User manual

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
TELEDYNE LECROY Appendicies
223 Frontline BPA low energy Hardware & Software User Manual