USER MANUAL
PTM 215B – BLUETOOTH
®
PUSHBUTTON TRANSMITTER MODULE
© 2019 EnOcean | www.enocean.com
F-710-017, V1.0
PTM 215B User Manual | v1.9 | June 2019 | Page 17/85
4.4.2
Resolvable private address mode
For some applications it is desirable to obfuscate the origins of PTM 215B data telegrams in
order to prevent tracking of its radio transmissions. This can be achieved by using resolva-
ble private addresses (RPA) as defined in the Bluetooth Core Specification.
PTM 215B can be configured to use resolvable private addresses by setting the
RPA ADDRESS
MODE
flag within the
Configuration
register (described in chapter 6.7.3) to
0b1
.
When using resolvable private addresses, the address used by PTM 215B is modified (rotat-
ed) according to a defined scheme which on one hand precludes determining the device
identity by unauthorized receivers while allowing authorized receivers (sharing a specific
security key with PTM 215B) to do so.
The shared security key – which has to be known by both PTM 215B and the authorized
receiver – is called the Identity Resolution Key (IRK). PTM 215B uses its device-unique ran-
dom key as identity resolution key. This key can be modified if needed via the NFC configu-
ration interface as described in chapter 6.7.5.
For each data telegram transmitted by PTM 215B (i.e. for every button push or release), a
new resolvable private address is generated. The 48 bit address field of such resolvable
private address 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.
Individual advertising events used to transmit one telegram (as described in chapter
3) use the same
prand
value.
hash
This field contains a verification value (hash) generated from
prand
using the IRK
The structure of a resolvable private address is shown in Figure 12 below.
Figure 12 – BLE resolvable private 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 private resolvable source address.
The receiver maintains a list of IRK for all transmitters that have been commissioned to
work with it.
