Webhook signature verification in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is webhook signature verification?

Webhook signature verification is a security method to confirm that a webhook request really comes from the expected sender by checking a special code called a signature.

beginner

Why do we verify webhook signatures?

We verify webhook signatures to protect our app from fake or harmful requests pretending to be from trusted sources.

intermediate

Which common method is used to create webhook signatures?

A common method is using HMAC (Hash-based Message Authentication Code) with a secret key shared between sender and receiver.

intermediate

What information do you usually need to verify a webhook signature?

You need the webhook payload (data sent), the secret key, and the signature sent with the webhook to compare and confirm authenticity.

beginner

What happens if webhook signature verification fails?

If verification fails, the webhook request should be rejected or ignored to keep your system safe from unauthorized actions.

What is the main purpose of webhook signature verification?

A To confirm the webhook is from a trusted sender

B To speed up webhook delivery

C To encrypt the webhook data

D To log webhook requests

Webhook signature verification confirms the webhook is from a trusted sender by checking the signature.

Which cryptographic method is commonly used for webhook signature verification?

A MD5 hashing without a key

B RSA encryption

C Base64 encoding

D HMAC

HMAC uses a secret key and hashing to create a secure signature for verification.

What should your app do if the webhook signature does not match?

A Process the webhook normally

B Reject or ignore the webhook request

C Send a thank you message

D Store the webhook for later

Rejecting or ignoring unverified webhooks protects your app from fake or harmful requests.

What two pieces of information are essential to verify a webhook signature?

A Payload and sender's email

B Sender's IP and timestamp

C Payload and secret key

D Secret key and sender's username

The payload and secret key are used to recreate and check the signature.

Where is the webhook signature usually sent?

A In the webhook request headers

B In the webhook URL

C In the webhook body as plain text

D In the webhook response

The signature is typically sent in the request headers for easy access and security.

Explain how webhook signature verification works step-by-step.

Think about how you check a secret code to trust a message.

Why is it important to keep the secret key safe in webhook signature verification?

Imagine the secret key as a password for trust.

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