Communication timeout when using micro ecc for secure connection

[jkorenj]

Describe the bug

I opened separated issue, to make it more clean.
Because I couldn't make bonding working as described in this two issues:
#732
espressif/esp-idf#18609 (comment)

I included the micro ecc library in host (STM32H563).

The problem now is that the connection timeouts at the beggining, and that is why our APP drops communication. We increased the timeout and noticed that it can take more than 20 second for connection to be establish. The delay is the same if we need to bond new device, or if the keys are already saved.

To Reproduce

Steps to reproduce the behavior:

1. #define ENABLE_MICRO_ECC_FOR_LE_SECURE_CONNECTIONS
2. include micro ecc library
3. Connect ESP32C3 mini, ESP-IDF v6.0.1

Expected behavior

Bonding and connection should happen in 5s. this is currently our timeout. We can increase it ofcourse, but 20s is a too much.

HCI Packet Logs
hci_uart_log.zip

Environment: (please complete the following information):

• Bluetooth Controller ESP32C3

Additional context
Log from the nrf connect APP

New device
nRF Connect, 2026-05-19
BLE pairing (28:37:2F:B5:A3:EA)
D 14:08:36.326 [Broadcast] Action received: android.bluetooth.device.action.ACL_CONNECTED
V 14:08:36.481 Connecting to 28:37:2F:B5:A3:EA...
D 14:08:36.481 gatt = device.connectGatt(autoConnect = false, TRANSPORT_LE, preferred PHY = LE 1M)
D 14:08:36.486 [Callback] Connection state changed with status: 0 and new state: CONNECTED (2)
I 14:08:36.486 Connected to 28:37:2F:B5:A3:EA
I 14:08:36.487 MTU changed to: 247
V 14:08:36.491 Discovering services...
D 14:08:36.491 gatt.discoverServices()
V 14:08:36.492 Discovering services...
D 14:08:36.492 gatt.discoverServices()
D 14:08:36.492 [Callback] Services discovered with status: 0
I 14:08:36.492 Services discovered
V 14:08:36.493 Unknown Service (11548c14-5c59-4bbb-aaa1-e05019060106)

• RX Characteristic [N R W WNR] (6e400002-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
• TX Characteristic [N R W WNR] (6e400003-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
• Unknown Characteristic [N R] (6e400004-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
• Unknown Characteristic [N R] (6e400005-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
  D 14:08:36.493 gatt.setCharacteristicNotification(6e400002-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:08:36.495 gatt.setCharacteristicNotification(6e400003-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:08:36.496 gatt.setCharacteristicNotification(6e400004-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:08:36.497 gatt.setCharacteristicNotification(6e400005-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:08:36.498 [Callback] Services discovered with status: 0
  I 14:08:36.498 Services discovered
  V 14:08:36.499 Unknown Service (11548c14-5c59-4bbb-aaa1-e05019060106)
• RX Characteristic [N R W WNR] (6e400002-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
• TX Characteristic [N R W WNR] (6e400003-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
• Unknown Characteristic [N R] (6e400004-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
• Unknown Characteristic [N R] (6e400005-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
  D 14:08:36.499 gatt.setCharacteristicNotification(6e400002-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:08:36.499 gatt.setCharacteristicNotification(6e400003-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:08:36.500 gatt.setCharacteristicNotification(6e400004-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:08:36.500 gatt.setCharacteristicNotification(6e400005-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:08:40.768 [Broadcast] Action received: android.bluetooth.device.action.BOND_STATE_CHANGED, bond state changed to: BONDING (11)
  D 14:08:40.797 [Broadcast] Action received: android.bluetooth.device.extra.PAIRING_VARIANT, pairing variant: CONSENT
  I 14:08:44.833 Connection parameters updated (interval: 7.5ms, latency: 0, timeout: 5000ms)
  D 14:08:44.963 [Broadcast] Action received: android.bluetooth.device.action.ENCRYPTION_CHANGE, encryption: ENABLED, algorithm: AES (16 bytes), transport: TRANSPORT_LE, status: 0
  D 14:08:47.721 [Broadcast] Action received: android.bluetooth.device.action.BOND_STATE_CHANGED, bond state changed to: BONDED (12)
  I 14:08:47.721 Device bonded
  I 14:08:47.756 Connection parameters updated (interval: 11.25ms, latency: 0, timeout: 2000ms)

