USER MANUAL
EMDCB
–
BLUETOOTH LOW ENERGY MOTION AND ILLUMINATION SENSOR
© 2019 EnOcean | www.enocean.com
EMDCB User Manual | v1.3 | August 2019 | Page 24/67
4.4.2
Resolvable private address mode
For some applications it is desirable to modify (rotate) the source address used by EMDCB
in order to prevent tracking of its radio transmissions. At the same time, each EMDCB de-
vice must remain uniquely identifiable by the receiver. To achieve these goals, EMDCB can
be configured via NFC to use resolvable private addresses (RPA).
Using resolvable private addresses requires that both EMDCB and the receiver both know a
common key
–
the so-called Identity Resolution Key (IRK). EMDCB uses its device-unique
random key as identity resolution key. This key can be configured via the NFC configuration
interface as described in chapter 7.5.
For resolvable private addresses, the 48 bit address field is split into two sub-fields:
◼
prand
This field contains a random number which always starts (two most significant bits)
with 0b10. The
prand
value is changed for each telegram that is transmitted. Indi-
vidual advertising events used to transmit one telegram use the same
prand
value.
◼
hash
This field contains a verification value (hash) generated from
prand
using the IRK
The structure of a random resolvable private address is shown in Figure 15 below.
Figure 15
–
BLE private resolvable source address structure
The
prand
value is encrypted using the IRK. The lowest 24 bit of the result (encrypted val-
ue) are then used as
hash
. The concatenation of 24 bit
prand
and 24 bit
hash
will be trans-
mitted as 48 bit resolvable private address.
The receiver maintains a list of IRK for all transmitters that are known to it (have been
commissioned to work with it). Whenever it receives a radio telegram with resolvable pri-
vate address (identified by the most significant bits being set to
0b10
), it will itself generate
a 24 bit
hash
from the 24 bit
prand
sequentially using the IRK of each device that it has
been learned into it. If an IRK matches (i.e. when
prand
is encoded with this specific IRK
then the result matches
hash
), then the receiver has established the identity of the trans-
mitter.
So conceptually the IRK takes the role of the device source address while
prand
and
hash
provide a mechanism to select the correct IRK among a set of IRK.
