###### Abstract  
The massive use of end information in the end hopping system not only significantly improves the proactive defense capability but also reveals great potential for covert communication. However, the development of existing network covert channels is hindered by various elimination techniques and a lack of robustness guarantees. In this paper, we first present a novel network covert channel model based on end spreading (CCES) in the end hopping system. We then propose a CCES-based scheme using m-sequence in the hypothetical scenario and theoretically analyze its characteristics, including eavesdropping resistance, loss tolerance, and robust synchronization. To evaluate the performance of the CCES scheme, three evaluation metrics are adopted: non-detectability, robustness, and transmission efficiency. Experimental results show that CCES achieves a bit error rate (BER) below 5% under 30% packet loss, entropy values ranging from 0.15 to 0.82 (comparable to normal traffic), and a transmission efficiency of up to 800 bits per second. These results confirm the CCES scheme's strong robustness and practical applicability, outperforming traditional covert channels in both reliability and stealth. network covert channel; end spreading; m-sequence; robustness; evaluation metrics 2025, 16, 589 2025 20  
while timing covert channels exploit packet intervals or the arrival times of data packets to transmit secret information [8; 9]. Although both types of covert channels have proven useful, they are not without their limitations. For example, network covert storage channels are highly susceptible to content-based detection techniques that inspect packet headers and look for anomalies in the data structure, such as irregular TCP flag distributions detected via relative entropy [10] or unusual sequence/ID field values filtered by tools like NetWarden [11], and extensive studies on IP-header-based covert channels [12]. Timing covert channels are often affected by network performance issues such as latency, packet loss, and noise, making them vulnerable to disruption. For example, network interference and pump could eliminate network timing covert channels with ease [13]. Moreover, recent advancements in countermeasures designed to mitigate covert channel communication have made it increasingly difficult for these channels to evade detection. Techniques such as traffic normalization [14], which alter packet patterns to mask covert channels, and network interference methods, which introduce random delays or noise, are often effective in neutralizing traditional covert channels [15; 16]. Despite these challenges, the need for robust covert communication channels that can operate effectively in high-speed and hostile network environments remains pressing. In light of these issues, this paper introduces a novel approach for constructing a highly robust network covert channel that leverages end spreading in end hopping systems. End hopping, which is inspired by frequency hopping communication [17], introduces an innovative mechanism to secure network communication by dynamically changing end information (such as IP addresses, source/destination ports) in a pseudo-random manner. As an extension of end hopping, end spreading [18] draws further inspiration from frequency hopping spread spectrum (FHSS) and is designed to achieve authentication and synchronization in high-speed end hopping systems, offering enhanced confidentiality and robustness. This paper proposes the CCES (covert channel based on end spreading) model, which addresses the critical challenges faced by existing network covert channels, namely, their low robustness, poor concealment, and vulnerability to detection. Although hopping techniques have been studied, this work focuses on the robust and quantifiable analysis of end spreading. The CCES model offers several key differences from previous methods:  
Eavesdropping Resistance: The secret messages are embedded into end spreading sequences, which are independent of each other.