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Software-based self-testing for the authentication of car components

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Abstract

We present a software solution for the authentication of ECUs based on hardware intrinsic authentication features of standard micro-controllers. It requires that the group of authentic ECUs is char- acterized by a dedicated MCU model and a group identifier in read-only memory. No secret ECU key is required. We make use of the fact that an MCU running a suitable self-test is a complex dynamical system that is hard to simulate in a cycle-accurate way. We demonstrate that software- based self-testing can serve as a “time-bounded” authentication method. One field of application is the detection or lock-out of counterfeits.
Software-based self-testing for the
authentication of car components
Frank.Schuhmacher@segrids.com
Segrids GmbH
Abstract. We present a software solution for the authentication of
ECUs based on hardware intrinsic authentication features of standard
micro-controllers. It requires that the group of authentic ECUs is char-
acterized by a dedicated MCU model and a group identifier in read-only
memory. No secret ECU key is required. We make use of the fact that an
MCU running a suitable self-test is a complex dynamical system that is
hard to simulate in a cycle-accurate way. We demonstrate that software-
based self-testing can serve as a “time-bounded” authentication method.
One field of application is the detection or lock-out of counterfeits.
1 Introduction
A “security control unit” (SCU) in an on-board network might be required to
authenticate ECUs at startup. One motivation is the detection of counterfeit
spare parts.
Key based cryptography is not a good choice for the authentication of an
ECU without a secure hardware element since it is most likely vulnerable to
probing, DPA, or fault injection attacks [1]. Cryptographic authentication is
broken as soon as a single secret ECU key is disclosed. Due to immense revenues
in product counterfeiting [2], a strong attack potential must be presumed. Secure
hardware elements providing a sufficient key protection level are expensive for a
price sensitive market.
Physical unclonable functions (PUFs) were originally developed as an alter-
native to key based cryptographic authentication. However, only “weak” PUF
solutions [3] play a practical role so far. Weak PUFs are not used directly as
an authenticator but serve as a secure key storage. They support the secure
key handling in a cryptographic solution but do not solve the problem of a key
leakage during a cryptographic computation.
The inventions of a “public” PUF (PPUF) in [4] and of “time-bounded”
authentication (TBA) in [5] are based on the insight that it is not mandatory
for a device authentication that its challenge-response-function is completely
unclonable. It is sufficient that it can be computed faster on an authentic device
than on any simulator, assuming that the challenger verifies that the response
time is below a suitable limit, additionally to the correctness of the response.
Both inventions are based on device unique signal propagation characteristics
of gate arrays due to production process variations. The article [6] suggests the
term SIMPL (simulation possible but laborious) instead of PPUF or TBA.
Our contribution is a new time-bounded authentication solution, which does
not require additional hardware or an FPGA. It perfectly suits for automotive
since it can be combined with software-based self-testing (SBST) for functional
safety [7] at start-up.
We consider an ECU as authentic if it has an authentic MCU. An MCU
is authentic if it is of the same model and has the same group identifier in a
read-only memory space as an authentic reference MCU. Our solution requires
a contract between the ECU manufacturer and the MCU manufacturer and
organizational means to ensure that not unauthorized third party gets access to
authentic MCU samples.
We use the fact that an MCU executing an SBST is a complex dynamical
system, and that it is hard to simulate its behavior in a cycle-accurate way. In
the authentication, the responder MCU interprets a received challenge as a test
pattern of an SBST. The SBST stimulates multiple parts of the MCU, covering a
set of peripheral modules characteristic for the MCU model, measures the MCU
behavior by sampling static and dynamic system states, and computes a hash
sum over the sampled data. The sequence of hash states serves as pseudo-random
stimulus data to achieve an unpredictable behavior and timing. Two kinds of
dynamic data will only be generated by an authentic MCU and will serve as a
timing signature: clock counter values sampled at dedicated program states, and
program counters sampled at dedicated runtimes.
Authentication by software-based self-testing of MCU intrinsic group features
is different in many aspects from state of the art authentication. Therefore,
we introduce a suitable formalism in Section 2, and provide a precise attacker
model in Section 3. We formally define four security objectives in Section 4, and
are able to prove the security of the authentication scheme provided that its
implementation achieves the four security objectives.
