Access control matrix in Operating Systems - Mini Project: Build & Apply

Access Control Matrix

📖 Scenario: You are managing a simple computer system where different users have different permissions on various files. To keep track of who can do what, you will create an access control matrix.This matrix shows which users have which permissions (like read, write, execute) on each file.

🎯 Goal: Build an access control matrix using a dictionary of dictionaries. Each key in the main dictionary is a user, and its value is another dictionary showing files and the permissions that user has on those files.

📋 What You'll Learn

Create a dictionary called access_matrix with exact users and files

Add a list of permissions for each file per user

Use a helper variable to list all possible permissions

Write a loop to check and list permissions for a specific user

Add a final step to show the permissions of a chosen user on all files

💡 Why This Matters

🌍 Real World

Access control matrices are used in computer systems to manage who can access or modify files and resources, ensuring security and proper authorization.

💼 Career

Understanding access control is essential for roles in system administration, cybersecurity, and software development to protect data and resources.

1

Create the access control matrix

Create a dictionary called access_matrix with these exact entries: 'Alice', 'Bob', and 'Charlie' as keys. Each user should have a nested dictionary with files 'file1' and 'file2'. Set the permissions as lists exactly like this: 'Alice': {'file1': ['read', 'write'], 'file2': ['read']}, 'Bob': {'file1': ['read'], 'file2': ['read', 'execute']}, 'Charlie': {'file1': [], 'file2': ['write']}.

Operating Systems

# Create the access_matrix dictionary with users and their file permissions
# Your code here

Hint

Use nested dictionaries where each user maps to another dictionary of files and their permission lists.

2

Define the list of all possible permissions

Create a list called all_permissions that contains these exact strings: 'read', 'write', and 'execute'.

Operating Systems

access_matrix = {
    'Alice': {'file1': ['read', 'write'], 'file2': ['read']},
    'Bob': {'file1': ['read'], 'file2': ['read', 'execute']},
    'Charlie': {'file1': [], 'file2': ['write']}
}
# Create the list all_permissions with 'read', 'write', 'execute'
# Your code here

Hint

Just list the three permission strings inside square brackets.

3

List permissions for a specific user

Write a for loop using variables file and permissions to iterate over access_matrix['Bob'].items(). Inside the loop, create a new dictionary called bob_permissions that stores each file as key and its permissions as value.

Operating Systems

access_matrix = {
    'Alice': {'file1': ['read', 'write'], 'file2': ['read']},
    'Bob': {'file1': ['read'], 'file2': ['read', 'execute']},
    'Charlie': {'file1': [], 'file2': ['write']}
}
all_permissions = ['read', 'write', 'execute']

bob_permissions = {}
# Use a for loop to fill bob_permissions with Bob's file permissions
# Your code here

Hint

Use for file, permissions in access_matrix['Bob'].items(): to loop through Bob's files and permissions.

4

Show Bob's permissions on all files

Add a final line that assigns bob_permissions to a variable called result. This will represent the completed access control information for Bob.

Operating Systems

access_matrix = {
    'Alice': {'file1': ['read', 'write'], 'file2': ['read']},
    'Bob': {'file1': ['read'], 'file2': ['read', 'execute']},
    'Charlie': {'file1': [], 'file2': ['write']}
}
all_permissions = ['read', 'write', 'execute']

bob_permissions = {}
for file, permissions in access_matrix['Bob'].items():
    bob_permissions[file] = permissions

# Assign bob_permissions to result
# Your code here

Hint

Just assign bob_permissions to result to complete the project.

Practice

1. What does an access control matrix primarily represent in operating systems?

easy

A. Which users have what permissions on which resources

B. The order in which processes are executed

C. The memory allocation for each program

D. The list of installed software on a system

Solution

1. Step 1: Understand the purpose of access control matrix

   An access control matrix is a table that shows permissions users have on resources.

2. Step 2: Match the description to the options

   Only Which users have what permissions on which resources correctly describes permissions of users on resources.

3. Final Answer:

   Which users have what permissions on which resources -> Option A

4. Quick Check:

   Access control matrix = user permissions on resources [OK]

Hint: Think: Who can do what on which resource? [OK]

Common Mistakes:

• Confusing access control matrix with process scheduling
• Thinking it shows memory or software lists
• Mixing permissions with system hardware details

2. Which of the following correctly represents a row in an access control matrix?

easy

A. Process1 -> Memory address 0x1000

B. User1: read, write

C. File size = 1024 bytes

D. CPU usage = 50%

Solution

1. Step 1: Identify what a row in access control matrix shows

   A row lists a user and their permissions on various resources.

2. Step 2: Check which option matches this format

   User1: read, write shows a user with permissions, matching the row concept.

3. Final Answer:

   User1: read, write -> Option B

4. Quick Check:

   Row = user permissions list [OK]

Hint: Rows list users and their permissions [OK]

Common Mistakes:

• Confusing resource details with user permissions
• Mixing process or CPU info with access control
• Choosing options unrelated to permissions

3. Given the access control matrix below:

Users\Files | File1 | File2
---------|--------|-------
Alice | read | write
Bob | write | execute

What permission does Bob have on File2?

medium

A. execute

B. write

C. read

D. no permission

Solution

1. Step 1: Locate Bob's row in the matrix

   Bob's permissions are listed in the second row.

2. Step 2: Find the permission under File2 column for Bob

   Bob has 'execute' permission on File2.

3. Final Answer:

   execute -> Option A

4. Quick Check:

   Bob on File2 = execute [OK]

Hint: Find user row, then resource column [OK]

Common Mistakes:

• Mixing up rows and columns
• Choosing Alice's permissions instead of Bob's
• Assuming default permissions if not listed

4. Identify the error in this access control matrix snippet:

Users\Resources | FileA | FileB
--------------|--------|-------
Carol | read | readwriteexecute

What is wrong with this matrix?

medium

A. The matrix must list users vertically, not horizontally

B. Carol cannot have read permission

C. Multiple permissions should be separated, not combined in one cell

D. FileB should not have any permissions listed

Solution

1. Step 1: Check how permissions are listed in each cell

   Permissions should be clearly separated, usually by commas or listed distinctly.

2. Step 2: Identify if combining multiple permissions in one cell is acceptable

   Combining multiple permissions in one cell without clear separation can cause confusion.

3. Final Answer:

   Multiple permissions should be separated, not combined in one cell -> Option C

4. Quick Check:

   Permissions must be clearly separated [OK]

Hint: Permissions must be clearly separated in matrix cells [OK]

Common Mistakes:

• Thinking users must be listed horizontally
• Assuming permissions can't be combined at all
• Ignoring formatting clarity in matrix

5. You have an access control matrix with users and resources. If you want to quickly check which users can write to a resource named Report.doc, what is the best approach?

hard

A. Look across the row for Report.doc and note users with write permission

B. Check the entire matrix for any write permission

C. Check the diagonal cells only for write permission

D. Look down the column for Report.doc and note users with write permission

Solution

1. Step 1: Understand matrix layout

   Rows represent users, columns represent resources.

2. Step 2: To find users who can write to Report.doc, check the column for that resource

   Looking down the column shows which users have write permission.

3. Final Answer:

   Look down the column for Report.doc and note users with write permission -> Option D

4. Quick Check:

   Users with write on resource = column check [OK]

Hint: Check down the resource column for permissions [OK]

Common Mistakes:

• Looking across a resource row instead of column
• Checking only diagonal cells
• Scanning entire matrix unnecessarily