INVALID_PARAMETER returned when setting CCCD to secure

[brianreinhold]

This question was asked, not answered, and locked. So I will ask it again

I changed the security settings on a CCCD for a heart rate measurement characteristic and got INVALID PARAMETER

Here is the code (comes from the BleHeartRateMonitor example in the pc-ble-driver).

static uint32_t characteristic_init()
{
uint32_t error_code;
ble_gatts_char_md_t char_md;
ble_gatts_attr_md_t cccd_md;
ble_gatts_attr_t attr_char_value;
ble_uuid_t ble_uuid;
ble_gatts_attr_md_t attr_md;
uint8_t encoded_initial_hrm[MAX_HRM_LEN];
uint16_t attr_char_value_init_len;

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

cccd_md.rd_auth = 1;
//BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.read_perm);
//BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.write_perm);

BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM(&cccd_md.read_perm);  // This is what I did
BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM(&cccd_md.write_perm);
cccd_md.vloc = BLE_GATTS_VLOC_STACK;

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

char_md.char_props.notify = 1;
char_md.p_char_user_desc = NULL;
char_md.p_char_pf = NULL;
char_md.p_user_desc_md = NULL;
char_md.p_cccd_md = &cccd_md;
char_md.p_sccd_md = NULL;

BLE_UUID_BLE_ASSIGN(ble_uuid, BLE_UUID_HEART_RATE_MEASUREMENT_CHAR);

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

BLE_GAP_CONN_SEC_MODE_SET_OPEN(&attr_md.read_perm);
BLE_GAP_CONN_SEC_MODE_SET_OPEN(&attr_md.write_perm);
attr_md.vloc = BLE_GATTS_VLOC_STACK;
attr_md.rd_auth = 0;
attr_md.wr_auth = 0;
attr_md.vlen = 1;

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

attr_char_value_init_len = heart_rate_measurement_encode(encoded_initial_hrm, 0);

attr_char_value.p_uuid = &ble_uuid;
attr_char_value.p_attr_md = &attr_md;
attr_char_value.init_len = attr_char_value_init_len;
attr_char_value.init_offs = 0;
attr_char_value.max_len = MAX_HRM_LEN;
attr_char_value.p_value = encoded_initial_hrm;

error_code = sd_ble_gatts_characteristic_add(m_adapter, m_heart_rate_service_handle,
&char_md,
&attr_char_value,
&m_heart_rate_measurement_handle);

if (error_code != NRF_SUCCESS)
{
printf("Failed to initialize characteristics. Error code: 0x%02X\n", error_code);
fflush(stdout);
return error_code;
}

printf("Characteristics initiated\n");
fflush(stdout);
return NRF_SUCCESS;
}

I know that I have tried to set the CCCDs on devices and got an insufficient authentication (5) error, so I SHOULD be able to do this. What is wrong?

[Einar Thorsrud]

Hi,

You can require security for writing to the CCCD, but it is not legal to require security for reading it. So the error you get is correct and expected, and you should fix it by replacing:

BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM(&cccd_md.read_perm);  // This is what I did

with:

BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.read_perm);  // This is what I did

[brianreinhold]

Yes, my bad on that. But I still need to give the HR characteristic read/write permission or I can't write to the descriptor. If I give read/write permission then I get the need for pairing. How do I prevent the characteristic value from being read or written to?

[Einar Thorsrud]

Hi,

You can set the properties to not allow write (something like char_md.char_props.write = 0). You can of course also just ignore writes to the characteristic, so that it does not have any practical consequences.