Apart from security, reliability is a central requirement. The complemen-
tary paper [8] describes an implementation of our authentication scheme on an
example MCU model, and reliability tests for the implementation. The test ob-
servations indicate that reliability can be achieved. It finally verifies that the
implementation satisfies the four security objectives. Section 5 summarizes the
results of this reference.
Section 6 provides automotive specific details, e.g. the provisioning of the
SCU with challenge-response-pairs, and the setup of a back-end server for garage
updates or updates over-the-air. The final Sections 7, 8, and 9 summarize advan-
tages and disadvantages with respect to the state of the art, draw conclusions,
and specify future work.
2 Definitions
2.1 Field of application
Afield of application is a pair (LCU,pth) of a set LCU of “limited resource”
control units, and a probability threshold pth defining the acceptable prob-
ability for a malicious LCU to pass an authentication. Informally, the set LCU
will be a set of ECUs with the following limitations: (1) The production cost per
piece is limited by the sales price of an original ECU. (2) The size is limited by
the size of an original ECU. (3) No online connectivity.
Examples of LCUs are MCUs if not over-sized for the field of application and
automotive ECUs. For counterfeit prevention, we only need to consider LCUs as
adversaries in the field.
2.2 Invariants
For a given MCU instance M, denote model(M) the set of all MCUs of the
same MCU product model. Fix a challenge space and a response space. A CRT
program is a binary for processing a challenge from the challenge space, and
computing a response
response =prog(M, challenge)
in the response space depending deterministically on the triple
(prog, M, challenge).
The response of a CRT program depends in general on the MCU hardware
instance. We set prog(L, challenge) = undef if an LCU Ldoesn’t support a
program prog. A CRT program prog is called an invariant of a subset group
MCU if
prog(M, challenge) = prog(M0,challenge),
for each M, M 0group and each challenge.
2.3 Authenticators
Fix a field of application, an invariant prog of a group MCU of “authentic”
MCUs, and a response time limit tlim. Introduce the relation
auth responder(response)
to state that the sender of a received response is a authentic, and the relation
match(challenge,response)
to state (a) that the responder is a an LCU, (b) that the response time is
below tlim, and (c) that the received response matches the authentic reference
response. The definition of an invariant requires the deterministic implication:
auth responder(response) =match(challenge,response) (1)
Implication (1) is referred to as reliability. Reliability needs to be supported
by testing, including environmental stress tests (refer to Section 5).
In the case ¬auth responder(response), we need to work with probabilistic
implications. Introduce the relation fresh(challenge) to state that challenge
was never used for an authentication in the field before. A pair (prog,tlim) of
an invariant prog of a subset group MCU and a response time limit tlim is an
authenticator for a given field of application (LCU,pth) if the response time of
each group element is below tlim, and the conditional probability of
fresh(challenge)match(challenge,response) (2)
under the condition ¬auth responder(response) is smaller than pth. An im-
portant aspect in the authenticator definition is that only malicious devices L
in the universe LCU are to be considered. To prevent ECU counterfeiting, an
authenticator doesn’t need to detect non-authentic big and expensive hardware.
2.4 Software-based self-tests
Consider a software based self-test (SBST) as a CRT program prog that
loads and executes a challenge dependent self-test binary (code,data), such
that the execution covers:
1. System stimulation by writing stimulus data into hardware registers at lo-
cations w0, ..., wM1;
2. Sampling a sequence of memory or register values s0, ..., sN1at locations
r0, ..., rN1and sample times t0, ..., tN1;
3. Computing the response response =prog(M, challenge) covering a hash
sum hash(r0, s0, ..., rN1, sN1) over locations and samples;
and satisfying the following requirements: (A) The code instruction flow uniquely
depends on the challenge. (B) The data depends on the challenge. (C) The se-
quence of locations w0, ..., wM1depends on data. (D) The sequence of locations
r0, ..., rN1depends on data.
An SBST measures a part of the system behavior. The self-test data defines
the test pattern.
