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Figure 11 - SMP Key Distribution Frames
A.1.7.2.2 Link Layer
The Link Layer (LL) protocol manages the Bluetooth low energy radio transmissions and is involved in starting link
encryption. To observe the decoded LL commands, click on the Frame Display LE LL tab, search for and select
ControlPkt “LL_ENC_REQ”. This command should originate with Side 1, the initiator of the encryption link. In
Figure 11 Frame# 39,617 is selected in the Summary pane and we see the decoded LE LL frame is display in the
Decoder pane. Shown in this frame packet is the SKDm that is the Master Session Key Diversifier (SKDmaster). In
Frame# 39,623 you will find SKDslave that is combined with SKDmaster to create the Session Key (SK). Both SDKs
were created using the LTK. Frame# 39,635 through 39,649 in the LE LL tab completes starting of the encryption
process. After the slave sends LL_START_ENC_RSP (Frame# 36,649) the Bluetooth devices can exchange
encrypted data, and the ComProbe sniffing device can also receive and decrypt the encrypted data because the
appropriate “key” is provided in the BPA 600 Datasource window.
Figure 12 - LE LL Tab Encryption Request (Frame# 39,617) from Initiator (Side 1)
A.1.7.3 Viewing Encryption in the Message Sequence Chart
Figure 13 - MSC SMP Paring (BPA 600 low energy capture)
The ComProbe software Message
Sequence Chart (MSC) links directly to
frames being viewed in the Frame Display.
Similarly MSC will display the same
information as the Frame Display
Decoder pane. Frames are synchronized
between the Frame Display Summary
pane and the MSC, so clicking on a frame
in either window will select that same
frame in the other window. Also the
protocol tabs are the same in each window.
To see the pairing process, click on the SMP
tab.
In the image above we see Frame# 35,539
initiating the pairing from the master
device. The response, SMP_Pairing
Response, is sent from the slave in Frame#
35,545. SMP_Pairing Confirm occurs
Appendicies TELEDYNE LECROY
Frontline BPA low energy Hardware & Software User Manual 218