User authentication and authorization in SCADA systems - Time & Space Complexity

Time Complexity: User authentication and authorization

O(n + m)

Understanding Time Complexity

When a SCADA system checks who a user is and what they can do, it runs certain steps repeatedly. Understanding how the time to do this grows helps us keep the system fast and safe.

We want to know: how does the time to authenticate and authorize a user change as the number of users or permissions grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// Authenticate user by checking username and password
function authenticate(userList, username, password) {
  for (let user of userList) {
    if (user.name === username && user.pass === password) {
      return true;
    }
  }
  return false;
}

// Authorize user by checking permissions
function authorize(userPermissions, requiredPermission) {
  for (let perm of userPermissions) {
    if (perm === requiredPermission) {
      return true;
    }
  }
  return false;
}
    

This code checks if a user exists with the right password and then checks if they have a needed permission.

Identify Repeating Operations

• Primary operation: Looping through the list of users to find a matching username and password.
• How many times: Up to once for each user in the user list until a match is found.
• Secondary operation: Looping through the user's permissions to find the required permission.
• How many times: Up to once for each permission the user has.

How Execution Grows With Input

As the number of users grows, the time to find the right user grows roughly in a straight line. The same happens with permissions.

Input Size (n)	Approx. Operations
10 users	Up to 10 checks to find user
100 users	Up to 100 checks to find user
1000 users	Up to 1000 checks to find user

Pattern observation: The time grows directly with the number of users and permissions checked.

Final Time Complexity

Time Complexity: O(n + m)

This means the time to authenticate and authorize grows linearly with the number of users (n) and the number of permissions (m).

Common Mistake

[X] Wrong: "Authentication time stays the same no matter how many users there are."

[OK] Correct: The system checks users one by one, so more users mean more checks and more time.

Interview Connect

Understanding how authentication and authorization scale helps you design systems that stay quick and secure as they grow. This skill shows you can think about real-world system behavior.

Self-Check

"What if the user list was stored in a way that lets us find users instantly? How would the time complexity change?"

Practice

1. What is the main purpose of user authentication in a SCADA system?

easy

A. To backup system data

B. To confirm the identity of a user

C. To monitor system performance

D. To control what actions a user can perform

Solution

1. Step 1: Understand authentication concept

   User authentication is about verifying who the user is.

2. Step 2: Differentiate from authorization

   Authorization controls permissions, not identity confirmation.

3. Final Answer:

   To confirm the identity of a user -> Option B

4. Quick Check:

   Authentication = Confirm identity [OK]

Hint: Authentication = Who you are, Authorization = What you can do [OK]

Common Mistakes:

• Confusing authentication with authorization
• Thinking authentication controls permissions
• Mixing system monitoring with user identity

2. Which of the following is the correct syntax to define a user role with read-only access in a SCADA system configuration file?

easy

A. role read_only { permissions: ["read"] }

B. role read_only = permissions: read

C. role: read_only { access = read }

D. read_only role { permissions = read }

Solution

1. Step 1: Identify correct role syntax

   The correct syntax uses 'role' keyword, role name, and permissions list in braces.

2. Step 2: Check option formats

   role read_only { permissions: ["read"] } correctly uses braces and array syntax for permissions.

3. Final Answer:

   role read_only { permissions: ["read"] } -> Option A

4. Quick Check:

   Correct role syntax uses braces and permission array [OK]

Hint: Look for braces and array syntax for permissions [OK]

Common Mistakes:

• Using '=' instead of ':' for permissions
• Missing braces around role definition
• Incorrect order of keywords

3. Given the following SCADA user permissions code snippet:

user = { name: "operator", roles: ["read_only", "control"] }
roles = {
  read_only: { permissions: ["read"] },
  control: { permissions: ["write", "execute"] }
}

What permissions does the user "operator" have?

medium

A. ["read"]

B. ["write", "execute"]

C. ["read", "write", "execute"]

D. ["read", "write"]

Solution

1. Step 1: Identify roles assigned to user

   User "operator" has roles "read_only" and "control".

2. Step 2: Combine permissions from both roles

   "read_only" grants ["read"], "control" grants ["write", "execute"]. Combined permissions are ["read", "write", "execute"].

3. Final Answer:

   ["read", "write", "execute"] -> Option C

4. Quick Check:

   Combine all role permissions for user [OK]

Hint: Add permissions from all user roles together [OK]

Common Mistakes:

• Selecting permissions from only one role
• Forgetting to merge permissions
• Confusing role names with permissions

4. A SCADA system admin tries to assign a user a role using this command:
assign_role user1 admin
But the system returns an error: "Role 'admin' not found." What is the most likely cause?

medium

A. The role 'admin' is not defined in the system

B. The user 'user1' does not exist

C. The command syntax is incorrect

D. The system is offline

Solution

1. Step 1: Analyze error message

   Error says "Role 'admin' not found," indicating the role is missing.

2. Step 2: Check command and user existence

   Command syntax looks correct and error does not mention user, so user likely exists.

3. Final Answer:

   The role 'admin' is not defined in the system -> Option A

4. Quick Check:

   Error message points to missing role [OK]

Hint: Check error message for missing role or user [OK]

Common Mistakes:

• Assuming user does not exist without checking
• Blaming command syntax without error details
• Ignoring error message content

5. In a SCADA system, you want to restrict a user so they can only read data but not control devices. Which approach best achieves this?

hard

A. Disable user authentication for faster access

B. Assign the user multiple roles including control

C. Grant the user admin privileges temporarily

D. Assign the user a role with only read permissions

Solution

1. Step 1: Understand requirement

   User must only read data, no control allowed.

2. Step 2: Choose role assignment

   Assigning a role with only read permissions restricts actions properly.

3. Step 3: Evaluate other options

   Assigning control roles or admin privileges contradicts restriction; disabling authentication is insecure.

4. Final Answer:

   Assign the user a role with only read permissions -> Option D

5. Quick Check:

   Restrict actions by role permissions [OK]

Hint: Use roles with minimal permissions to restrict users [OK]

Common Mistakes:

• Giving users more permissions than needed
• Disabling authentication to simplify access
• Using admin roles for normal users