# On IRS-Assisted Covert Communication with a Friendly UAV
## 1 Introduction  
Traditional cryptography mechanisms protect sensitive data by the encryption of messages into cipher text, thereby preventing access by unauthenticated users. However, cipher texts generated with cryptography mechanisms exhibit a high degree of randomization, which can easily arouse the suspicion of adversaries (e.g., eavesdroppers, hackers, crackers, etc.) [1]. In this case, more powerful decryption methods and mechanisms may be employed to crack randomized data streams, which can severely threaten the security of the information protected by the cryptography regime [2]. In this scenario, covert communication which can be used to transmit sensitive information without being perceived attacks tremendous attention [3]. In general, a transmitter can use covert communication to transmit sensitive data to legal receivers without the communication process being detected by malicious adversaries, which can provide a higher level of security to transmitters [4]. In covert communication systems, covert information can be carried by normal messages and transmitted together with these normal messages to avoid being perceived by adversaries [5]. Recently, covert communications have been intensively investigated in different scenarios. As an example, a covert communication system has been developed to conceal covert information in sound documents by utilizing sound information concealing systems [6]. However, the covert rateof this system is significantly limited by the implementing software. Furthermore, a large amount of public data are required to carry a very small amount of covert information. To cope with this problem, the authors of [7] proposed a system in which covert communication and secure transmission are jointly implemented in untrusted relaying networks by adopting a power allocation strategy. In this approach, there is a tradeoff between covertness and covert rate for covert communication systems. To increase the covert rate, multiple jammers have been introduced to the covert communication system [8], where friendly relaying nodes operate as jammers to cooperatively interfere with the adversary in order to aid in the receipt of private information. However, the small capacity and high bit error rate (BER) limits the application of traditional covert communication systems (CCS). Intelligent reflecting surfaces (IRS) have been introduced into CCS to improve the efficiency and reliability of systems [9]. IRS, sometimes known as reconfigurable intelligent surfaces, is a low-cost technology that integrates a large number of passive reflecting units to intelligently adjust the reflected phase shift of signals in the environment [10]. Recently, IRS has been gradually applied to wireless CCS to improve its performance. Initially, an IRS-based method was proposed to take advantage of the smart controlled surfaces to modify unforeseen propagation conditions that could reveal messages [11]. The authors of [12] proposed an IRS-aided CCS system and jointly optimized the data rate, transmitting power, and reflection matrix of the IRS. To reduce computational complexity, a low-complexity penalty-based successive convex approximation (SCA) algorithm was proposed in [13] to jointly optimize the communication schedule and IRS reflection matrix. In [14; 15], an IRS-aided CCS with multiple-input multiple-output (MIMO) was proposed; high coupling was exploited for alternative optimization to find the optimal solution. Similarly, non-orthogonal multiple access methods were adopted in [16; 17] to design an IRS-assisted CCS, exploiting the phase shift uncertainty of the IRS as a cover medium to hide the existence of covert transmission. A passive IRS-assisted CCS was proposed in [18] to adjust the phase shifts of the IRS to align the phases of the received reflected signals while adjusting the reflection amplitude to satisfy the covert constraint. However, the covert information in IRS-assisted wireless CCS is vulnerable to adversaries.