2.5 MIGA
Consider an identifier of a model(M) as a map id from model(M) to a set of bit
strings. A subset of the form
group(M, model,id) := {M0model(M)|id(M0) = id(M)}
will be called an MCU intrinsic group. Provided a field of application (LCU,pth),
we call an authenticator (prog,pth) of an MCU intrinsic group, where prog is
an SBST an MCU intrinsic group authenticator (MIGA).
Obviously, the identifier id(M) needs to be sampled by any authenticator
of group(M, model,id), and it needs to be ensured that the sampling is done
from the authentic identifier address. We will show that at least for some MCU
models, a MIGA can be realized without the need of a secret key. A core element
will be the timing signature as defined in the following subsection.
2.6 Timing signature
In our MIGA construction, the self-test code will sample some states siin order
to measure the timing of the system (M, prog) in a clock-cycle accurate way.
Timing covers the instruction flow of code and the cycle-accurate behavior of the
peripheral MCU modules. There are two complementary variants of timing data:
system states changing every clock cycle (e.g. clock counters) measured at ded-
icated program states, and program counters measured at dedicated runtimes.
In [8], we will realize the second variant by sampling stacked return addresses
via timer interrupt service routines. The fastest changing bit of a timing sample
is bit zero (LSB) for the first variant, and bit one for the second variant if the
width of most CPU instructions is 16 bit.
In our MIGA construction, the self-test code will successively append ded-
icated fast changing bits to a bit sequence called the timing signature and
denoted σ(challenge). In an SBST with timing signature, we assume that
the response contains the timing signature in addition to the hash sum.
A necessary condition for a unique timing signature is that all involved hard-
ware modules (in particular the timers and busses) are clocked without prescaler
by the same clock as the CPU. If for example timers are clocked with prescaler
2, in the best case the response takes one out of two possible values.
Definition 1. For an SBST prog with timing signature σand integers m
nN, denote [m:n]the set {m, m + 1, ..., n 1}, and
K(challenge, m, n)[m:n]
the subset of indices iwhere the element siof the sequence s1, ..., sNof sampled
data contributes a bit to the timing signature, and call the minimal dsuch that
for each challenge:
mnd=⇒ |K(m, n, challenge)| ≥ 1
the mesh size d(σ).
The security of MIGA is based on the hardness of cycle-accurate system
emulation, i.e. on the hardness to determine the timing signature on any non-
authentic hardware.
2.7 Synchronization
In order to get a deterministic response in an SBST with timing signature, the
MCU must execute code synchronously to a reference MCU. A synchronous
execution includes a synchronous instruction execution timing, and synchronous
read and write accesses to the system bus. It requires starting from a known
state [9]. The known state can only be achieved deterministically after receiving
the challenge if it is very simple, e.g. if memory and bus controllers have a known
initial configuration, the peripheral modules to be used in the self-testing are
clocked but disabled and in a known initial configuration, no interrupt is pending
or active, and no DMA process is running. The observations in [8] indicate that
– in combination with the the no prescaler assumption – these conditions are
also sufficient, at least for two tested MCU models (HT32F52 and SAM3x8e).
We will implement an SBST in such a way that after some system stimuli,
these prerequisites are not true any more: DMA processes will compete with the
CPU and memories for bus services, and timer interrupts of pseudo-randomly
configured timer modules will occur frequently. This makes the system state at
a dedicated code instruction or dedicated runtime thard to predict. Runtime
always refers to the number of CPU clock cycles counted from the known state.
It is even harder for any system (authentic or malicious) to synchronize with
a reference system at a runtime tif running some asynchronous code0before
t: since this would not only require the knowledge but also the capability to
establish an equivalent system state. The security of MIGA also depends on the
hardness of such a synchronization in the middle of a self-test execution.
