## 1 Introduction  
The security and privacy of information is one of the most critical issue of today's fast moving world. Innovative and novel techniques are being explored to keep information safe from intruders [1]. The demand of secretly transmitting data in underwater acoustic channels is rapidly increasing in oceanography, military and naval operations [2; 3]. Mariners desire to transmit their information clandestinely to secure it from data theft and malicious attacks [4]. Military and naval personnel need to hide their communication to prevent the leakage of such information to the enemy [5]. Covert Underwater Acoustic Communication (CUAC) is the key research area where information is transmitted furutively in an open underwater acoustic environment [6]. It is the concept of communicating secretly in an underwater acoustic channel. An underwater channel is considered as the preeminent medium for acoustic communication as sound transmitsfardistances [7; 8]. The key challenge is to keep acoustic information safein the far flung sea. Variety of ways have been explored by scientists which will be discussed in detail. CUAC is classified into two major constraints as Low Probability of Detection (LPD) and Low Probability of Interception (LPI). LPD means that communication signals have a less chance ofdetection by eavesdroppers however LPI defines that communication signals can be detected by the intruder but that the enemy has a lesser chance ofextracting such information [9]. History of covert communication goes back a long way with the invention of communication; as a user needs to hide sensitive information from the enemy [10]. During World War I and II, Morse codes were extensively used for transmitting secret messages [11]. It consists of two elements termed as a dot and dash. The American Standard Code for Information Interchange (ASCII) characters are represented by a combination of these elements. Covert communication is attained by the coded combinations of Morse elements. The enemy can detect the communication signal but has less of a chance to decode the information. Perfect LPI constraint communication is achieved in underwater telephone and telegraph systems using Morse coding. The drawback of this technology is that the enemy can detect the existence of the communication and could try to decode the coded format. In an underwater acoustic channel, earliest LPD constraint covert communication with best of our knowledge was conducted back in 1986 by [PERSON] by spreading the waveform [12]. Until the last decade, CUAC was established by reducing the SNR and spreading the waveform through Direct Sequence Spread Spectrum (DSSS) [13], Orthogonal Frequency Division Multiplexing (OFDM) [14] and other modulation techniques. It was realized however that as the enemy approaches towards the transmitter the LPD constraint communication signal is perceived by the eavesdropper as the SNR is increased by limiting the distance [15]. It is similar to the situation of two people having a secret talk and when a third person approaches them, their communication doesnot remain covert. Scientists have given a lot of contributions to increase the covert distance but it cannot be true for all regions [15; 16; 17]. This technique gives perfect CUAC for a bounded region but it doesnot remain covert for all communication ranges. To trounce back above limitation, biological inspired mimicry CUAC was introduced. It is an entirely different approach where the communication signal is made akin to the natural sounds of the sea [18]. A large variety of natural noises are present in the ocean such as shipping noises, cetacean vocals, shrimp noises, wind noises, sea piling noises, etc. The natural ocean noise is mimicked by modulating or watermarking the covert information in it. The mimicked communication signal is detected by the enemy but it is excluded from the process of recognition due to its identicalness with the natural sound. The intruder assumes it as the sea natural noise due to its characteristics.