Wrong disconnected reason due to encryption with incorrect LTK

[Remo]

Hi

I'm using the nRF52832 with the SoftDevice S132 V6.0.0 in the central role and would like to encrypt a link with a stored LTK.

The encryption with the function sd_ble_gap_encrypt() with the correct LTK works fine. But when I start the encryption procedure with a peripheral that has an incorrect LTK, I would have expected that the BLE_GAP_EVT_DISCONNECTED event with the reason 0x3D (MIC failure) occurs, as it is described in this MSC.

Instead, I get the BLE_GAP_EVT_DISCONNECTED event with the reason 0x08 (Connection Timeout) after the supervision timeout expired. Am I doing something wrong or is this a bug in the SoftDevice?

EDIT:

I have a further question. Is that correct that the parameters lesc, auth and ltk_len in the ble_gap_enc_info_t struct are not needed/respected by the function sd_ble_gap_encrypt?

Many thanks in advance for your help.

Best regards

[Kenneth]

Not really, we do believe the root cause is as I wrote earlier. 

Best regards,
Kenneth

[Remo]

How does the S132 V6.1.0 in the peripheral role behave in case of a MIC failure? Does it also not send an answer to the central? Because in this case the central device also reached the supervision timeout.

[Kenneth]

The peripheral should immediately disconnect by spec, no further communication. I suspect the out-going packet from the peripheral is invalid here, thereby it's discarded (not mic failure), and you end up with supervisor timeout (as expected) on the central.

Best regards,
Kenneth

[Remo]

Thanks for the explanation. This would mean that the variant #3 (incorrect peripheral keys) in this MSC is wrong, isn't it? Shouldn't it be BLE_GAP_EVT_DISCONNECTED(Connection Timeout) instead of BLE_GAP_EVT_DISCONNECTED(MIC failure)?