Access control is fundamental to operating system (OS) security. It ensures that only authorized processes can perform specific actions on system resources.

One key tool used to manage access control is the access matrix. This article explores an access matrix, how it functions, and the various methods used to implement it in an operating system.

Understanding Access Matrix

An access matrix is a conceptual model used to define the permissions and rights of processes within a computing environment. It is a two-dimensional matrix where:

• Rows represent domains. A domain is a set of processes or users with the same access rights.
• Columns represent objects. Objects are resources or entities such as files, devices, or other data that processes might need to access.

Each cell in the matrix, denoted as an entry (i,j)(i,j), specifies the set of operations that processes in domain DiDi​ can perform on object OjOj​. These operations might include reading, writing, executing, or deleting an object.

Purpose of the Access Matrix

Access control is a fundamental aspect of cybersecurity, and it revolves around three key principles: availability, integrity, and confidentiality.

1. Availability

• Availability ensures that authorized users can access the system whenever needed.
• To maintain availability, organizations implement strategies to prevent and recover from hardware or software failures, network issues, and cyberattacks like Denial-of-Service (DoS) attacks. This might include regular backups, redundancy, and robust network security measures.

2. Integrity

• Integrity ensures that the data within a system remains accurate and unaltered, whether intentionally (e.g., by an attacker) or unintentionally (e.g., due to human error).
• Measures to protect integrity include using checksums, encryption, access controls, and regular audits to detect and correct unauthorized changes or errors in data.

3. Confidentiality

• Confidentiality ensures that sensitive information is only accessible to those authorized to see it.
• Organizations use encryption, strong authentication methods, access control lists, and secure communication channels to maintain confidentiality to prevent unauthorized access or data breaches.

Implementation of Access Matrix in OS

Due to its potential size and sparsity, directly implementing an access matrix can be inefficient. Several methods have been developed to implement access matrices more effectively, optimizing storage and performance.

1. Global Table

The global table is a straightforward implementation where each entry is an ordered triple: ⟨Domain(Di),Object(Oj),Right set(Rk)⟩⟨Domain(Di​),Object(Oj​),Right set(Rk​)⟩. When an operation is requested, the system checks for the presence of a corresponding triple. If found, the operation proceeds; otherwise, it is denied.

Limitations:

• The global table can become very large, making storing in the main memory difficult.
• It may require additional input/output operations, slowing down the system.

2. Access Lists

In the access list approach, the matrix is decomposed into columns. Each object OjOj​ has an associated list that specifies which domains have access rights to it. When a process attempts an operation, the system checks this list to determine if the action is permitted.

Advantages:

• Efficient for systems where a few domains access many objects.
• It is easy to manage and update access rights for individual objects.

3. Capability Lists

Capability lists are the opposite of access lists. Each domain DiDi​ has a list of capabilities: the objects it can access and the operations it can perform on them. This approach associates each row of the matrix with its domain.

Advantages:

• It is useful in systems where domains frequently access many objects.
• Capabilities can be easily transferred between domains, allowing for flexible access control.

4. Lock-Key Mechanism

This method combines aspects of both access lists and capability lists. Each object has a unique lock (a bit pattern), and each domain has a unique key. A process can only access an object if its domain’s key matches its lock.

Advantages:

• It provides a simple and effective way to manage access.
• Ensures that only authorized processes can access specific objects.

Example of Access Matrix Implementation

To better understand how access matrices work, consider a scenario with three domains (D1, D2, D3) and four files (f1, f2, f3, f4). The access rights might be defined as follows:

• D1: Can read any file and write to f1.
• D2: Can read f1, write to f2, and execute f4.
• D3: Can write to f1 and f3, and execute f4.

This setup ensures each domain has specific permissions tailored to its needs, enhancing security by restricting unauthorized access.

Dynamic and Static Access Rights

Access matrices can support both static and dynamic access rights:

• Static Access Rights: These are predefined and do not change during the system’s operation. They provide a stable security environment.
• Dynamic Access Rights: These can change based on certain conditions or operations, such as domain switching. For example, a process might move from D1 to D2, gaining new access rights.

Domain Switching and Rights Duplication

Access matrices also support domain switching, allowing processes to change domains and acquire different access rights.

Additionally, rights can be duplicated across domains, indicated by an asterisk (*) in the matrix. This feature allows for flexible permissions management, enabling processes to adapt to different operational contexts.

Access matrices are vital components of operating system security. They provide a structured approach to managing permissions and enforcing access control policies.

Access matrices help maintain system integrity and prevent unauthorized access by clearly defining what each process can do with each object.

Access matrices can be tailored to meet the specific needs of different systems through various implementation methods, such as global tables, access lists, capability lists, and lock-key mechanisms.

Whether dealing with static or dynamic rights, access matrices offer a robust framework for managing the complex interactions between processes and resources in an operating system.