BLE encryption link

Question

Hello, I'm developing application starting from blinky example extracted from nRF5_SDK_15.3.0_59ac345 SDK.
I see (using nRF Connect) that the connection result as "unencrypted link" (image here below)
How can I make the link secured with encryption? There is an example on SDK that show how to make it?
Thanks
A. Barbieri - Italy

Amanda Hsieh · Answer

Hi Abele, 
 You don't have to bond to get an encrypted link. You will get an encrypted link if you pair, but if you don't bond, you will have to pair every time you connect to get an encrypted link. Also, see my colleague explained in this post . 
 The Peer manager implements pairing and has a bond field in the security parameters that you pass when it is initialized. By setting that to 0 you indicate that bonding is not supported. With this change, pairing will take place as before, but there will be no bonding (storing of bonding information such as keys) 
 
 -Amanda H.

abe · Answer

Hi Amanda, 
 Using the macro BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM on characteristic configuration, GATT write is enabled ONLY with pairing (unauthenticated encrypted link). here my code: 
 static uint32_t custom_value_char_add(ble_cusi_t * p_cusi, const ble_cusi_init_t * p_cusi_init)
{
 uint32_t err_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;

 // Add Custom Value characteristic
 memset(&cccd_md, 0, sizeof(cccd_md));

 // write operation on cccd should not be possible without authentication.
 BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.read_perm);
 BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM(&cccd_md.write_perm);
 
 cccd_md.write_perm = p_cusi_init->custom_value_char_attr_md.write_perm;
 cccd_md.read_perm = p_cusi_init->custom_value_char_attr_md.read_perm;
 cccd_md.vloc = BLE_GATTS_VLOC_STACK;

 memset(&char_md, 0, sizeof(char_md));

 char_md.char_props.read = 1;
 char_md.char_props.write = 1;
 char_md.char_props.notify = 0; 
 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; 
 I reached may goal, thanks for your help. Abele