Authentication vs authorization in HLD - When to Use Which

The Big Idea

What if you could stop guessing who someone is and what they can do, and instead have a clear, secure way to check both every time?

The Scenario

Imagine you run a busy office where everyone needs to enter a building and access different rooms. Without a system, you have to check each person's ID and then remember which rooms they can enter. This manual checking slows everything down and causes confusion.

The Problem

Manually verifying who someone is and what they can do is slow and error-prone. People might get access to places they shouldn't, or get denied entry by mistake. It's hard to keep track of permissions and identities separately, leading to security risks and frustration.

The Solution

Authentication and authorization split the job into two clear steps: first, confirm who the person is (authentication), then decide what they are allowed to do (authorization). This separation makes systems faster, safer, and easier to manage.

Before vs After

✗ Before

if user_id == known_id and room in allowed_rooms:
    allow_entry()

✓ After

if authenticate(user_credentials):
    if authorize(user, resource):
        allow_entry()

What It Enables

This clear separation lets systems securely control access at scale, ensuring only the right people do the right things.

Real Life Example

When you log into your email, the system first checks your password (authentication). Then, it decides if you can read emails, send messages, or change settings (authorization).

Key Takeaways

Authentication verifies identity; authorization controls access.

Separating these improves security and clarity.

It helps build scalable, safe systems for many users.

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