Signal Processing - September 2017 - 30
about their antennas' phase centers and the target antenna's
phase center, thus being able to defeat complex countermeasures, as those based on the angle-of-arrival estimation.
With these definitions in mind, Table 1 is helpful to benchmark the attacks against one another or rank them with respect to
their cost and complexity, as in these illustrative examples:
1) Typically, meaconing and spoofing attacks requiring multiple high-gain antennas are expensive, according to all of
the three cost items. Apart from the high costs related to
the high-gain antennas themselves, they require a complex
set-up, only possible if an elevated expertise is available. In
addition, the cost of operating the attack is high, due to the
complexity associated with its actual implementation.
2) A further meaningful example can be done through the
comparison between intermediate spoofing and intermediate self-spoofing: the costs of HW and required expertise
are obviously the same for the two types of attacks, but the
connivance of the victim eases the complexity of operation.
3) Among the cited attacks, meaconing is considered the least
expensive one: the costs associated with the HW are limited
since it requires few RF components, the required expertise
to build the attack is moderate, and the costs associated to its
implementation are very low. Two of these cost items slightly increase in the case of meaconing with variable delay. In
Table 1. A list of meaconing and spoofing attacks
with associated cost items.
Cost/Complexity at the Attacker Side
List of Meaconing and
Spoofing Attacks
Developing/
Buying HW
Expertise
Required
Complexity of
Operation
Meaconing
VL
M
VL
Meaconing with variable
delay
L
M
L
Meaconing with modem
L
M
M
Simplistic spoofing-
custom low-cost HW
L
H
M
Simplistic spoofing-
commercial HW
simulator
H
M
M
Intermediate self-spoofing
M
H
M
Intermediate spoofing
M
H
H
Estimation and replay
M
VH
H
FEA
M
VH
H
Meaconing with multiple
receiver antennas
M
VH
H
Meaconing/spoofing
with multiple transceiver
antennas
M
VH
H
Meaconing/spoofing
with high-gain antennas
VH
H
H
Nulling attack
M
VH
VH
Sophisticated spoofing
VH
VH
VH
VH: very high, H: high, M: medium, L: low, VL: very low.
30
particular, there is an additional cost associated with the
HW used to control the signal delay and an increased complexity associated with its implementation.
Also, notice that emerging needs and technological trends
continue to raise the bar in terms of feasible attacks and possible
countermeasures. Depending on the specific application requirements, some future GNSS receivers may need to be protected
from these attacks as well as from their possible combinations.
For the sake of brevity, Table 1 only lists each attack separately.
Obviously, the combination of two or more attacks potentially
increases their separate effectiveness, at the cost of a higher level
of complexity.
Cryptographic defenses for civil GNSS receivers
A first macroclassification for possible countermeasures,
understood in general as techniques to protect against the previously mentioned attacks, divides them in two main classes,
cryptographic and noncryptographic defenses [12]. The former class takes advantage of cryptographic features added to
GNSS signals, as encryption, digital signatures, or other solutions borrowed from cryptography. The latter class includes
some common antispoofing methods suitable to standalone
GNSS receivers (e.g., consistency checks on different measurements, spatial processing with multiple antennas, signal
quality monitoring techniques, etc.) and also some methods
leveraging the presence of external sensors and complementary technologies to GNSS (i.e., hybrid positioning techniques,
such as the integration with inertial systems or wireless communications) [3]. A general summary of GNSS cryptographic
and noncryptographic defenses can be found in [9], while the
rest of the article will focus on cryptographic defenses only.
The core objective of cryptographic solutions is to ensure
information security, adding signal features useful to authenticate the received signals and the information they provide
and to make it difficult for an attacker to alter them. A simple
definition for signal authentication is given in [12]: "GNSS
signal authentication means certifying that a received signal is
not counterfeit, that it originates from a GNSS satellite and not
a spoofer." Cryptographic defenses relate to specific features
of the GNSS signals, as designed by the GNSS provider, and
to how these features are treated at the receiver. According to
the conventional nomenclature adopted in cryptography [23],
cryptographic functions provide the following services:
■ Authentication: verifying the authenticity of information
and of the entity transmitting it (i.e., entity authentication
and data origin authentication)
■ Confidentiality: protecting the information from disclosure
to nonauthorized parties
■ Integrity of information: allowing the detection of any
unauthorized information modification
■ Nonrepudiation: preventing an originator of information
from denying being the source of that information (e.g., one
entity may authorize a purchase of property by another entity and later deny such authorization was granted [23]).
In GNSSs, the first three services are especially relevant to increase the robustness against spoofing. In detail,
IEEE SIGNAL PROCESSING MAGAZINE
|
September 2017
|
Table of Contents for the Digital Edition of Signal Processing - September 2017
Signal Processing - September 2017 - Cover1
Signal Processing - September 2017 - Cover2
Signal Processing - September 2017 - 1
Signal Processing - September 2017 - 2
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Signal Processing - September 2017 - Cover3
Signal Processing - September 2017 - Cover4
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