User manual

.
Figure 3 - Responder Pairing Confirm Example (ComProbe Frame Display, BPA 600 low energy capture)
Figure 4 - Message Sequence Chart: SMP
Pairing
The initiating device will generate a 128-bit random number that is
combined with TK, the Pairing Request command, the Pairing
Response command, the initiating device address and address type,
and the responding device address and address type. The resulting
value is a random number Mconfirm that is sent to the responding
device by the Pairing Confirm command. The responding device will
validate the responding device data in the Pairing Confirm command
and if it is correct will generate a Sconfirm value using the same
methods as used to generate Mconfirm only with different 128-bit
random number and TK. The responding device will send a Pairing
Confirm command to the initiator and if accepted the authentication
process is complete. The random number in the Mconfirm and
Sconfirm data is Mrand and Srand respectively. Mrand and Srand
have a key role in setting encrypting the link.
Finally the master and slave devices exchange Mrand and Srand so
that the slave can calculate and verify Mconfirm and the master can
likewise calculate and verify Sconfirm.
A.1.4 Encrypting the Link
The Short Term Key (STK) is used for encrypting the link the first time the two devices pair. STK remains in each
device on the link and is not transmitted between devices. STK is formed by combining Mrand and Srand which
were formed using device information and TKs exchanged with Pairing Confirmation (Pairing Confirm).
A.1.5 Encryption Key Generation and Distribution
Figure 5 - Encryption Request from Master, Example (ComProbe
Frame Display, BPA 600 low energy capture
To distribute the LTK, EDIV, and Rand values an
encrypted session needs to be set up. The
initiator will use STK to enable encryption on the
link. Once an encrypted link is set up, the LTK is
distributed. LTK is a 128-bit random number that
the slave device will generate along with EDIV
and Rand. Both the master and slave devices can
distribute these numbers, but Bluetooth low
energy is designed to conserve energy, so the
slave device is often resource constrained and
does not have the database storage resources
for holding LTKs. Therefore the slave will
distribute LTK, EDIV, and Rand to the master device for storage. When a slave begins a new encrypted session
with a previously linked master device, it will request distribution of EDIV and Rand and will regenerate LTK.
Appendicies TELEDYNE LECROY
Frontline BPA low energy Hardware & Software User Manual 214