3 Attacker model
In this section, we need to establish a sufficiently generic attacker model. Con-
sider a triple (prog, σ, tlim) of an SBST with timing signature, and a time limit
implemented for the authentication of group(M, model,id) elements in a field
of application (LCU,pth). An attacker is a counterfeit manufacturer, i.e. the im-
plementer of malicious systems (L, prog0) with
LLCU \group(M, model,id)
trying to pass challenge-response-tests with probability higher pth. For passing
a challenge-response-test, the malicious Lneeds to determine the hash sum and
the timing signature. The SBST shall be designed in such a way that for each
malicious Lthere exists at least one isuch that Lwill not sample authentic
system states siat the authentic location ri. For achieving the correct hash
result nevertheless, the malicious Lmight try to solve this by “spoofing”, to be
defined in Section 3.1. Introduce the relation recovery(response) to indicate
that the responder correctly determined – deterministically or by guessing – the
timing signature. By construction:
match(challenge,response) =recovery(response) (3)
If the challenge is fresh, a malicious responder has three possibilities for the
recovery of the timing signature: first, by running a code0synchronously to an
authentic reference MCU running the authentic self-test code. In this case, we
assume that Lis able to sample the authentic timing values. It will be formalized
in Section 3.2. Secondly, by running an asynchronous code0and figuring out
the authentic timing samples by a mix of simulation and guessing. This case
will be formalized in Section 3.3. Third, by switching between synchronous and
asynchronous sampling. This case will be formalized in Section 3.4.
3.1 Spoofing
Introduce the relation spoof(m, n, response) to indicate that for some i[m:
n] the responder did not sample the authentic siat the authentic location ribut
nevertheless performed a hashing of each authentic pair (ri, si) for i[m:n] to
obtain the authentic hash states hashiof an authentic MCU at sample time ti.
3.2 Synchronous sampling
Introduce the relation sync(m, n, response) to state that the responder has the
same processor model than an authentic MCU, and for integers 0 m<nN
the responder sampled the timing values within sm, ..., sn1synchronously to the
authentic reference device. This includes that the responder CPU executed an in-
struction sequence synchronously to an authentic MCU, that the responder sam-
pled a sequence s0
m, ..., s0
n1of system states at the authentic sampling runtimes
tm, ..., tn1, and that for iK(m, n, challenge), we have s0
i=si. Introduce
the logical relation sound(m, n, response) to state that sync(m, n, response)
holds, and for i[m:n], we have s0
i=siand r0
i=ri.
Introduce the relation auth binary(m, n, response) to state that the respon-
der executed the authentic self-test binary (code,data) without interruption for
sampling s0
m, ...., s0
n1.
3.3 Asynchronous sampling
Introduce the relation async(m, n, response) to state that the responder is ma-
licious, and that for integers m<nNit correctly determined the authentic
timing samples within sm, ..., sn1asynchronously, or that the responder has
a different processor model than an authentic MCU and sampled these timing
samples correctly by chance. The case m=nis used if a synchronous sampling
is interrupted and restarted after a re-synchronization, the case m=n= 0 shall
indicate that the responder started the sampling asynchronously. An example
for an asynchronous sampling would be a cycle-accurate emulation of an authen-
tic system. This is out of the scope for elements LLCU due to sophisticated
mechanism for branch prediction, bus arbitration, caching effects and latencies
of peripheral responses [10], [11].
A malicious Lwill not be able to do cycle-accurate system emulation within
the time limit tlim but in the worst case it can do something close: mix functional
system emulation and guessing of timing data in such a way that if it has already
determined s1, ..., sn1correctly, then it is able to determine non-timing snin
real-time and guess snfor nK(challenge) with a guess rate limited by some
global px <1.
Axiom 1 (No cycle accurate emulation). There exists a global boundary
px <1 such that for 0 mnN, the probability of
async(m, n, response)
under the condition of a malicious responder is smaller equal pxK(m,n,response).
If a synchronous program sequence runs after an asynchronous one, a malicious
Lmust also first do a synchronization, see Section 2.7. Such a synchronization
is a heuristic process that will only succeed with a small success rate.
Axiom 2. There exists a global boundary py <1/2 such that for 0 lm <
nN, the probability of
async(l, m, response)sync(m, n, response)
under the condition of a malicious responder is smaller equal py.
The minimal pair (px,py) can be considered as the attack potential of the
worst malicious LLCU. For the example implementation in [8], we assume as
worst case attack potential (px,py) = (0.9,0.1).
