Camouflage is a defense and survival strategy that organisms use to disguise their appearance and to blend in with their habitat, such that they can hide from predators and sneak up on prey. For example, a stick insect camouflages by having its appearance resemble a tree branch. The arctic hare grows grey fur in the summer, but their fur is white in winter to blend in with snow.

In communication systems, camouflage of information is done via steganography or stealth transmission, which is a
technology to conceal the presence of a secret message in publicly available media, such that the eavesdropper cannot tell if there is a secret message to look for. Steganography [1] has a long history dating back to the Greeks. The word steganography has two Greek roots: “stegos” means “cover” and “grafia” means “write.” Secret messages were written on a wooden folding table and then covered over with wax to make the table appear to be blank. Furthermore, Histiaeus, a ruler in ancient Greece, shaved the head of a messenger and wrote a secret message on it. Once the messenger’s hair grew back, the messenger was dispatched with the message. During the High Renaissance,
Leonardo Da Vinci hid secret messages in his paintings, including the famous Mona Lisa and the Last Supper.

In modern society, communication networks are an inseparable part of human society and are responsible for supporting applications including telemedicine, online banking, and augmented reality learning [2], [3]. Needless to say, it is essential to protect the massive amount of sensitive and personal information against attackers. Although encryption is usually performed at the media access control (MAC) layer or above, the physical layer is one of the most vulnerable places to attack that usually could result in total exposure.


Figure 1. Effective cryptography requires both encryption
and steganography. It is like storing valuables in a locked safe
(encryption) hidden behind a secret bookcase door (steganography).


Physical encryption techniques have been studied intensively to secure the physical layer [4]–[6]; however, physical
layer steganography [7] has always been overlooked and its development is still lagging behind. Effective cryptography
requires two major components: encryption and steganography. Encryption scrambles the sensitive information so that it is unreadable without the key, while steganography hides the sensitive information within ordinary information so that the attacker will not even know there is a signal to look for. It is like storing valuables in a locked safe (encryption) hidden behind a secret bookcase door (steganography)—Figure 1.

Turning to nature for a solution, the marine hatchetfish (Figure 2) is an expert at hiding its appearance in the deep ocean. No matter if you are the predator that is swimming next to it or the prey that is wandering underneath it, the hatchetfish is invisible to you. The marine hatchetfish does not have an invisibility cloak or a computer to change the bits of its image. Instead, its body performs important camouflage strategies—silvering and counter-illumination—that conceal the appearance of the hatchetfish in all directions.

How could the marine hatchetfish’s camouflage ability help with security in our communication systems? If we could borrow both the ideas of silvering and counter-illumination and implement them in our frequency bands, then we could facilitate a secure physical transmission medium that supports stealth transmission in all domains, making the stealth information disappear from the attacker’s eyes.

Marine Hatchetfish Camouflage Strategies
Among the different types of camouflage, underwater camouflage is powerful because of the multi-dimensional concealment it can achieve. Underwater camouflage helps sea animals to hide from predators from above the water, to appear invisible from its sides, and to remove its dark appearance when seen from below. The marine hatchetfish’s [8] skin has microstructure features that reflect light and create constructive interference only at the color of the surrounding medium—a color that does not reveal the presence of the fish. In other words, any colors that could reveal the presence of the fish are under destructive interference. This camouflage strategy is called silvering. Furthermore, the marine hatchetfish also has a line of photophores that produces and directs light to the bottom part of its body. The illumination of the fish matches the color and intensity of its surroundings as seen from below,
essentially removing any darkness due to the blockage of light from above, which is referred to as counter-llumination.
Therefore, the camouflage strategies of the marine hatchetfish provide it with multi-dimensional invisibility, concealment, and protection. If we are able to learn from the marine hatchetfish, borrow their camouflage strategies, and transform it into a steganography scheme for our communication channels, an effective multi-domain concealment of sensitive information could be achieved.

From Marine Hatchetfish to Optical RF Steganography
The fiber optic network is the backbone of most communication systems, supporting radio-over-fiber transmission of mobile radio frequency signals (i.e. 5G and beyond). Both silvering and counter-illumination would be the perfect solutions for providing multi-domain steganography to the sensitive information in the fiber optics network. Photonics is well-known for its wideband, flexible, and dynamic properties, making it a promising candidate for implementing the bio-inspired steganography in fiber transmission.

First, silvering—constructive interference occurring at the surroundings’ color and destructive interference occurring at
the fish’s color due to the microstructured skin of the marine hatchetfish—can be achieved using an optical finite impulse response (FIR) structure. In FIR, multiple copies of the signal are created, weighted, and delayed, such that interference of the signal can be set to be constructive or destructive at a designed frequency depending on the FIR parameters. To achieve steganography, the stealth signal frequency should be set to interfere destructively in the attacker’s view at any point of the transmission. Next, counter-illumination—color and intensity matched illumination—can be achieved using a wideband noise-like optical carrier for the stealth signal so that its intensity and spectral distribution is similar to the inherent system noise during transmission. Unlike most existing optical steganography
schemes, this bio-inspired steganography scheme does not just bury the stealth signal underneath system noise,
but it also uses destructive interference at the stealth signal frequency to make the signal disappear in the attacker’s eyes.


Figure 2. Image of a Marine Hatchetfish (Wikipedia).

Figure 4 shows the detailed design of the marine hatchetfish- inspired optical steganography scheme [9]. To achieve
counter-illumination, a wideband optical noise source is used as the optical carrier for the stealth signal, which could be amplified spontaneous emission (ASE) noise. The low-intensity, wideband noise source has the same spectral characteristic that “illuminates” at the same wavelength and intensity as the system background noise, such that no distinct optical spectral component can be observed, mimicking counter-illumination in the marine hatchetfish. To achieve silvering, the broadband optical noise source is spectrally sliced into a comb to achieve a desired photonic FIR in the RF domain when passing through a negative dispersive media (DCF1) at the stealth transmitter. The constructive interference (fc) is set to be way above the frequency range of interest that will hide the presence of the stealth signal (Figure 4(i)). The stealth signal frequency (fs) experiences destructive interference in the attacker’s view at any point of the transmission, hiding the stealth signal in both the RF spectral domain and the temporal domain. Before the stealth signal is transmitted, it is combined with the public channel(s) and broadband optical noise from signal amplification. With silvering and counter-illumination, the stealth signal is concealed in the optical spectral, RF spectral, and temporal domains—which are all the possible domains that an eavesdropper could be listening to.