![](bg7.png)
ping these useless steps induces significant energy
savings. Besides, the update server processes only 1
instead of n connections. For a fleet of 4 devices, al-
most 750 kB of useless data transfer is saved. These
energy and bandwidth savings evolve in O(n), so
the result is worth the messaging overhead. The sys-
tem adds 10 kB of code in FLASH, for a reference
firmware size of 237 kB (sensor management code,
MQTT, TLS and device drivers libraries) and no ex-
tra RAM budget is required.
5 CONCLUSION
In this paper, we studied a way to group IoT de-
vices to strengthen the security of the firmware up-
date procedure. The method establishes a common
cryptographic key, and uses it to group IoT nodes.
This technique requires no compute-intensive pro-
cessing and no specific provisioning of the devices.
The secure group concept is used to create a syn-
chronized firmware update procedure. Besides, this
system requires very few server interactions as being
fully based on a device to device approach. This prop-
erty brings additional autonomy and robustness bene-
fits.
In future work, a more efficient device to de-
vice communication could be exploited to improve
power consumption and server independence, by us-
ing Long Range Wide Area Network (LoRaWAN) or
Bluetooth Mesh networking. Last but not least, we
may refine the heuristics to improve the system effi-
ciency: guard periods, unreachable devices manage-
ment, power saving impact on procedures.
ACKNOWLEDGEMENTS
The authors would like to thank Julien Montmasson
(ST Microelectronics) and G
´
eraud Plagne (ST Micro-
electronics) for their valuable comments and sugges-
tions to improve this contribution. The authors would
also like to thank Eloise Cheval (ST Microelectron-
ics) for her thorough reviews of this paper.
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