3.4 Mixed attack strategy
We state that the recovery of the authentic timing signature requires a mix
between synchronous and asynchronous sampling, if the challenge is fresh.
Axiom 3. If fresh(challenge) and recovery(response), then there exists a
Y0 and a sequence
0 = n0n1... n2Y+2 =N1 (4)
with sync(ni, ni+1,response) for even i,async(ni, ni+1,response) for odd i,
and nistrictly smaller ni+1 for even i2.
We call Ythe re-synchronization count. It is zero if the responder is authen-
tic. If the responder is a malicious LCU, the sequence (4) can be considered as
the attack strategy. An attack strategy might be challenge dependent.
Remark 1. For an attacker with potential (px,py), the probability for an attack
with strategy (4) is limited by
pxX·pyY,
where Xis the number of timing indices in
[n1:n2][n3:n4]... [n2Y+1 :n2Y+2].
4 Security architecture
Fix a field of application (L, pth). Section 4.1 defines security objectives for a
pair (prog,tlim) of a group(M, model,id) invariant SBST and a response time
limit. We prove in Section 4.2 that the security objectives are suitable for a
MIGA if the minimal samples count Nis chosen big enough.
4.1 Security objectives
Objectives 1. Provide a “coverage constant” cand a “feedback parameter” o,
such that for each m, n with mn > c, and assuming a malicious responder,
the probability for each of the following implications is greater 1 pth/4:
1. Hardware coverage: if the responder generated the response by faithful exe-
cution of the authentic binary, and sampled the authentic system states at
the authentic locations, then it is authentic. More precisely:
auth binary(m, n, response)
sound(m, n, response)
=auth responder(response)
2. Code coverage: if the responder sampled the authentic system states at the
authentic locations, it necessarily executed the authentic binary faithfully.
More precisely:
sound(m, n +o, response) =auth binary(m, n, response)
3. Feedback: if the responder was in sync with an authentic reference MCU,
then it fed the authentic data into the authentic hash function. More pre-
cisely:
sync(m, n +o, response) =
sound(m, n, response)
spoof(m, n, response)
4. Spoofing detection: if the responder did not read the authentic data, or not
from the authentic addresses, but computed the hash sum nevertheless over
authentic data, then this was not in sync with an authentic reference MCU.
More precisely:
spoof(m, n, response) =⇒ ¬sync(m, n +o, response)
Remark 2. If Objectives 1 are satisfied for parameters (c, o), the implication
sync(m, n, response) & mn>c+o=auth responder(response)
holds with probability greater 1 pth.
4.2 Rationale
By the following theorem, the four objectives are sufficient for a MIGA imple-
mentation if the minimal samples count Nis big enough. The latter only depends
on the maximal assumed attack potential (px,py).
Theorem 1. Let (LCU,pth)be a field of application, prog an SBST invariant
of group(M, model,id)with minimal samples count Nand mesh width d, and
tlim a response time limit. Assume that for natural numbers c, o, the sum c+o
is a multiple of dand that Objectives 1 are satisfied. Assume a maximal attack
potential (px,py)for malicious devices in LCU. Set p:= max{px(c+d+o)/d ,py}
and let Zbe the smallest integer with pZpth. If N > (c+d+o)·(Z+ 1),
then the probability for (2) is smaller pth, i.e. (prog,tlim)is an MCU intrinsic
group authenticator.
Proof. We have to show that the probability for (2) under the condition of a
malicious responder is smaller equal pth. By implication (3), the assumption
(2) implies recovery(response). By Axiom 3 and Remark 2 we can assume an
attack strategy (4) with ni+1 nic+ofor even i. By Definition 1, we get
X(NX
ieven
(ni+1 ni+d)) div d
(N(Y+ 1)(c+d+o)) div d
>(ZY)(c+d+o)/d
In consequence, the probability for the attack is limited by:
pxX·pyY<px(ZY)(c+d+o)/d ·pyY
pZY·pYpZpth.
ut
5 Proof of concept
A proof of concept is provided in the reference [8]. This section briefly summarizes
its results.
