Webhook signature verification in Rest API - Step-by-Step Execution

Concept Flow - Webhook signature verification

Receive webhook request

↓

Extract signature from headers

↓

Compute expected signature using secret and payload

↓

Compare expected signature with received signature

↓

Process webhook

The server receives a webhook, extracts the signature, computes the expected signature using a secret and payload, then compares both to verify authenticity.

Execution Sample

Rest API

import hmac
import hashlib

def verify_signature(secret, payload, signature):
    expected = hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest()
    return hmac.compare_digest(expected, signature)

This code checks if the webhook signature matches the expected signature computed with a secret key and payload.

Execution Table

Step	Action	Input	Computed Value	Comparison Result	Decision
1	Receive webhook	payload='data', signature='abc123'	-	-	-
2	Extract signature	signature='abc123'	abc123	-	-
3	Compute expected signature	secret='key', payload='data'	expected='def456'	-	-
4	Compare signatures	expected='def456', received='abc123'	-	No Match	Reject request
5	End	-	-	-	Stop processing

💡 Signatures do not match, so the webhook request is rejected.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
secret	key	key	key	key
payload	data	data	data	data
signature	-	abc123	abc123	abc123
expected	-	def456	def456	def456
comparison_result	-	-	No Match	No Match
decision	-	-	Reject request	Reject request

Key Moments - 2 Insights

Why do we compute the expected signature instead of trusting the received one?

Because the received signature could be fake. Computing the expected signature with the secret ensures the webhook is from a trusted source, as shown in step 3 and 4 of the execution_table.

What happens if the signatures match?

If they match, the webhook is accepted and processed. This is the 'Match' branch in the concept_flow diagram, opposite to the 'No Match' path shown in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'expected' after step 3?

A def456

B key

C abc123

D data

💡 Hint

Check the 'Computed Value' column at step 3 in the execution_table.

At which step does the program decide to reject the webhook?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Decision' column in the execution_table.

If the received signature matched the expected signature, what would the decision be?

A Compute signature again

B Reject request

C Process webhook

D Stop processing

💡 Hint

Refer to the concept_flow diagram where 'Match' leads to 'Process webhook'.

Concept Snapshot

Webhook signature verification:
- Receive webhook with payload and signature
- Compute expected signature using secret + payload
- Compare expected and received signatures
- If match, process webhook
- If no match, reject request

Full Transcript

Webhook signature verification is a process to confirm that a webhook request is from a trusted source. When the server receives a webhook, it extracts the signature sent in the headers. Then, it computes the expected signature by using a secret key and the payload data with a hashing function. The server compares the expected signature with the received one. If they match, the webhook is accepted and processed. If not, the webhook is rejected to prevent unauthorized actions. This ensures security by verifying authenticity before processing.

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