# A Proposal for a Zero-Trust-Based Multi-Level Security Model and Its Security Controls
## 3 Introduction of MLS Classification Criteria and Proposed Zero-Trust-Based MLS Model
### Proposed Zero-Trust-Based MLS Model  
Section 4.2 presents an MLS model based on Zero Trust. The proposed security model incorporates PDP and PEP, the core logical components of the Zero Trust Architecture, and classifies resources into Classified, Sensitive, and Open categories according to the MLS framework of the Republic of Korea. Furthermore, if data of different classifications are stored within a single system, lateral movement between classifications within the same system is prohibited, in accordance with Zero Trust principles. Figure 4 depicts the proposed Zero-Trust-based MLS model and data flow. The proposed security model categorises data flow into three broad types based on the resources accessed. The three categories are as follows:  
* When an external subject accesses C-level resources (Orange line): C-level resources represent the highest level of data or systems, such as national secrets, requiring security equivalent to the traditional network segregation framework. Therefore, external users who are not located within the same network as the C-level data or systems can only transmit data via network bridging solutions within the network segregation system. * When an internal subject accesses C-level resources (Green line): This type of subject resides within the same network as the C-level resources and refers to users or systems authorised to access C-level data or information systems. Since they are located within the same network as the C-level resources, there is no need for network bridging solutions during communication. However, access to the resources is granted only after validation by the PDP and PEP, in accordance with Zero Trust principles. * When a subject accesses S- and O-level resources, regardless of being internal or external (Blue line): This type involves access to resources other than C-level resources (i.e., S- and O-level resources). Access is dynamically controlled by the PDP and PEP based on the subject's identity and authorisation, as well as the state of the resource being accessed. Figure 3: Security principles according to information and system level [32]. The MLS model offers the advantage of systematically managing and categorising an organisation's assets and data. However, it is primarily based on static security controls, making it challenging to adapt to dynamic environmental changes. The ZT model provides robust security through its inherent characteristic of validating all access requests. However, excessive security measures may potentially decrease operational productivity [33]. Therefore, when adopting a Zero-Trust-based model, resolving the trade-off between security and operational efficiency is crucial. Also, there are several studies that have explored approaches and technologies, such as multi-factor authentication (MFA) and adaptive authentication, to balance between these two models [34; 35; 36]. As such, the proposed model in this study aims to leverage the advantages of these two models, seeking to overcome the limitations of static security controls in the MLS through the automated dynamic access control of Zero Trust. One of the key elements of a Zero-Trust-based security model is the automated decision making and enforcement of security policies. This involves controlling and continuously validating access to networks and resources and is particularly related to dynamic access control. Dynamic access control does not rely on static rules. Instead, it dynamically adjusts access permissions based on the status of users and systems, environmental factors, and real-time active logs [37]. In essence, dynamic access control underpins automated security policy decision making and enforcement, contributing to the establishment of a more efficient security environment. This enables security administrators and operators to reduce the need for manually managing access permissions, as the system automatically applies and enforces security policies.