IEEE Solid-States Circuits Magazine - Fall 2020 - 72

of the current data bit for random
channel selection provides physicallayer data encryption, since the at-
tacker cannot decode whether the
data bit sent is a bit 1 or bit 0 without
obtaining the private key of the Tx.
One future research direction is
to explore how such devices employ-
ing a CS bit-level FH Tx will coexist
in a network. If they operate with
independent hopping patterns, this
scheme will provide high security
against selective jamming attacks at
the expense of producing more col-
lisions. Hence, investigating how to
avoid collisions at scale without com-
promising security is a critical next
step for enabling wide adoption of
these proposed techniques in future
wireless communication systems.

Securing Low-Power, Implantable
Wireless Devices Through a DualFactor Authentication
IMDs with wireless control promise
better disease management in a more
continuous and connected fashion. In
the same way that fitness trackers are
used to encourage our daily health,
wirelessly controllable IMDs, such as
on-demand drug delivery systems,
allow continuous monitoring and ad-

justment of healthcare delivery, po-
tentially resulting in improved health
outcomes. For example, for the ondemand drug delivery systems, these
IMDs can be embedded in the arm of a
patient and directed to deliver medica-
tion as approved by the physician and
requested by the patient. Users will
control the drug delivery schedule
and dosage via external handheld de-
vices, such as cellphones. However, it
has been shown that these life-critical
implantable devices can be attacked,
e.g., by exploiting weaknesses in the
authentication protocols, motivating
the need to establish end-to-end se-
cure solutions [24]. In this work [25],
researchers developed a dual-factor
authentication mechanism to ensure
the security of these IMDs by adding
another layer of security in addition
to an energy-efficient first-factor au-
thentication achieved via symmet-
ric cryptography.

Threat Model
It is envisioned that three parties
will be involved in these wireless
drug delivery systems: the user, the
IMD, and a secure cloud server. The
secure cloud server is envisioned to
authenticate the IMD, approve the

80-Way PR-FSK Modulation
Target Base-Station Receiver View
Noncoherent Decoder
Energy (Ch31) > Energy (Ch4)

Ch1

Ch4
Bit 0

Ch17

Ch31
Bit 1

Ch80

Ch31
Unknown
Bit X

Ch80

Ch23

Attacker View in 80 Channels
Unpredictable Bit Location

Ch1

Ch4

Ch17

Ch23

FIGURE 10: The CS 80-way PR-FSK modulation countermeasure. Two random channels are
designated to encode bit 1 and bit 0 in every hop, thus making channel localization and selective overwriting of data bits infeasible. Moreover, as shown in the bottom attacker view, the
attacker can sense the energy in Ch31, but the value of the transmitted bit is still encrypted
in the physical layer through the random channel assignments using a secret key [6].

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IEEE SOLID-STATE CIRCUITS MAGAZINE	

control commands by the user, and
then securely communicate the drug
delivery parameters, as determined
by the physician, and also keep a
record of the drug delivery transac-
tions for disease progress monitor-
ing. However, these IMDs typically
cannot execute complex communi-
cation protocols or computation due
their resource constraints, e.g., size
and battery constraints. To commu-
nicate with the cloud server, they
utilize a cellphone as the relay. For
example, the communication be-
tween the IMD and cellphone can be
performed using the BLE protocol,
and then these messages can be re-
layed to the cloud server through
the Internet. The attacker targets
the relay device between the three
parties to harm the life-critical com-
munication link. The threat model
considered in [25] is summarized in
the following.
■■ A type-1 adversary, i.e., stealthy
adversary, can control the IMD to
execute malicious actions without
being detected by relaying the com-
munication via a malware- or ran-
somware-infected cellphone or by
simply keeping the cellphone in its
physical possession. In this threat
model, the adversary can maintain
its deniability, and even an IMD
utilizing a traditional cryptogra-
phy-based authentication protocol
cannot distinguish the command
relayed by this malicious cellphone
from an intended command re-
ceived from a legitimate user.
■■ A type-2 adversary aims to de-
grade the performance of the
drug delivery system by schedul-
ing frequent wake up of the IMD
over the BLE link to increase the
battery usage, creating a need for
frequent battery replacement.

Countermeasures
A dual-factor authentication protocol
addresses this threat model, where
the attacker exploits the weakness in
the protocol relying on the authentic-
ity of the commands from the relay
device. Traditional two-factor authen-
tication is used widely, for example,
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