Validating Twilio Authy Callbacks

When using Webhooks with push authentications, Twilio will send a callback to your application's exposed URL when a user interacts with your ApprovalRequest. While testing, you can accept all incoming webhooks, but in production, you'll need to verify the authenticity of incoming requests.

Twilio sends an HTTP Header X-Authy-Signature with every outgoing request to your application. X-Authy-Signature is a HMAC signature of the full message body sent from Twilio hashed with your Application API Key (from Authy in the Twilio Console).

You can find complete code snippets here on Github.

Verify a Twilio Authy Callback

Checking the authenticity of the X-Authy-Signature HTTP Header is a 6 step process.

• Create a string using the Webhook URL without any parameters

• Flatten the received JSON body and sort this list in case-sensitive order and convert them to URL format

• Grab the nonce from the X-Authy-Signature HTTP Header

• Join the nonce, HTTP method ('POST'), and the sorted parameters together with the vertical pipe, ('|') character

• Use HMAC-SHA256 to hash the string using your Application API Key

• Base64 Encode the digest (as described in RFC 4648 - do not include line breaks)

Here is every step summarized so you can get an idea of the whole process.

Once you have encoded the digest, you can compare the resulting string with the X-Authy-Signature HTTP Header. If they match, the incoming request is from Twilio. If there is a mismatch, you should reject the request as fraudulent.