PUF based Implantable Medical Device Security

Seonghan Ryu

Department of Information and Communication Engineering, Hannam University, Daejeon, Republic of Korea

Keywords: PUF, IMD, Dynamic Divider, CMOS, SoF.

Abstract: For the resource-constrained device such as Implantable Medical Device(IMD), lightweighting cryptographic

methods are required. Physical unclonable function(PUF) is promising hardware based lightweight security

solution, which makes use of the inherent process variation in semiconductor fabrication process to generate

unique ID. This paper presents an PUF based IMD security with local oscillator(LO) chain composed of VCO

and dynamic divider(DDiv), which use self oscillation frequency(SoF) variation characteristics. In the LO

chain PUF implementation, the output bits are obtained by comparing the oscillation frequencies of different

VCO L banks or dynamic dividers. For the lightweight operability, simple VCO and DDiv-PUF based

authentication protocol is also proposed.

1 INTRODUCTION

Physical unclonable function is a hardware based

promising security primitives, which is favored by

resource constrained devices such as IMD. PUF

makes use of the inherent process variation in

semiconductor process such as CMOS technology.

Implantable medical devices are widely used these

days. In general, only monitoring function such as

EEG, ECG and EMG are major application category,

however stimulator such as deep brain

stimulator(DBS) and pacemaker are also widely

adopted. Therefore, the IMD security is becoming

essential and cardinal issues. Most secure

cryptographic algorithms use a private secret value,

secret key for encryption and decryption. The key is

stored in a non-volatile based memory and inherently

vulnerable to invasive attacks such as tampering(Suh,

2003)-(Mangard, 2007). However, PUF uses the

physical fingerprint of the silicon chip, variation of

each device to generate a set of unique data. Though

identical PUF chips are implemented, the variation

between devices generate unique difference and

duplication of PUF characteristics is inherently

impossible. Figure 1 shows the local oscillator(LO)

chain for RF transceiver of IMD for wireless

connectivity. This paper presents a PUF based IMD

security using these LO chain block circuitry with

inherent silicon fabrication process variation

Figure 1: RF transceiver local oscillator(LO) chain

structure.

2 LO-CHAIN BASED PUF

IMPLEMENTATION

Figure 2 shows the proposed LO chain PUF

implementation composed of VCO and DDiv-PUF

circuit.

Figure 2: VCO and DDiv-PUF circuit implementation.

Each VCO and dynamic divider have unique

oscillation(OSC) frequency, even within same chip,

Ryu, S.

PUF based Implantable Medical Device Security.

DOI: 10.5220/0008983701650168

In Proceedings of the 13th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2020) - Volume 1: BIODEVICES, pages 165-168

ISBN: 978-989-758-398-8; ISSN: 2184-4305

165

the imitation of unpredictable variation caused by

process variation is impossible. The two multiplexers

select two VCOs or two DDivs by challenge bit

stream and two OSC frequencies are compared after

two counting blocks during a fixed comparison time

interval. The output is set to 0 or 1, depending on

which counter value is high

3 DYNAMIC DIVIDER BASED

PUF

Each divider has exactly same circuit structure and

device size, however each self oscillation frequency

of divider has small device to device variation and

results in unique characteristics such as silicon

fingerprint.

Figure 3: Schematic diagram of dynamic divider.

PUF does not store any secret key in device, the PUF

device itself generates unique response immediately

for the given challenge. Any invasive or semi-

invasive attack will cause permanent alteration of the

device physical characteristics and alter the PUF

operation permanently. Figure 3 shows a schematic

diagram of dynamic divider. Utilizing structures

similar to digital logic gates, a rail-to-rail signal swing

can be maintained by the energy from bias

current(Conroy, 2009)~(Kim, 2006). Full signal

swing can be attained in this structure. The flip-flops

in dynamic divider have a feedback connection and

generate self oscillation, which is depicted in Figure

4. Self oscillation frequency range of the divider can

be largely varied by using current starved structure as

dipicted in Fig. 3, and Ibias can also change SOF by

varying bias voltage of CLK and CLKB nodes(Ryu,

2009)~(Koukab, 2006). This wide tunability is

helpful for enhancing random variation for PUF.

