FrontLine BPA LOW ENERGY, Hardware User Manual

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Figure 17 - Chappe's Telegraph Code
Of course anyone positioned between the telegraph stations that
had Chappe's telegraph code in hand could decode the transmission.
So securing the code was of paramount importance in Chappe's
protocol.
Modern wireless networks
such as Bluetooth low energy
employ security measures to prevent similar potentially man-in-the-
middle attacks that may have malicious intent.
Bluetooth low energy devices connected in a link can pass sensitive
data by setting up a secure encrypted link. The process is similar to
but not identical to
Bluetooth BR/EDR Secure Simple Pairing. One
difference is that in Bluetooth low energy the confidential payload
includes a Message Identification Code (MIC) that is encrypted with
the data. In
Bluetooth BR/EDR only the data is encrypted. Also in
Bluetooth low energy the secure link is more vulnerable to passive
eavesdropping, however because of the short transmission periods
this vulnerability is considered a low risk. The similarity to BR/EDR
occurs with "shared secret key", a fundamental building block of
modern wireless network security.
This paper describes the process of establishing a Bluetooth low
energy secure link.
A.2.1 How Encryption Works in Bluetooth low energy
Data encryption is used to prevent passive and active—man-in-the-middle (MITM) — eavesdropping attacks
on a
Bluetooth low energy link. Encryption is the means to make the data unintelligible to all but the Bluetooth
master and slave devices forming a link. Eavesdropping attacks are directed on the over-the-air transmissions
between the
Bluetooth low energy devices, so data encryption is accomplished prior to transmission using a
shared, secret key.
A.2.2 Pairing
A Bluetooth low energy device that wants to share secure data with another device must first pair with that
device. The Security Manager Protocol (SMP) carries out the pairing in three phases.
1. The two connected Bluetooth low energy devices announce their input and output capabilities and
from that information determine a suitable method for phase 2.
2. The purpose of this phase is to generate the Short Term Key (STK) used in the third phase to secure
key distribution. The devices agree on a Temporary Key (TK) that along with some random numbers
creates the STK.
3. In this phase each device may distribute to the other device up to three keys:
a. the Long Term Key (LTK) used for Link Layer encryption and authentication,
b. the Connection Signature Resolving Key (CSRK) used for data signing at the ATT layer, and
c. the Identity Resolving Key (IRK) used to generate a private address.
Of primary interest in this paper is the LTK. CSRK and IRK are covered briefly at the end.
Bluetooth low energy uses the same pairing process as Classic Bluetooth : Secure Simple Pairing (SSP). During
SSP initially each device determines its capability for input and output (IO). The input can be None, Yes/No, or
TELEDYNE LECROY Appendicies
205 Frontline BPA low energy Hardware & Software User Manual