5.1 Platform
The reference specifies an SBST design, and describes an example implemen-
tation for an MCU intrinsic group with the Holtek HT32F52 as MCU model,
and the default “custom ID” 0xFF...FF as group identifier. The runtime for the
example implementation is 30ms for a clock rate of 8MHz.
5.2 Reliability
The reference describes reliability tests performed with six HT32 starter kits
as test boards. The testing covered (a) repeated power-on-power-off cycles, (b)
voltage variations (2V-6V), and (c) temperature variations (20C120C). At
least 10 mio. challenge-response-tests were executed per sample. In none of the
tests a mismatch between a response and a reference response was observed,
except the case of missing responses due to a complete failure of the MCU
functionality, e.g. when VDD supply voltage was smaller 2V3 or greater 5V2.
Equivalent test observations were made for a second implementation on an
Atmel SAM3x8e (Arduino Due) with Cortex-M3 processor. We consider these
observations as promising. Further tests are work in progress. Before relying on
a MIGA scheme for a dedicated MCU model, reliability testing on a larger scale
is mandatory.
5.3 Security
The reference assumes an attack potential of (px,py) = (0.9,0.1), and requires
a maximal rate pth = 0.001 of false positives. The reference proves that the ex-
ample implementation satisfies Objectives 1 with coverage parameter c= 1548,
feedback parameter o= 220, mesh width d= 52, and minimal samples count
N= 7280. It follows from Theorem 1 that the example implementation is an
MCU intrinsic group authenticator for the group of all HT32F52 samples with
default custom ID.
6 Application to car components
In this Section, we want to show that the MIGA scheme is applicable to the
automotive use case sketched in the introduction.
6.1 Challenger application
A security control unit SCU will act as the challenger in CRTs with the to-be-
authenticated ECUs as responders. The challenger maintains for each responder
an individual set of CRPs consisting of a challenge and reference response, and
a response time limit. For the authentication, the challenger sends an individual
challenge to each of the responders, collects the responses, and measures the
response times. If a response time exceeds the response time limit, the response
is withdrawn. Otherwise, an authentication passes if the response matches its
reference value.
Since the to-be-authenticated ECUs can compute responses in parallel, and
the response time is about 30ms for each ECU, the time for the authentication
of multiple ECUs is only limited by the bandwidth of the on-board network.
6.2 Back-end
The implementer can choose a strategy for the reuse of CRPs, or for “offline
CRP updates”, where an already authenticated ECU is misused as response
generator for future CRTs, see [12]. Nevertheless, the SCU might need to update
sets of CRPs sometimes. This requires a CRP update server in a back-end of the
manufacturer. CRP update requests can be sent over-the-air, if applicable, or by
a diagnosis tool in the garage. The garage update is applicable in particular after
replacing an ECU with an original spare part, and the SCU is not yet equipped
with CRPs for the new ECU model.
The CRP update server has a random number generator for the generation
of uniformly distributed challenges, and reference MCUs for generating reference
responses for each ECU model in the field. It serves its client SCUs with CRPs
upon request. Each CRP must only be sent once to a client.
MIGA and symmetric key authentication can be mixed without any overhead
in the SCU software. In this case, the back-end is also used for the generation of
CRPs for ECUs with a symmetric group key as authentication factor. This only
requires reference MCUs with valid group key in the back-end.
7 Pros and cons
This section describes advantages and disadvantages of the MIGA scheme with
respect to the state of the art.
7.1 Compared to key based authentication
An obvious advantage of the MIGA scheme is that the costs per piece are sig-
nificantly smaller compared to secure hardware elements.
As a counterfeit prevention, key based authentication is broken as soon as
a single key is disclosed. An attacker can buy and analyze authentic ECUs in
his laboratory in order to disclose an authentication key with methods including
DPA and DFA and probing. To achieve a suitable protection level, a certified
secure element is mandatory. But even certified secure elements are sometimes
vulnerable [13].
The advantage of the MIGA scheme is that no secret key is required, thus
there is no risk of a key disclosure. As shown in the present article, it achieves
nevertheless a high security level for the prevention of ECU counterfeits. Note
that the MIGA scheme does not provide data encryption.
