## 1 Introduction  
The way information is shared has significantly changed due to the advancement of communication technologies. These technologies enable interaction across several platforms and devices [1; 2; 3; 4]. However, there are increasing concerns about individual privacy. Additionally, in some contexts, communications can be restricted due to the presence of a censor that is able to monitor the entire network traffic. In such contexts, ensuring secure and undetectable communication is crucial, as traditional methods may expose sensitive information [5; 6; 7]. For example, standard encryption-based solutions only conceal the exchanged content but not the act of communication itself; i.e., an adversary knows the communicating parties. Additionally, an adversary can analyze metadata such as the timing, frequency, and patterns of communication to infer sensitive information. Alternative solutions consist of anonymous communication protocols [8; 9; 10; 11] whose aim is to hide the identities of the communicating parties and metadata. However, these approaches often require a complex network setup involving a high number of communicating parties. Additionally, the use of such protocols may act as a red flag to tensors, who are capable of detecting their distinct traffic patterns, making it easier for them to block or restrict the use of these methods [5; 6; 7]. In recent years, covert channel methods [12; 13] have gained increasing attention as a means to enhance privacy and security. These methods enable the hiding of messages within regular communication channels. This makes it difficult for third parties to detect the presence of any secret communications. Covert channels can be classified into two types: storage channels and timing channels [14]. Storage channels hide data within network packets, such as the headers or the payload [15; 16]. For example, unused header fields can be manipulated to carry hidden messages. Instead, timing channels alter the timing of packet transmissions to encode information [17; 18]. For instance, the presence or absence of a packet at a specific time can be encoded by a binary value, thus creating a hidden communication channel within the normal flow of traffic. This paper proposes a novel approach to create covert communication by leveraging the construction of a storage channel within MQTT messages. Specifically, our solution leverages the field length within these messages to exfiltrate data, which are in turn related to the actual data through known one-way mathematical functions. Through this method, we aim to allow for the secret exchange of information between parties while obfuscating the communication itself. This approach enables safe information exchange in high-risk environments. For instance, it can bypass censorship activities since the information is completely hidden in covert channels carried by commonly adopted protocols. In this work, we adopt the MQTT protocol as a carrier protocol since it is commonly used in the IoT context. Therefore, with the increased spread of MQTT, covert communications can be embedded within regular MQTT traffic without raising any suspicion. Our overhead analysis shows that our solution does not impose any significant time cost for the required operations. However, there is some overhead in terms of the number of bytes transmitted. Nevertheless, this overhead appears to be acceptable. Finally, our security analysis shows that our solution is effective in protecting the transmission of information in high-risk environments, where attackers can observe all network traffic, even in the clear. In addition, it is effective against more powerful adversaries in the middle of the communication that may attempt to tamper with the messages exchanged between publishers and subscribers. This also includes compromised brokers natively in the middle of the communication. To conclude, we summarize the main contributions of this paper.