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Optical Layer Security in Fiber-Optic Networks
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INTRODUCTION

OPTICAL communication systems have foundwidespread
adoption in a variety of applications, ranging from personal
to commercial to military communications. Due to the
dramatic increase in network usage and the increased accessibility
of optical networks, it is important that communications
crossing these networks are properly secured. As with any other
type of network, the first line for securing communications starts
with employing cryptographic protocols at higher layers of the
protocol stack. However, building security on top of an insecure
foundation is a risky practice, and for this reason it is desirable
to make certain that the physical layer of an optical system



Confidentiality
Although optical networks do not emit an electromagnetic
signature, an attacker can eavesdrop on an optical system using
a variety of approaches, including physically tapping into the
optical fiber [14], or by listening to the residual crosstalk from
an adjacent channel while impersonating a legitimate subscriber
[15]. Tapping optical fiber is not difficult if the fiber itself is exposed
and without physical protection. For example, fiber can
be tapped by peeling off the protective material and cladding of
the fiber, so that a small portion of the light escapes from the
optical fiber. By placing a second fiber directly adjacent to the
place where light escapes from the first fiber, it is possible to
capture a small amount of the desired optical signal.


Authentication
Authentication requires the use of a unique coding/decoding
scheme between the desired users. The coding scheme forms
an identity for the user. In the physical optical link, an optical
signal travels freely in the network and can reach any destination
as long as it has the correct wavelength (for a WDM network),
or a correct temporal synchronization (for a time-division-multiplexing
(TDM) network). With an OCDMA coding/decoding
scheme, a certain level of authentication can be achieved by
using a unique OCDMA code that is agreed upon by the sender
and designated recipient. Without knowledge of that code, an
unauthorized user cannot decode the OCDMA signal in the
presence of other OCDMA traffic. In other words, in addition to
providing multiaccess capability, OCDMA codes also provide
a means for authentication between two users.


Availability
Optical networks are susceptible to a variety of attacks on
their physical infrastructure as well as signal jamming attacks
[18]. The net result in either case can be a denial of service.
Although denial of service does not necessarily result in the theft
of information, it can translate into loss of network resources
(such as bandwidth), impact many users, and could result in
significant fiscal losses to the network provider.

Optical Layer Security in Fiber-Optic Networks

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Abstract

The physical layer of an optical network is vulnerable
to a variety of attacks, including jamming, physical infrastructure
attacks, eavesdropping, and interception. As the demand for network
capacity grows dramatically, the issue of securing the physical
layer of optical network cannot be overlooked. In this survey
paper, we discuss the security threats in an optical network as well
as present several existing optical techniques to improve the security.
In the first part of this paper, we discuss various types of
security threats that could appear in the optical layer of an optical
network, including jamming, physical infrastructure attacks,
eavesdropping, and interception. Intensive research has focused
on improving optical network security, in the above specific areas.
Real-time processing of the optical signal is essential in order to
integrate security functionality at the physical layer while not undermining
the true value of optical communications, which is its
speed.

INTRODUCTION

Due to the dramatic increase in network usage and the increased accessibility of optical networks, it is important that communications
crossing these networks are properly secured. As with any other
type of network, the first line for securing communications starts
with employing cryptographic protocols at higher layers of the
protocol stack. However, building security on top of an insecure
foundation is a risky practice, and for this reason it is desirable
to make certain that the physical layer of an optical system
(which we shall refer to as the optical layer in this paper) ismade
secure against threats that might target .

THREATS AND DEFENSES IN OPTICAL NETWORKS AT THE
OPTICAL LAYER

There are many types of optical networks, ranging from local
area networks to optical networks that form the backbone of the
Internet. For each of these networks, the actual implementation
of a particular type of threatmay vary. However, in spite of these
many different modalities, the threat categories can loosely be
categorized as threats where an adversary tries to listen in on
communications (confidentiality), where an unauthorized entity
tries to communicate (authentication), where an entity alters
or manipulates communication (integrity), where an adversary
tries to subvert the successful delivery of communications
(availability), and privacy risks associated with an adversary observing
the existence of communications (privacy and traffic
analysis). In the remainder of this section, we quickly survey
confidentiality, authentication, privacy, and availability threats
and solutions at the optical layer.

OPTICAL LAYER SECURITY: CONFIDENTIALITY

Optical Encryption

In encryption, the data cannot be recovered from the ciphertext
by an eavesdropper without knowledge of the encryption
key. Thismakes encryption an effectiveway of securing a signal
and enhancing the confidentiality of a network. There has been
considerable effort to develop optical architectures for implementing
fast encryption functions in the optical domain. One
motivation for such work is that optical processing can operate
at data rates far in excess of what is capable with electronic
components. Further, optical components do not have electromagnetic
emissions that are observable from a distance, and
hence pose less side-channel risk than their electrical counterparts.
As an example, the investigation of optical XOR logic
has been carried out by several researchers as a starting point
for building optical encryption algorithms. The resulting optical
XOR gates do not have electromagnetic signatures that can
be monitored by an eavesdropper. Optical XOR gates have been
proposed and demonstrated using various techniques.

OPTICAL LAYER SECURITY: AVAILABILITY

Survivable Ring

To provide high survivability and ensure service availability,
self-healing ring architectures are a good candidate compared
to other architectures [19]. As discussed in Section IV, the large
code cardinality of OCDMA not only increases the difficulty in
channel-detection by brute-force, it also enhances service availability
while minimizing the use of bandwidth. Thus, the use
of an OCDMA-based backup channel to implement a bandwidth-
efficient bidirectional OCDMA ring network has been
proposed [13]. With large cardinality, a survivable ring network
can be built such that there is no need to reserve separate
bandwidth or a separate path for link failure. Conventional
backup paths require the permanent reservation of all or part
of their bandwidth. The reserved bandwidth is wasted unless
failure occurs.One unique characteristic of incoherent OCDMA
networks is “soft blocking” [1].

PRINCIPLE OF OPTICAL LAYER SECURITY: PRIVACY

Steganography is one way to improve the privacy of a signal
by hiding the stealth signal underneath the public transmission
and noise level. Although steganography does not completely
ensure signal privacy, it does provide it with an additional layer
of protection. Optical steganography was first proposed by
Wu et al. [2] and the performance of the stealth channel was
theoretically analyzed. [54], [55]. Experimental investigations
of optical steganography illustrate that optical steganography
has good compatibility with various types of public channels.
Examples include transmitted SPE encoded stealth signal in an
RZ-OOK public channel [56], RZ-OOK stealth signal under
a NRZ-OOK public channel [57], WHTS encoded OCDMA
stealth signal through another WHTS public channel [58], and
RZ-OOK stealth signal transmission through a NRZ-DPSK
public channel [59]. Optical steganography is particularly suitable
where the signals are not filtered or digitally regenerated
at nodes, which is the case of many of today’s passive optical
networks (e.g., FIOS).

SUMMARY AND DISCUSSION

In this survey paper, we discuss the vulnerability of optical
networks towards various types of security threats that
could appear in the optical layer of a network, and present an
overview of various optical techniques for defending against
the corresponding security threats. With the use of optical techniques,
real-time signal processing is realized to improve the
security of optical networks. In this paper, we discussed optical
encryption to enhance confidentiality at line rates, while posing
less side-channel risk than its electrical counterparts. Various
types of optical XOR gates with and without feedback have been
built experimentally. These techniques enable the generation
of long key streams from smaller keys or for processing registers
used in the process of encipherment by Vernam ciphers
to enable a secure optical encryption.