Webhook signature verification in Rest API - Time & Space Complexity

Time Complexity: Webhook signature verification

O(n)

Understanding Time Complexity

When verifying webhook signatures, we want to know how the time to check the signature changes as the message size grows.

We ask: How does the verification time grow when the webhook payload gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// Assume payload is a string and secret is a key
const crypto = require('crypto');

function verifySignature(payload, secret, signature) {
  const hmac = crypto.createHmac('sha256', secret);
  hmac.update(payload);
  const digest = hmac.digest('hex');
  return digest === signature;
}

This code creates a hash of the payload using a secret key and compares it to the given signature to verify authenticity.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The hashing process reads every character of the payload once.
• How many times: Exactly once for each character in the payload string.

How Execution Grows With Input

As the payload size grows, the hashing work grows proportionally because each character is processed once.

Input Size (n)	Approx. Operations
10	~10 hash steps
100	~100 hash steps
1000	~1000 hash steps

Pattern observation: The time grows linearly with the size of the payload.

Final Time Complexity

Time Complexity: O(n)

This means the verification time increases directly in proportion to the payload size.

Common Mistake

[X] Wrong: "Signature verification takes the same time no matter how big the payload is."

[OK] Correct: The hashing must read the entire payload, so bigger payloads take more time.

Interview Connect

Understanding how signature verification scales helps you explain security checks clearly and shows you can reason about performance in real systems.

Self-Check

"What if the payload was streamed in chunks instead of a full string? How would the time complexity change?"

Practice

1. What is the main purpose of webhook signature verification?

easy

A. To confirm the webhook is from a trusted source

B. To speed up the webhook delivery

C. To encrypt the webhook payload

D. To log the webhook request time

Solution

1. Step 1: Understand webhook security

   Webhook signature verification is used to check if the webhook request is really from the expected sender.

2. Step 2: Identify the purpose of verification

   This process confirms the webhook is trusted by comparing the signature with a secret key.

3. Final Answer:

   To confirm the webhook is from a trusted source -> Option A

4. Quick Check:

   Verification = Trusted source confirmation [OK]

Hint: Verification means checking trust, not speed or encryption [OK]

Common Mistakes:

• Confusing verification with encryption
• Thinking it speeds up webhook delivery
• Assuming it logs request time

2. Which of the following is the correct Python code snippet to compute an HMAC SHA256 signature for webhook verification?

easy

A. hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest()

B. hmac.new(payload.encode(), secret.encode(), hashlib.md5).hexdigest()

C. hashlib.sha256(secret + payload).hexdigest()

D. hmac.sha256(secret, payload).hexdigest()

Solution

1. Step 1: Understand HMAC function parameters

   The correct order is secret key first, then message (payload), and the hashing algorithm.

2. Step 2: Identify correct syntax and algorithm

   hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest() uses hmac.new with secret encoded, payload encoded, and hashlib.sha256, which is correct.

3. Final Answer:

   hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest() -> Option A

4. Quick Check:

   Secret first, payload second, SHA256 used [OK]

Hint: Secret key is first argument in hmac.new() [OK]

Common Mistakes:

• Swapping secret and payload order
• Using wrong hash function like md5
• Incorrect function call syntax

3. Given the following Python code snippet, what will be the output if the signature matches the computed HMAC?

import hmac
import hashlib

secret = "mysecret"
payload = "testdata"
signature = hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest()
computed = hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest()
if computed == signature:
    print("Signature verified")
else:
    print("Signature mismatch")

medium

A. None

B. "Signature verified"

C. RuntimeError

D. "Signature mismatch"

Solution

1. Step 1: Analyze the signature comparison

   The code compares the computed signature with the received signature and prints "Signature verified" if they match.

2. Step 2: Determine output when signatures match

   Since the signatures match, the output will be the success message "Signature verified".

3. Final Answer:

   "Signature verified" -> Option B

4. Quick Check:

   Matching signatures print verification message [OK]

Hint: Match means print verified message [OK]

Common Mistakes:

• Confusing mismatch message as output
• Expecting an error instead of message
• Assuming no output (None)

4. Identify the error in this Python webhook verification snippet:

import hmac
import hashlib

secret = "mysecret"
payload = "data"
signature = "abc123"

computed = hmac.new(secret, payload.encode(), hashlib.sha256).hexdigest()
if computed == signature:
    print("Verified")
else:
    print("Not verified")

medium

A. payload should not be encoded

B. hashlib.md5 should be used instead of sha256

C. secret should be encoded before use in hmac.new

D. signature comparison should use != instead of ==

Solution

1. Step 1: Check data types for hmac.new

   hmac.new requires the secret key as bytes, but secret is a string and not encoded.

2. Step 2: Identify correct fix

   Encoding secret with secret.encode() fixes the error; payload encoding is correct.

3. Final Answer:

   secret should be encoded before use in hmac.new -> Option C

4. Quick Check:

   Secret must be bytes, so encode it [OK]

Hint: Encode secret string before hmac.new call [OK]

Common Mistakes:

• Not encoding secret key
• Changing payload encoding incorrectly
• Using wrong hash function
• Wrong comparison operator

5. You receive webhook requests with a signature header. To securely verify the webhook, which steps should you perform in order? Select the correct sequence.

hard

A. 1. Retrieve secret key
2. Compare signatures
3. Compute HMAC of payload
4. Accept webhook if signatures do not match

B. 1. Accept webhook
2. Compute HMAC of payload
3. Compare signatures
4. Log the webhook

C. 1. Compute HMAC of payload
2. Retrieve secret key
3. Compare signatures
4. Reject webhook if signatures match

D. 1. Retrieve the secret key
2. Compute HMAC of the payload using the secret
3. Compare computed signature with header signature
4. Accept webhook if signatures match

Solution

1. Step 1: Retrieve the secret key

   You need the secret key first to compute the HMAC signature.

2. Step 2: Compute HMAC of the payload using the secret

   Use the secret key and payload to compute the signature.

3. Step 3: Compare computed signature with header signature

   Check if the computed signature matches the one sent in the webhook header.

4. Step 4: Accept webhook if signatures match

   Only accept the webhook if the signatures are the same to ensure trust.

5. Final Answer:

   1. Retrieve the secret key 2. Compute HMAC of the payload using the secret 3. Compare computed signature with header signature 4. Accept webhook if signatures match -> Option D

6. Quick Check:

   Secret -> Compute -> Compare -> Accept [OK]

Hint: Always compute signature before comparing and accepting [OK]

Common Mistakes:

• Accepting webhook before verification
• Comparing before computing signature
• Rejecting on matching signatures