FrontLine BPA LOW ENERGY, Hardware User Manual

Page: 204 / 228

message is transmitted from the responder and contains much of the same information as the initiators
Pairing Request message thus confirming that a pairing is successfully negotiated.
Figure 1 - Sample Initiator Pairing Request
Decode (ComProbe Frame Display, BPA 600
low energy capture)
In the sample SMP decode, in the figure at the right, note
the “keys” identified. Creating a shared, secret key is an
evolutionary process that involves several intermediary
keys. The resulting keys include,
1. IRK: 128-bit key used to generate and resolve
random address.
2. CSRK: 128-bit key used to sign data and verify
signatures on the receiving device.
3. LTK: 128-bit key used to generate the session key
for an encrypted connection.
4. Encrypted Diversifier (EDIV): 16-bit stored value
used to identify the LTK. A new EDIV is generated
each time a new LTK is distributed.
5. Random Number (RAND): 64-bit stored value
used to identify the LTK. A new RAND is
generated each time a unique LTK is distributed.
Of particular importance to decrypting the encrypted data on a Bluetooth low energy link is LTK, EDIV, and
RAND.
A.1.3 Pairing Methods
The two devices in the link use the IO capabilities from Pairing Request and Pairing Response packet data to
determine which of two pairing methods to use for generation of the Temporary Key (TK). The two methods
are
Just Works and Passkey Entry
1
. An example of when Just Works method is appropriate is when the IO
capability input = None and output = None. An example of when Passkey Entry would be appropriate would
be if input= Keyboard and output = Display. There are 25 combinations that result in 13
Just Works methods
and 12 Passkey Entry methods.
In Just Works the TK = 0. In the Passkey Entry method,
Figure 2 - Initiator Pairing Confirm Example (ComProbe Frame Display, BPA 600 low energy capture)
.
Figure 3 - Responder Pairing Confirm Example (ComProbe Frame Display, BPA 600 low energy capture)
1
A third method, Out Of Band (OOB), performs the same as Pass Key , but through another external link such
as NFC.
TELEDYNE LECROY Appendicies
197 Frontline BPA low energy Hardware & Software User Manual