7.2 Compared to PUFs
The advantage of the MIGA scheme with respect to non-cryptographic PUF
solutions [14], [15] is that it is much more practical due to the fact that it uses
a group authentication feature. A CRP can be used to authenticate any ECU of
the given product model. A responder ECU can be replaced by an ECU of the
same product model without the need of a CRP update. Non-cryptographic PUF
solutions require responder individual CRPs. The pre-generation and storage of
device individual CRPs in a back-end is very complex. Non-cryptographic PUF
solutions are not very common on the market. Available PUF solutions use a
weak PUF, e.g. an SRAM-PUF. Weak PUFs have too little entropy to serve for
challenge-response-testing. They are used instead as a secure key storage. Since
keys can be attacked not only at their memory location but also during crypto
operations, weak PUFs do not make secure hardware elements redundant.
7.3 Counterfeit MCUs
A disadvantage of the MIGA scheme is that it does not apply if a counterfeit
ECU manufacturer can produce or purchase MCU counterfeits behavioral equiv-
alent to the the authentic MCU model, and with the authentic group identifier.
This would apply to (1) overproduced MCUs of the original model on behalf
of the counterfeit ECU manufacturer, (2) cloned MCUs based on illegal copies
of the authentic design data, (3) cloned MCUs based on layer-by-layer reverse
engineering of the original MCU. We consider recycled MCUs of the authentic
model as a minor problem. Recycled MCUs will not be available on a large scale
with the valid group identifier.
The described disadvantage will decrease in the future due to advances in
MCU cloning prevention technologies, such as hardware obfuscation with key
based locking [16].
7.4 Combination of functional testing and authentication
An advantage of the MIGA approach is the possible combination of SBST driven
by functional safety requirements and authentication. Due to the coverage ob-
jective, an SBST implemented for the MIGA scheme already provides evidence
that the MCU is working properly. Bad responses in the MIGA scheme should
always be considered as a possible safety issue in safety-critical applications.
7.5 Crypto agility
The MIGA authentication permits the challenger device to choose the security
level dynamically. This only requires that the samples count Nis chosen by the
challenger (as a challenge parameter). The challenger could decide to choose
a small Nas long as no counterfeit ECUs were observed on the market, and
increase Nin dependency of an increased attack potential (px,py) of counterfeit
manufacturers. The suitable Nis provided by Theorem 1.
8 Conclusions
We introduced MIGA as a time bounded authentication scheme based on hard-
ware intrinsic characteristic group features of MCUs of the same model, and
with a common group identifier. It is based on the fact that an MCU is a com-
plex dynamical system, and hard to emulate in a cycle-accurate way. The MIGA
scheme is optimized for counterfeit prevention. To our knowledge, it is the first
authentication method based on digital hardware intrinsic group features.
We provided a generic attacker model, and defined four security objectives
on a MIGA implementation in software. We proved the security of the MIGA
scheme provided that the four security objectives are satisfied. We referred to
[8] for a practical implementation and the proof that the four security objectives
can be achieved for a specific MCU model. In particular, the reference provides
promising results on the reliability of the MIGA scheme. We indicated how
the authentication scheme can be applied as an anti-counterfeiting solution to
automotive ECUs.
9 Future work
Planned work items cover: (1) Large scale reliability tests with the support of
an MCU manufacturer. (2) Testing if MIGA can help in detecting some class
of MCU counterfeits. (3) Prove the security of authentication with “relaxed
freshness” similar to the result in [12] for an embedded system with multiple to-
be-authenticated device components. (4) Build a demonstrator for a complete on-
board network where MIGA and key based authentication are mixed. (5) Publish
a MIGA implementation, and organize a hacker competition for circumventing
MIGA with a low resource control unit.
References
1. “Requirements to perform integrated circuit evaluations,” May 2013. https://
www.commoncriteriaportal.org/files/supdocs/CCDB-2013-05-001.pdf.
2. Michigan State University, “Defining the threat of product counterfeiting,”
2019. https://www.michiganstateuniversityonline.com/resources/acapp/
threat-of-product-counterfeiting/.
