nRF52840 Cannot Sniff Encrypted Connection

RE: nRF52840 Cannot Sniff Encrypted Connection

sroyinct — Thu, 05 Jan 2023 21:00:41 GMT

Thanks for the help.

RE: nRF52840 Cannot Sniff Encrypted Connection

Vidar Berg — Thu, 05 Jan 2023 19:42:36 GMT

Yes, LESC pairing should be used if both devices include the "Secure Connection Flag" in their pairing request/response. You can also tell that LESC is used by the SMP packets sent after the pairing response. Legacy pairing will not include the public key exchange or the DHKey check at the end.

With regards to the nRF Connect issue, try to press the Reset button on the dongle to make it enter DFU mode (The red LED indicates DFU mode). Then select the "open bootloader" device from the dropdown menu and click yes when prompted to program it. This will allow the app to replace the sniffer FW with the connectivity FW which is needed for BLE connectivity.

RE: nRF52840 Cannot Sniff Encrypted Connection

sroyinct — Thu, 05 Jan 2023 18:39:45 GMT

Thanks, Vidar. A couple of followup questions:

1. I wasn't aware that the watch is using LESC. How can you tell? Is it from the "Secure Connection Flag" in the pairing request (packet 3053)?

2. I don't have a spare dongle, but I thought I would at least try the nRF Connect Bluetooth Low Energy app as you suggested. But I am getting a error as shown in the attached screenshot and log file. Does this indicate a problem with the dongle?

devzone.nordicsemi.com/.../2023_2D00_01_2D00_05T18_5F00_28_5F00_06.366Z_2D00_log.txt

RE: nRF52840 Cannot Sniff Encrypted Connection

Vidar Berg — Thu, 05 Jan 2023 09:10:56 GMT

Hello,

Unfortunately, a BT sniffer will not be able to pick up the encryption key when the devices use LE secure connections pairing (LESC) to establish the bond like in your case. LESC uses a Diffie-Hellman key exchange that protects against passive eavesdropping. For comparison, with LE Legacy pairing, the encryption key would be sent in cleartext on air. Thus, making it easy for a passive sniffer to retreive the key.

If you have a spare DK/Dongle, you may try to use the nRF Connect Bluetooth Low Energy app in nRF Connect for Desktop. This allows you to configure the security capabilities for pairing so you can force LE legacy pairing mode.

Initiate Pairing in nRF Connect Bluetooth Low Energy app

Best regards,

Vidar