Role of Keys kdist_central and kdist_periph in sd_ble_gap_authenticate

[Darrew]

My Central is willing to perform a bond (pairing with bond) to a Peripheral.

As an example and according to

Message Sequence Chart > Central Security Procedures > Central Legacy Pairing > Bonding: Just Works,

the central calls sd_ble_authenticate(bond, no_mitm, no_io_caps) and it seems that the rest of the security parameters in p_sec_params (including kdist_central and kdist_periph) are of no importance.

It seems that keys will be stored in p_keyset if the following call to sd_ble_gap_sec_params_reply supplies the pointer p_keyset.

I wonder what the role of these keys kdist_central and kdist_periph is.

[Hung Bui]

Hi Darrew,

Note that inside p_sec_params, the description of kdist_central and kdist_periph say:

 ble_gap_sec_kdist_t   kdist_own;                 /**< Key distribution bitmap: keys that the local device will distribute. */
  ble_gap_sec_kdist_t   kdist_peer;                /**< Key distribution bitmap: keys that the remote device will distribute. */

They are bitmap (flags) telling which key the peers will distribute and expect to be distributed.

[Darrew]

Hi Hung Bui,

It would be more clear to have a Message Sequence Chart that exactly defines the correct and needed arguments on the call to sd_ble_authenticate(). I find sd_ble_authenticate(bond, no_mitm, no_io_caps) ambiguous (it will not work if the correct keys are not distributed!).

Same problem -ambiguity- for sd_ble_gap_encrypt(ediv, rand, ltk) in MSC Central Legacy Pairing > Bonding: Just Works; it would help to mention that those arguments correspond to the peer device i.e. Peripheral.

[Hung Bui]

Hi Darrew,

It's hard for us to include all arguments into the message sequence chart. We want to mention only those arguments that make the difference between Just Work, Passkey, OOB, Legacy pairing etc.

This is when you should have a look at the example to get the detail of how to set the arguments. Most of the arguments have the definition to explain what they do, as mentioned in the answer above. If you need more detail on how it works, you may need to have a look at the Bluetooth Core Spec.