.
Figure 20 - Responder Pairing Confirm Example (ComProbe Frame Display, BPA 600 low energy capture)
Figure 21 - 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.2.4 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.2.5 Encryption Key Generation and Distribution
Figure 22 - 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.
TELEDYNE LECROY
Appendicies
207
Frontline BPA low energy Hardware & Software User Manual
Summary of Contents for BPA LOW ENERGY
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