FrontLine BPA LOW ENERGY, Hardware User Manual

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Figure 23 - Encryption Response from Slave, Example
(ComProbe Frame Display, BPA 600 low energy capture)
A.2.6 Encrypting The Data Transmission
Data encryption begins with encrypting the link. The Session Key (SK) is created using a session key diversifier
(SKD). The first step in creating a SK is for the master device to send Link Layer encryption request message
(LL_ENC_REQ) that contains the SKD
master
. The SKD
master
is generated using the LTK. The slave receives
SKD
master
, generates SKD
slave
, and generates SK by concatenating parts of SKD
master
and SKD
slave
. The slave
device responds with an encryption response message (LL_ENC_RSP) that contains SKD
slave
; the master will
create the same SK.
Now that a SK has been calculated, the master and slave devices will now begin a handshake process. The
slave will transmit unencrypted LL_START_ENC_REQ, but sets the slave to receive encrypted data using the
recently calculated SK. The master responds with encrypted LL_START_ENC_RSP that uses the same SK just
calculated and setting the master to receive encrypted data. Once the slave receives the master’s encrypted
LL_START_ENC_RSP message and responds with an encrypted LL_START_ENC_RSP message the
Bluetooth
low energy devices can now begin transmitting and receiving encrypted data.
A.2.7 IRK and CSRK Revisited
Earlier in this paper it was stated that LTK would be the focus, however the IRK and CSRK were mentioned.
We revisit these keys because they are used in situations that require a lesser level of security. First let us
note that IRK and CSRK are passed in an encrypted link along with LTK and EDIV.
Use of the IRK and CSRK attempt to place an identity on devices operating in a piconet. The probability that
two devices will have the same IRK and generate the same random number is low, but not absolute.
IRK and Bluetooth low energy Privacy Feature
Bluetooth low energy has a feature that reduces the ability of an attacker to track a device over a long period
buy frequently and randomly changing an advertising device's address. This is the privacy feature. This feature
is not used in the discovery mode and procedures but is used in the connection mode and procedures.
If the advertising device was previously discovered and has returned to an advertising state, the device must
be identifiable by trusted devices in future connections without going through discovery procedure again. The
IRK stored in the trusted device will overcome the problem of maintaining privacy while saving discovery
computational load and connection time. The advertising devices IRK was passed to the master device during
initial bonding. The a master device will use the IRK to identify the advertiser as a trusted device.
CSRK and Signing for Authentication
Bluetooth low energy supports the ability to authenticate data sent over an unencrypted ATT bearer between
two devices in a trust relationship. If authenticated pairing has occurred and encryption is not required
(security mode 2) data signing is used if CSRK has been exchanged. The sending device attaches a digital
signature after the data in the packet that includes a counter and a message authentication code (MAC). The
key used to generate MAC is CSRK. Each peer device in a piconet will have a unique CSRK.
The receiving device will authenticate the message from the trusted sending device using the CSRK
exchanged from the sending device. The counter is initialized to zero when the CSRK is generated and is
incremented with each message signed with a given CSRK. The combination of the CSRK and counter
mitigates replay attacks.
Appendicies TELEDYNE LECROY
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