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Verifying Webhooks Manually

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The recommended way to verify webhooks is using our official libraries as outlined in the How to Verify section.

However, here are instructions for verifying manually for those who need it.

Verifying signatures manually​

Each webhook call includes three headers with additional information that are used for verification:

  • svix-id: the unique message identifier for the webhook message. This identifier is unique across all messages, but will be the same when the same webhook is being resent (e.g. due to a previous failure).
  • svix-timestamp: timestamp in seconds since epoch.
  • svix-signature: the Base64 encoded list of signatures (space delimited).
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Professional tier customers can have the headers white-labeled to use the webhook- prefix instead of the svix- prefix used above. The Svix libraries support both.

Constructing the signed content​

The content to sign is composed by concatenating the id, timestamp and payload, separated by the full-stop character (.). In code, it will look something like:

signedContent = "${svix_id}.${svix_timestamp}.${body}"

Where body is the raw body of the request. The signature is sensitive to any changes, so even a small change in the body will cause the signature to be completely different. This means that you should not change the body in any way before verifying.

Determining the expected signature​

Svix uses an HMAC with SHA-256 to sign its webhooks.

So to calculate the expected signature, you should HMAC the signed_content from above using the base64 portion of your signing secret (this is the part after the whsec_ prefix) as the key. For example, given the secret whsec_MfKQ9r8GKYqrTwjUPD8ILPZIo2LaLaSw you will want to use MfKQ9r8GKYqrTwjUPD8ILPZIo2LaLaSw.

For example, this is how you can calculate the signature in Node.js:

const crypto = require('crypto');

signedContent = `${svix_id}.${svix_timestamp}.${body}`
const secret = "whsec_5WbX5kEWLlfzsGNjH64I8lOOqUB6e8FH";

// Need to base64 decode the secret
const secretBytes = new Buffer(secret.split('_')[1], "base64");
const signature = crypto
  .createHmac('sha256', secretBytes)
  .update(signedContent)
  .digest('base64');
console.log(signature);

This generated signature should match one of the ones sent in the svix-signature header.

The svix-signature header is composed of a list of space delimited signatures and their corresponding version identifiers. The signature list is most commonly of length one. Though there could be any number of signatures. For example:

v1,g0hM9SsE+OTPJTGt/tmIKtSyZlE3uFJELVlNIOLJ1OE= v1,bm9ldHUjKzFob2VudXRob2VodWUzMjRvdWVvdW9ldQo= v2,MzJsNDk4MzI0K2VvdSMjMTEjQEBAQDEyMzMzMzEyMwo=

Make sure to remove the version prefix and delimiter (e.g. v1,) before verifying the signature.

Please note that to compare the signatures it's recommended to use a constant-time string comparison method in order to prevent timing attacks.

Verify timestamp​

As mentioned above, Svix also sends the timestamp of the attempt in the svix-timestamp header. You should compare this timestamp against your system timestamp and make sure it's within your tolerance in order to prevent timestamp attacks.