
    memset(&cccd_md, 0, sizeof(cccd_md));

    // Read operation on Cccd should be possible without authentication
    BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.read_perm);
    BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.write_perm);
    cccd_md.read_perm  = p_info_init->btp_info_value_char_attr_md.read_perm;
    cccd_md.write_perm = p_info_init->btp_info_value_char_attr_md.write_perm;
    cccd_md.vloc       = BLE_GATTS_VLOC_STACK;

    char_md.char_props.write = 0;
    char_md.char_props.read  = 1;
    char_md.char_props.notify = 1;
    char_md.p_char_pf = NULL;
    char_md.p_cccd_md = &cccd_md;
    char_md.p_sccd_md = NULL;
    char_md.p_user_desc_md = NULL;
    
    // Characteristic Description
    static char desc[] = "Tracking State";
    char_md.p_char_user_desc = (uint8_t *)desc;
    char_md.char_user_desc_size = strlen(desc);
    char_md.char_user_desc_max_size = strlen(desc);

    attr_md.read_perm  = p_info_init->btp_info_value_char_attr_md.read_perm; //dis_attr_md->read_perm;
    attr_md.write_perm = p_info_init->btp_info_value_char_attr_md.write_perm; //dis_attr_md->write_perm;
    attr_md.vloc       = BLE_GATTS_VLOC_STACK;
    attr_md.rd_auth    = 0;
    attr_md.wr_auth    = 0;
    attr_md.vlen       = 0;

    uint32_t            err_code;
    ble_uuid128_t       base_uuid = BTP_INFO_TRACKING_STAT_BASE;
    ble_uuid.uuid      = BTP_UUID_INFO_TRACKING_STAT;
    err_code = sd_ble_uuid_vs_add(&base_uuid, &ble_uuid.type);
    APP_ERROR_CHECK(err_code);  

    memset(&attr_char_value, 0, sizeof(attr_char_value));

    attr_char_value.p_uuid    = &ble_uuid;
    attr_char_value.p_attr_md = &attr_md;
    attr_char_value.init_len  = sizeof(uint8_t);
    attr_char_value.init_offs = 0;
    attr_char_value.max_len   = sizeof(uint8_t);
    //attr_char_value.p_value   = p_char_value;

    return sd_ble_gatts_characteristic_add(p_info->service_handle, &char_md,
                                           &attr_char_value,
                                           &p_info->btp_info_trek_stat_handles);
}

Got it with second answer, thanks.

[Max Liao]

Hi,

I found the problem is because the cccd_md.read_perm level is not 1 (open).

In my code I set it as 1 (open level) first (line 14 in my previous reply), but then I set it as level 2 (encryption level) on line 16 that pass from main->service_init function.

So the final cccd_md.read_perm level is 2, then the application will crash.

Now the problem seems been fixed, but I'm not sure is the cccd read permission only can set as OPEN level? please help to answer it.

Thanks.

[Einar Thorsrud]

Hi,

You are correct. The read permissions must be open for notifications/indications. This makes sense, as you can require an encrypted link in order to write to the CCCD to enable notifications, as described in this post.

[Max Liao]

Got it, very thanks for your answer.