Verifying a webhook

If you have created your webhook subscription using a signingKey you can validate the authenticity of the webhook by validating the request signature sent in the request header.

From the incoming webhook, parse the following request headers:

messagebird-signature

messagebird-request-timestamp

In addition, parse the request URL and the request Body.

To calculate the request signature:

1. Base64 decode the messagebird-signature header;

2. Create a SHA256 hash checksum of the request body as a binary result;

3. Join the request timestamp (messagebird-request-timestamp header) with the request URL and request body checksum computed in step 2, separated by a new line (\n);

4. Calculate HMACSHA256 using the signing key as the secret and the joined payload from step 3 to calculate the signature;

5. Compare the output of step 4 to the signature from step 1;

Examples

The code examples below are pseudo code to help explain what needs to happen. We advise you to amend the example of your preferred language to what suits your codebase or framework.

PHP

<?php

function verifyWebhookSignature(
    string $headerSignature,
    string $headerTimestamp,
    string $receivedBody,
    string $requestedUrl,
    string $signingKey
): bool {
    $receivedDecodedSignature = base64_decode($headerSignature);

    $bodyHash = hash('sha256', $receivedBody, true);
    $computedSignature = hash_hmac(
        'sha256',
        sprintf("%s\n%s\n%s", $headerTimestamp, $requestedUrl, $bodyHash),
        $signingKey,
        true
    );

    return hash_equals($computedSignature, $receivedDecodedSignature);
}

// Example usage
$isVerified = verifyWebhookSignature(
    $_SERVER['HTTP_MESSAGEBIRD_SIGNATURE'],
    $_SERVER['HTTP_MESSAGEBIRD_REQUEST_TIMESTAMP'],
    file_get_contents('php://input'),
    'https://domain.com/webhook/bird',
    'secureSigningKey'
);

Golang

package main

import (
    "bytes"
    "crypto/hmac"
    "crypto/sha256"
    "encoding/base64"
    "fmt"
    "os"
)

func main() {
    var key string
    var signatureHeader string
    var timestampHeader string
    var url string
    var body []byte

    // Example usage
    match, err := checkSignature(key, signatureHeader, timestampHeader, url, body)
    if err != nil {
       // Error out
       fmt.Printf("something went wrong: %v", err)
       os.Exit(1)
    }

    if !match {
       // Signature doesn't match
       fmt.Println("Signatures don't match!")
    } else {
       // Signature matches
       fmt.Println("Signatures match!")
    }
}

// checkSignature verifies the Bird style HMAC-SHA256 signature for the given data.
func checkSignature(signingKey, signatureHeader, timestampHeader, requestURL string, data []byte) (bool, error) {
    // Step 1: Decode the signature from the header
    actualSignature, err := base64.StdEncoding.DecodeString(signatureHeader)
    if err != nil {
       return false, err
    }

    // Step 2 - 4: Calculate the expected signature
    expectedSignature, err := signSha256(signingKey, timestampHeader, requestURL, data)
    if err != nil {
       return false, err
    }

    // Step 5: Compare the expected signature with the actual signature
    return hmac.Equal(expectedSignature, actualSignature), nil
}

// signSha256 returns the Bird style HMAC-SHA1 signature for the given data.
func signSha256(signingKey, timestamp, requestURL string, data []byte) ([]byte, error) {
    // Step 2: Calculate the SHA256 hash for the given data
    bh := sha256.Sum256(data)

    // Step 3: Concatenate the timestamp, request URL, and the SHA256 hash, separated by a newline character
    var m bytes.Buffer
    _, err := fmt.Fprintf(&m, "%s\n%s\n%s", timestamp, requestURL, bh)
    if err != nil {
       return []byte{}, err
    }

    // Step 4: Calculate the HMAC-SHA256 hash for the concatenated string using the signing key
    mac := hmac.New(sha256.New, []byte(signingKey))
    if _, err := mac.Write(m.Bytes()); err != nil {
       return []byte{}, err
    }
    return mac.Sum(nil), nil
}

Python

import hashlib
import hmac
import base64
import time

class SignedRequest:
    def __init__(self, requestSignature, requestTimestamp, requestBody, requestUrl):
        self._requestSignature = requestSignature
        self._requestTimestamp = str(requestTimestamp)
        self._requestBody = requestBody
        self._requestUrl = requestUrl

    def verify(self, signing_key):
        payload = self._build_payload()
        expected_signature = base64.b64decode(self._requestSignature)
        calculated_signature = hmac.new(signing_key.encode('latin-1'), payload.encode('latin-1'),
                                        hashlib.sha256).digest()
        return expected_signature == calculated_signature

    def is_recent(self, offset=10):
        return int(time.time()) - int(self._requestTimestamp) < offset

    def _build_payload(self):
        checksum_body = hashlib.sha256(self._requestBody.encode('latin-1')).digest()
        str_checksum_body = checksum_body.decode('latin-1')
        parts = [self._requestTimestamp, self._requestUrl, str_checksum_body]
        return "\n".join(parts)