## 1 Introduction  
Currently, the field of Integrated Circuit (IC) technology is undergoing rapid advancement, leading to increased commercial cooperation within the IC industry. Given that Register Transfer Level (RTL) code is the most prevalently utilized description language in digital circuits and System on Chip (SoC) designs, the industry hopes to realize win-win business cooperation through Intellectual Property (IP) of RTL code projects [1]. However, due to security concerns related to IP, conflicts of interest often arise among companies in practical collaborations. In August 2022, ARM filed a lawsuit against Qualcomm, alleging unauthorized usage of ARM's IP in chip design [2]. Therefore, it is very important to ensure IP security during business cooperation. In the preliminary communication stage of business cooperation, if RTL code appears in plain text, the internal information security of the code is difficult to guarantee. Regarding IP authorization issues, RTL code without any protective measures faces challenges in ensuring the post-authorization interests of the IP provider remain unharmed. To circumvent these infringements, researchers employ methods such as code encryption and obfuscation to secure the RTL code, thereby promoting secure and equitable business practices within the IC industry. Code obfuscation and code encryption represent two distinct methodologies for safeguarding RTL code. Code obfuscation functions as a countermeasure against human analysis and reverse engineering by transforming the source code into a protected format with indiscernible semantics [3]. Obfuscated code thwarts attackers from analyzing the control flow and data interaction patterns within the code, thereby preserving the intellectual property of the developer or organization. Obfuscation focuses on making the code less readable, which is more appropriate when the developer does not want others to be able to analyze the code to obtain the fruits of their labor. Code encryption is a method of converting code into ciphertext by using a key, and is preferred to prevent intellectual property leakage through key management. Nevertheless, due to the necessity of compliance with the regulations of the Institute of Electrical and Electronics Engineers (IEEE) [4], the encryption can only rely on the key of the electronic design automation (EDA) software company. This arrangement introduces a potential vulnerability, as there exists a risk of information disclosure from the EDA software company endowed with key management responsibilities. Code obfuscation is more advantageous than code encryption in preventing code leakage and avoiding code parsing. Current obfuscation studies of RTL code are still problematic. Several popular obfuscation methods that are widely used are not well-suited for RTL code. For instance, data obfuscation techniques designed to complicate mathematical operations, when applied to RTL code, tend to increase the number of operation units in the circuit. This is often conflicts with practical development scenarios with area, timing, and power requirements, and requires obfuscation methods that are more compatible with RTL code. Simultaneously, certain existing RTL code obfuscation tools have the problem of wrong code analysis, leading to the obfuscation of information that should be preserved. The obfuscation process of these tools is also unreasonable, and through careful observation, patterns may be found and thus the possibility of cracking exists. These flaws reduce the efficiency and accuracy of RTL code obfuscation. In response to the industry's demand for code obfuscation, this paper proposes RTL code obfuscation methods based on Extensible Markup Language (XML) syntax tree. The introduction of these methods is anticipated to yield benefits and changes for practitioners in related fields.