Authentication vs authorization in HLD - Architecture Trade-offs

Problem Statement

Without clear separation, systems confuse verifying who a user is with what they can do. This leads to security risks like unauthorized access or users being blocked from allowed actions, causing failures in protecting sensitive data or resources.

Solution

Authentication confirms the user's identity by checking credentials like passwords or biometrics. Authorization then decides what actions or resources the authenticated user can access based on their permissions or roles.

Architecture

User

→Authentication

↓

Identity Data

This diagram shows the flow where a user first passes through authentication to verify identity, then through authorization to check access rights.

Trade-offs

✓ Pros

→

Separates identity verification from permission checks, improving security clarity.

→

Allows flexible permission management without re-verifying identity.

→

Supports scalable systems by decoupling user login from access control.

✗ Cons

→

Requires additional infrastructure to manage both authentication and authorization.

→

Complexity increases when integrating multiple authentication methods with fine-grained authorization.

→

Misconfiguration can lead to security gaps if either step is weak.

Always use when building systems with user access control, especially when multiple roles or permissions exist and security is critical.

For simple public systems with no user-specific access control, where all users have the same permissions.

Real World Examples

Amazon

Separates customer login (authentication) from their ability to purchase or view order history (authorization) to protect user data.

Google

Uses OAuth for authentication to verify user identity, then applies authorization to control access to services like Gmail or Drive.

Netflix

Authenticates users to confirm identity, then authorizes content access based on subscription level and region.

Alternatives

Role-Based Access Control (RBAC)

Focuses on authorization by assigning permissions to roles rather than individuals, but still requires authentication first.

Use when: When managing permissions for large groups with similar access needs.

Attribute-Based Access Control (ABAC)

Authorization decisions are made based on user attributes and environment, adding more dynamic control after authentication.

Use when: When access depends on multiple contextual factors beyond roles.

Summary

Authentication verifies the user's identity using credentials.

Authorization determines the user's permissions after identity is confirmed.

Separating these improves security and system clarity.

Practice

1. Which of the following best describes authentication in system design?

easy

A. Encrypting user data

B. Granting access to resources

C. Logging user activities

D. Verifying the identity of a user

Solution

1. Step 1: Understand authentication purpose

   Authentication is the process of confirming who the user is by checking credentials like username and password.

2. Step 2: Differentiate from authorization

   Authorization controls what the user can do after identity is confirmed, not the identity check itself.

3. Final Answer:

   Verifying the identity of a user -> Option D

4. Quick Check:

   Authentication = Verifying identity [OK]

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

Common Mistakes:

• Confusing authentication with authorization
• Thinking authentication controls permissions
• Mixing identity verification with data encryption

2. Which of the following is a correct example of an authorization check in code?

easy

A. if user.password == input_password:

B. if user.session_token is not None:

C. if user.role == 'admin':

D. if user.email_verified:

Solution

1. Step 1: Identify authorization logic

   Authorization checks what actions a user can perform, often by checking roles or permissions like user.role == 'admin'.

2. Step 2: Differentiate from authentication checks

   Authentication checks credentials like passwords or tokens, not roles.

3. Final Answer:

   if user.role == 'admin': -> Option C

4. Quick Check:

   Authorization checks roles/permissions [OK]

Hint: Authorization checks roles or permissions, not passwords [OK]

Common Mistakes:

• Using password checks as authorization
• Confusing session token presence with authorization
• Checking email verification as authorization

3. Consider this code snippet:

def access_resource(user):
    if not user.is_authenticated:
        return "Access Denied"
    if user.permission != 'read':
        return "Permission Denied"
    return "Resource Content"

What will be the output if user.is_authenticated is True but user.permission is 'write'?

medium

A. "Permission Denied"

B. "Resource Content"

C. "Access Denied"

D. Error due to missing permission

Solution

1. Step 1: Check authentication condition

   The user is authenticated (True), so the first if condition fails and does not return "Access Denied".

2. Step 2: Check authorization condition

   The user's permission is 'write', which is not equal to 'read', so the second if returns "Permission Denied".

3. Final Answer:

   "Permission Denied" -> Option A

4. Quick Check:

   Authenticated but wrong permission = Permission Denied [OK]

Hint: Authenticated? Check permission next for authorization [OK]

Common Mistakes:

• Ignoring permission check and returning resource
• Confusing authentication failure with permission failure
• Assuming 'write' permission equals 'read'

4. A developer wrote this code to check access:

if user.is_authorized:
    print("Access granted")
else:
    print("Access denied")

But the system always denies access even for valid users. What is the likely issue?

medium

A. The variable is_authorized is misspelled

B. The code checks authorization without authenticating the user first

C. The print statements are reversed

D. The user object is not defined

Solution

1. Step 1: Analyze authentication and authorization order

   Authorization should only be checked after confirming the user is authenticated. Skipping authentication can cause authorization to fail.

2. Step 2: Understand why access is always denied

   If is_authorized depends on authentication, but authentication is missing, it will always be false or unset, denying access.

3. Final Answer:

   The code checks authorization without authenticating the user first -> Option B

4. Quick Check:

   Authenticate before authorizing [OK]

Hint: Always authenticate before authorization checks [OK]

Common Mistakes:

• Checking authorization without authentication
• Assuming authorization implies authentication
• Ignoring user object initialization

5. In designing a secure web service, which approach correctly applies authentication and authorization to protect user data?

hard

A. Authenticate users with passwords, then authorize access based on roles before serving data

B. Authorize users first, then authenticate them before data access

C. Allow data access after authentication without any authorization checks

D. Skip authentication if the user has a valid session token

Solution

1. Step 1: Understand correct security flow

   First, confirm user identity by authentication (e.g., passwords). Then, check what the user is allowed to do by authorization (e.g., roles).

2. Step 2: Evaluate options for security best practice

   Authenticate users with passwords, then authorize access based on roles before serving data follows the correct order: authenticate then authorize. Other options either reverse order, skip authorization, or skip authentication improperly.

3. Final Answer:

   Authenticate users with passwords, then authorize access based on roles before serving data -> Option A

4. Quick Check:

   Authenticate first, then authorize [OK]

Hint: Authenticate identity first, then authorize actions [OK]

Common Mistakes:

• Reversing authentication and authorization order
• Skipping authorization after authentication
• Trusting session tokens without authentication