IEEE Consumer Electronics Magazine - November/December 2019 - 100

Special Section on Emerging Paradigms in Vehicular Cybersecurity

as a type of hash function in which a given input
will result in a specific output. In PUFs, inputs
are known as "challenges" and outputs are
"responses." Collectively, a challenge and its associated response are known as a challenge
response pair (CRP). A PUF is considered a
strong PUF if it has a large number of CRPs. Ideally, it would have an exponential number of
CRPs so that an n-bit strong PUF would have 2n
CRPs. Conversely, a weak PUF has a very limited
number of CRPs (typically just one).2
PUFs are designed to have a uniqueness
property where each copy of the PUF will
have a unique response for a given challenge.
These inherent properties of PUFs make them
attractive for use in vehicle security applications. An attacker would be forced to obtain
the actual PUF itself if he or she wanted to
use it in an attack as it would be impossible
to create an exact copy of the desired PUF.
Researchers have explored incorporating
PUFs into such areas as key storage and generation, signature creation, and authenticity
verification.

Security Modules
Trusted platform modules (TPM) are cryptographic co-processors designed to integrate
security into larger computer systems. TPMs are
physically wired into the system and are not
removable. The specifications for a TPM were
standardized by ISO/IEC 11889. The current standard requires AES-128 for symmetric cryptography, RSA and ECC algorithms for asymmetric
cryptography, and the SHA-1 and SHA-256 hashing functions in addition to other hardware features such as a random number generator and
secure nonvolatile storage.
The TPM specification was designed for a
generic computing system. Directly integrating
TPMs and vehicles is difficult due to the limited
computing resources and real-time constraints
that are uniquely present in vehicles. Instead,
researchers have used the TPM specification as
a blueprint of sorts to design vehicular hardware security modules (HSMs). These modules
can be easily integrated into vehicles and tend
to only include some of the cryptographic
components that are required to be included
in TPMs.

100

HARDWARE MODULES
Vehicular security depends on the creation of
security modules that can be introduced to the
unsecured buses found in vehicles.1 These modules attempt to implement a measure of security
by providing a secure hardware device for secure
communication between nodes in both internal
and external vehicular communication networks
without incurring significant costs. The unique
security challenges facing vehicles prevent the
direct adaptation of existing TPMs from normal
computers to vehicles. For this reason, HSMs have
been proposed. HSMs are specially designed so
that the modules may be effectively integrated into
vehicular communication networks. However, this
does not mean that vehicular TPMs will be unique
from existing implementations. Both existing TPMs
and their proposed vehicular implementations
tend to share multiple hardware components such
as storage mediums and crypto engines.

Hardware Security Modules
Researchers from the E-Safety Vehicle Intrusion Protected Applications (EVITA) project have
previously proposed a vehicular HSM.3 Their
module is a general-purpose cryptographic coprocessor designed to be connected to an in-vehicle communication network and comes in three
variations: "full," "medium," and "light." The "full"
version has everything needed to secure vehicleto-everything communications. The "medium"
variation is focused only on the security of communication within the vehicle. The "light" variation is designed to only secure interactions
between ECUs and the sensors and actuators.
The module uses ECC-256 for asymmetric
cryptography, AES-128 for symmetric encryption
and decryption, and WHIRLPOOL as its hash
function. The module also includes a pseudorandom number generator, unsecured RAM, and
nonvolatile memory (NVM). These features are
kept behind a so-called cryptographic boundary
that is separate from the actual application core
that communicates with other ECUs attached to
the in-vehicle communication system.
This design has since become an inspiration
for more recent vehicular HSM implementations.
For example, the HSM included in NXP Semiconductor's MPC5748x family of automotive microcontrollers is a direct implementation of the

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IEEE Consumer Electronics Magazine - November/December 2019

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