Figure 4: Self oscillation frequency characteristics of DDiv.

4 VCO BASED PUF

The proposed PUF VCO structure is shown in Figure

5. A switched bondwire inductor bank is used for

wide frequency tunability, which enhances random

variation.

Figure 5: Proposed PUF VCO structure with bond-wire

inductor bank.

As depicted in Figure 5, mid and short length

bondwire inductors are shunt - connected to long

bondwire inductor. When all MOS switches are on

state, switched inductor bank has lowest total

inductance value, which causes highest OSC

frequency. When all MOS switches in switched

inductor bank are off, mid and short length bondwire

inductors are disconnected and highest inductance

value, therefore lowest OSC frequency can be

achieved. The challenge bit stream could select the L

bank bit which would be turned on, even if the same

L bank bit is selected and turned on, each VCO

2 4 6 8 101214161820222426

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Self Oscillation Frequency [GHz]

Total DC Current [mA]

Divider SOF

BIODEVICES 2020 - 13th International Conference on Biomedical Electronics and Devices

166

generates a little bit different OSC frequency and

could show the silicon fingerprint. Though bondwire

inductors are connected through MOS switches at on

state, the Q factor degradation from MOS Ron

resistance can be mitigated due to shunt connection

with long bondwire inductor which is directly

connected to VCO oscillation node without MOS

switch. Therefore, oscillation is maintained during

and after bondwire L bank switching.

Figure 6: The fabricated PUF chip on board.

For minimizing power consumption, the VCO bias

current is varied between each frequency band by

controlling the 3-bit binary weighted bias resistors.

This programmability allows power consumption

minimization. Considering these PUF VCO design

issues, the proposed PUF VCO is implemented in

65nm CMOS technology. Figure 6 shows the

fabricated chip on board with bondwire incuctor. The

chip size is 0.75 × 0.75 mm

Figure 7 depicts the measured frequency tuning

range for the proposed PUF VCO. The carrier signal

frequency of the PUF VCO is tunable from 1.91GHz

when all MOS switches are at off state to 6.42GHz

when all MOS switches are at on state. The whole

frequency band can be covered by controlling each

MOS switch in the inductor bank separately. The full

tuning range can also be covered by utilizing both

switched capacitor bank and switched inductor bank.

The VCO core operates from 1.2V supply and biases

at 6 mA.

(a)

(b)

(c)

Figure 7: Measured oscillation frequency of the proposed

PUF VCO when (a) all MOS switches are at off state (b)

one switch is at on and another is at off state (c) all MOS

switches are at on state.

5 LO-CHAIN PUF BASED

AUTHENTICATION

Figure 8 shows a proposed simple authentication

protocol for LO(Sx) Chain PUF based implantable

medical device security. At first, IMD gives a

challenge(C) to remote health monitor/stimulator,

The PUF in monitor/stimulator generates unique

response(R) and transfer this silicon fingerprint to

IMD. And IMD check the C to R data with PUF

database(DB). If the acquired C to R data is matched

PUF based Implantable Medical Device Security

167

with DB, IMD authenticate the health

monitor/stimulator. And then, in the same way, health

monitor/stimulator gives a challenge to IMD. The

PUF in IMD generates unique response and transfers

the response to remote monitor. If the C to R is

matched with DB in remote monitor, the monitor

authenticate the IMD, and the whole authentication

process is finalized.

Figure 8: LO(Sx) chain PUF based authentication protocol.

6 CONCLUSIONS

An LO chain based PUF security for implantable

medical device is proposed. Thanks to the oscillation

frequency variation characteristics caused by

semiconductor fabrication process, the lightweight

and unclonable authentication method for resource-

constrained IMD security application is proposed.

The simple authentication protocol for the LO chain

PUF based IMD security is also presented.

ACKNOWLEDGEMENTS

This work was supported by the National Research

Foundation of Korea under Grant NRF-

2017R1D1A1B03036412; IDEC(EDA Tool, MPW).

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