Signed requests

Sign events and webhooks

Signing requests

Both webhooks and events can be signed for extra security.

Signing webhooks

For webhooks if you'd like the requests to be signed to ensure that they are authentic requests from formsort, you can enable the sign security. Read more about creating a webhook here.

Signing events

All events for variant_revision_published are signed by default. The signature for verfication can be found on the "Events" page.

Click "Show signing key"

Note: The signing key is generated by Formsort, and is different across events and webhooks.

How to verify a signature

To generate the signature for verification, use the original HTTP request body (aka JSON payload) and:

1. Hash the request body with SH256, encrypting it with signing key.

   1. You can obtain the key in the Integrations tab when this option is enabled.

2. Base64 encode the result in a URL-safe-way, using - and _ instead of the + and / characters. This is necessary as Formsort sends the signature in an HTTP header.

3. Remove the trailing = signs. Those are often generated by the hashing libraries as padding.

Sample implementations of the signature code follow:

Python

import base64
import hashlib
import hmac


def as_bytes(v):
    return v.encode("utf8")


def hmac_sign(signing_key, original_request_body):  # unmodified, a jsonified string
    key = as_bytes(signing_key)
    message = as_bytes(original_request_body)
    return (
        # Note:
        # 1. for url safety, use - and _ characters instead of + and / respectively
        # 2. remove the padding = signs at the end of the signature
        base64.urlsafe_b64encode(
            hmac.new(key, message, hashlib.sha256).digest())
            .rstrip(b"=")
            .decode("utf8")
    )

Javascript (Express)

To verify a signature in Javascript, the original request buffer can be verified with the signing key like follows:

import crypto from 'crypto';

const hmacSign = (signingKey, originalRequestBuffer) => {
    const key = Buffer.from(signingKey, 'utf8');
    return (
        crypto
            .createHmac('sha256', key)
            .update(originalRequestBuffer)
            .digest('base64')
            .replace(/\+/g, '-')
            .replace(/\//g, '_')
            .replace(/=+$/, '')
    );
}

If you are using express as your webserver, and have enabled JSON body parsing with express.json() be careful that you are signing the original body buffer and not the parsed JSON. Signing a stringified req.body is not reliable due to differences in whitespace between implementations and settings.

The simplest way to do this is to ensure that routes for receiving Formsort webhooks do not apply the JSON middleware.

Using the hmacSign function above, assuming a SIGNING_KEY environment variable, the following will sign requests from Formsort:

import crypto from 'crypto';
import express from 'express';
import bodyParser from 'body-parser';
const PORT = 8080;

const app = express();

app.use((req, res, next) => {
  if (req.header('x-formsort-secure') === 'sign') {
    // Signed formsort requests require verification of the raw body content
    bodyParser.raw({ type: 'application/json' })(req, res, next);
  } else {
    express.json()(req, res, next);
  }
});

const hmacSign = (signingKey, originalRequestBuffer) => {
    const key = Buffer.from(signingKey, 'utf8');
    return (
        crypto
            .createHmac('sha256', key)
            .update(originalRequestBuffer)
            .digest('base64')
            .replace(/\+/g, '-')
            .replace(/\//g, '_')
            .replace(/=+$/, '')
    );
}

app.post('/formsort-webhook', (req, res) => {
  const signature = hmacSign(process.env.SIGNING_KEY, req.body);
  if (signature !== req.header('x-formsort-signature')) {
    return res.status(401).json({ message: 'Formsort signature mismatch' });
  }

  const answers = JSON.parse(req.body);
  return res.json({
    message: 'Well now, it looks like you are all good.',
    answers,
  });
});

app.listen(PORT, () => {
  console.log(`listening on port ${PORT}`);
});

Note

When signing is enabled, Formsort will send the following additional HTTP headers on webhook requestsIf signature is enabled, then X-Formsort-Secure will have the value of "sign" and X-Formsort-Signature will have the signature itself.

Header	Value
X-Formsort-Secure	sign
X-Formsort-Signature	{the hash}