3. T. McGrath, I. E. Bagci, Z. M. Wang, U. Roedig, and R. J. Young, “A PUF
taxonomy,” Applied Physics Reviews, vol. 6, no. 1, p. 011303, 2019.
4. N. Beckmann and M. Potkonjak, “Hardware-based public-key cryptography with
public physically unclonable functions,” Information Hiding, Springer-Verlag,
p. 206-220, 2009.
5. M. Majzoobi and F. Koushanfar, “Time-bounded authentication of FPGAs,” IEEE
Transactions on Information Forensics and Security, vol. 6, no. 3, pp. 1123–1135,
2011.
6. U. R¨uhrmair, “SIMPL systems as a keyless cryptographic and security primitive,”
D. Naccache (Editor), Cryptography and Security: From Theory to Applications -
Essays Dedicated to Jean-Jacques Quisquater on the Occasion of His 65th Birthday.
Lecture Notes in Computer Science, Vol. 6805, Springer, 2012.
7. M. Psarakis, D. Gizopoulos, E. Sanchez, and M. Sonza Reorda, “Microprocessor
software-based self-testing,” IEEE Design Test of Computers, vol. 27, no. 3, pp. 4–
19, 2010.
8. F. Schuhmacher, “A MIGA design and example implementation,” 2020.
https://www.researchgate.net/publication/344257849_A_MIGA_design_and_
example_implementation.
9. F. Reimann, M. Glaß, J. Teich, A. Cook, L. R. G´omez, D. Ull, H. Wunderlich,
U. Abelein, and P. Engelke, “Advanced diagnosis: Sbst and bist integration in au-
tomotive e/e architectures,” in 2014 51st ACM/EDAC/IEEE Design Automation
Conference (DAC), pp. 1–6, 2014.
10. J. Bauer and F. Freiling, “Towards cycle-accurate emulation of cortex-M code
to detect timing side channels,” 11th International Conference on Availability,
Reliability and Security, IEEE, 2016.
11. M. Reshadi and N. Dutt, “Generic pipelined processor modeling and high per-
formance cycle-accurate simulator generation,” Proceedings of the Conference on
Design, Automation and Test in Europe—Volume 2, Washington, DC, USA: IEEE
Computer Society, p. 786–791, 2005.
12. F. Schuhmacher, “Relaxed freshness in component authentication,” 2020. https:
//eprint.iacr.org/2020/106.
13. M. Wagner and S. Heyse, “Single–trace template attack on the DES round keys of
a recent smart card,” Cryptology ePrint Archive, Report 2017/057, 2017. https:
//eprint.iacr.org/2017/057.
14. R. S. Pappu, “Physical one-way functions,” PhD thesis, MIT, 2001.
15. S. Devadas, G. E. Suh, S. Paral, R. Sowell, T. Ziola, and V. Khandelwal, “Design
and implementation of PUF-based “unclonable” RFID ICS for anti-counterfeiting
and security applications,” International Conference on RFID, pages 58–64, 2008.
16. S. Amir, B. Shakya, D. Forte, M. Tehranipoor, and S. Bhunia, “Comparative anal-
ysis of hardware obfuscation for ip protection,” in Proceedings of the on Great Lakes
Symposium on VLSI 2017, GLSVLSI ’17, (New York, NY, USA), p. 363–368, As-
sociation for Computing Machinery, 2017.
ResearchGate has not been able to resolve any citations for this publication.
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Cryptography and Security: From Theory to Applications -Essays Dedicated to Jean-Jacques Quisquater on the Occasion of His 65th Birthday
  • U Rührmair
U. Rührmair, "SIMPL systems as a keyless cryptographic and security primitive," D. Naccache (Editor), Cryptography and Security: From Theory to Applications -Essays Dedicated to Jean-Jacques Quisquater on the Occasion of His 65th Birthday. Lecture Notes in Computer Science, Vol. 6805, Springer, 2012.
A MIGA design and example implementation
  • F Schuhmacher
F. Schuhmacher, "A MIGA design and example implementation," 2020. https://www.researchgate.net/publication/344257849_A_MIGA_design_and_ example_implementation.