CSRF attacks and token protection in PHP - Time & Space Complexity

Time Complexity: CSRF attacks and token protection

O(1)

Understanding Time Complexity

We want to understand how the time it takes to check CSRF tokens grows as the number of requests or tokens increases.

How does the token verification process scale when handling many requests?

Scenario Under Consideration

Analyze the time complexity of the following PHP code snippet for CSRF token verification.

// Assume session token is stored
session_start();

function verifyCsrfToken($token) {
    if (!isset($_SESSION['csrf_token'])) {
        return false;
    }
    return hash_equals($_SESSION['csrf_token'], $token);
}

$userToken = $_POST['csrf_token'] ?? '';
if (verifyCsrfToken($userToken)) {
    // Proceed with request
} else {
    // Reject request
}
    

This code checks if the CSRF token sent by the user matches the one stored in the session.

Identify Repeating Operations

Look for operations that repeat or take time depending on input size.

• Primary operation: Comparing two fixed-length strings with hash_equals.
• How many times: Once per request, no loops or recursion involved.

How Execution Grows With Input

The token comparison always takes about the same time because tokens have fixed length.

Input Size (n)	Approx. Operations
10 requests	10 token comparisons
100 requests	100 token comparisons
1000 requests	1000 token comparisons

Pattern observation: The time grows linearly with the number of requests, but each check is very fast and constant time.

Final Time Complexity

Time Complexity: O(1)

This means each token check takes the same small amount of time, no matter what.

Common Mistake

[X] Wrong: "Checking CSRF tokens gets slower as more users use the site."

[OK] Correct: Each token check is independent and constant time, so more users do not slow down a single check.

Interview Connect

Understanding how security checks like CSRF token verification scale helps you write safe and efficient web applications.

Self-Check

What if the token was stored in a database and you had to search for it each time? How would the time complexity change?

Practice

1. What is the main purpose of a CSRF token in a PHP web application?

easy

A. To speed up the loading time of the website

B. To prevent unauthorized commands from being transmitted from a user the website trusts

C. To encrypt user passwords before storing them

D. To validate user input format on forms

Solution

1. Step 1: Understand CSRF attack basics

   CSRF attacks trick a user into submitting unwanted actions on a trusted site.

2. Step 2: Role of CSRF token

   A CSRF token is a unique value that verifies the request is from the legitimate user, preventing unauthorized commands.

3. Final Answer:

   To prevent unauthorized commands from being transmitted from a user the website trusts -> Option B

4. Quick Check:

   CSRF token purpose = prevent unauthorized commands [OK]

Hint: CSRF tokens stop fake requests from trusted users [OK]

Common Mistakes:

• Confusing CSRF tokens with password encryption
• Thinking CSRF tokens speed up websites
• Believing CSRF tokens validate input format

2. Which PHP function is commonly used to generate a CSRF token?

easy

A. random_bytes()

B. md5()

C. htmlspecialchars()

D. strtoupper()

Solution

1. Step 1: Identify secure token generation

   CSRF tokens must be random and hard to guess, so secure random functions are used.

2. Step 2: Choose the correct function

   random_bytes() generates cryptographically secure random bytes, suitable for tokens.

3. Final Answer:

   random_bytes() -> Option A

4. Quick Check:

   Secure token generation = random_bytes() [OK]

Hint: Use random_bytes() for secure token generation [OK]

Common Mistakes:

• Using md5() which is not secure for tokens
• Confusing htmlspecialchars() with token generation
• Using strtoupper() which only changes case

3. What will the following PHP code output if the CSRF token in session matches the token sent in POST?

$_SESSION['csrf_token'] = 'abc123';
if (isset($_POST['csrf_token']) && $_POST['csrf_token'] === $_SESSION['csrf_token']) {
    echo 'Valid request';
} else {
    echo 'Invalid request';
}

medium

A. No output

B. Invalid request

C. Valid request

D. Undefined index error

Solution

1. Step 1: Check token comparison logic

   The code compares the POST token with the session token for equality.

2. Step 2: When tokens match

   If tokens match, it echoes 'Valid request'. If not, 'Invalid request'.

3. Final Answer:

   Valid request -> Option C

4. Quick Check:

   Matching tokens = Valid request [OK]

Hint: Matching tokens means valid request output [OK]

Common Mistakes:

• Assuming error if POST token is set correctly
• Ignoring isset() check causing errors
• Thinking output is empty when tokens match

4. Identify the error in this PHP CSRF token validation snippet:

session_start();
if ($_POST['csrf_token'] == $_SESSION['csrf_token']) {
    echo 'Token valid';
} else {
    echo 'Token invalid';
}

medium

A. session_start() should be called after token check

B. Using == instead of === causes token mismatch

C. Tokens should be compared with strtolower()

D. Missing check if $_POST['csrf_token'] is set before comparing

Solution

1. Step 1: Analyze token existence check

   The code compares $_POST['csrf_token'] without verifying if it exists, risking undefined index error.

2. Step 2: Importance of isset()

   Using isset() prevents errors by ensuring the token is present before comparison.

3. Final Answer:

   Missing check if $_POST['csrf_token'] is set before comparing -> Option D

4. Quick Check:

   Check token existence before compare = Missing isset() [OK]

Hint: Always check if POST token exists before comparing [OK]

Common Mistakes:

• Ignoring isset() causing runtime errors
• Thinking == vs === affects token validation here
• Calling session_start() after token check

5. You want to protect a PHP form from CSRF attacks. Which sequence correctly implements token generation, embedding, and validation? A) Generate token on form display, store in session, embed in hidden input, then check token on form submission. B) Generate token on form submission, embed in hidden input, then store in session. C) Store token in session after form submission, then generate token and embed in form. D) Embed a static token in the form and check it on submission without session.

hard

A. Generate token on form display, store in session, embed in hidden input, then check token on form submission.

B. Generate token on form submission, embed in hidden input, then store in session.

C. Store token in session after form submission, then generate token and embed in form.

D. Embed a static token in the form and check it on submission without session.

Solution

1. Step 1: Token generation and storage

   Token must be generated before showing the form and stored in session to verify later.

2. Step 2: Embedding and validation

   Embed the token in a hidden form field and check it against the session token on submission.

3. Final Answer:

   Generate token on form display, store in session, embed in hidden input, then check token on form submission. -> Option A

4. Quick Check:

   Token lifecycle = generate, store, embed, validate [OK]

Hint: Generate and store token before form, validate after submission [OK]

Common Mistakes:

• Generating token after form submission
• Not storing token in session
• Using static tokens without session storage