already bonded

nRF Connect, 2026-05-19
BLE pairing (28:37:2F:B5:A3:EA)
D 14:10:49.199 [Broadcast] Action received: android.bluetooth.device.action.ACL_CONNECTED
V 14:10:49.370 Connecting to 28:37:2F:B5:A3:EA...
D 14:10:49.370 gatt = device.connectGatt(autoConnect = false, TRANSPORT_LE, preferred PHY = LE 1M)
D 14:10:49.389 [Callback] Connection state changed with status: 0 and new state: CONNECTED (2)
I 14:10:49.389 Connected to 28:37:2F:B5:A3:EA
D 14:10:49.389 wait(1600ms)
V 14:10:50.989 Discovering services...
D 14:10:50.989 gatt.discoverServices()
I 14:10:51.572 Connection parameters updated (interval: 7.5ms, latency: 0, timeout: 5000ms)
D 14:10:52.712 [Callback] Services discovered with status: 0
I 14:10:52.712 Services discovered
V 14:10:52.716 Unknown Service (11548c14-5c59-4bbb-aaa1-e05019060106)

• RX Characteristic [N R W WNR] (6e400002-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
• TX Characteristic [N R W WNR] (6e400003-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
• Unknown Characteristic [N R] (6e400004-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
• Unknown Characteristic [N R] (6e400005-b5a3-f393-e0a9-e50e24dcca9e)
     Client Characteristic Configuration (0x2902)
  D 14:10:52.716 gatt.setCharacteristicNotification(6e400002-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:10:52.719 gatt.setCharacteristicNotification(6e400003-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:10:52.726 gatt.setCharacteristicNotification(6e400004-b5a3-f393-e0a9-e50e24dcca9e, true)
  D 14:10:52.727 gatt.setCharacteristicNotification(6e400005-b5a3-f393-e0a9-e50e24dcca9e, true)
  I 14:10:52.831 Connection parameters updated (interval: 11.25ms, latency: 0, timeout: 2000ms)
  I 14:11:07.299 Connection parameters updated (interval: 7.5ms, latency: 0, timeout: 5000ms)
  I 14:11:08.171 Connection parameters updated (interval: 11.25ms, latency: 0, timeout: 2000ms)

[mringwal]

Hi. The timeout is in your app, not in the OS, correct? The official timeout for Bluetooth pairing is 30 seconds. Where does the 5s timeout come from?

Anyway, in your log with bonding, AES128 is calculated by the Controller over HCI. Please try to calculate it on the STM32 by adding ENABLE_SOFTWARE_AES128 and 3rd-party/rijendael files. This could be faster.

If really needed, you could use an AES128 or even AES-CMAC engine of the STM32 as well. btstack_crypto.c already uses async HCI commands for AES128, so it could be extended to use hardware crypto operations.

You're updating the advertising data every second, although it doesn't change. Please only update if needed - it makes the log better to read.

Ah.. the main issue in the logs is that each write request takes about 7 seconds to get answered. It looks like this is related to storing data in the TLV, between line 1045 and line 1107 in att_server.c. What do you use as TLV implementation? it seems to be quite slow. Please debug this on your system, there's no reason it would take that long.

[jkorenj]

Hello,

Thank you for fast response.

We were using EEPROM to read TLV data. I changed this to read data from EEPROM at boot, and it looks like the connection is now instant.

I will test more tomorrow, but it looks very promising now.