Checking webhook signatures (HMACs)
When Kindly sends data to external services (e.g. when triggering a webhook to a service owned by you), the payload will be authenticated with a hash-based message authentication code (HMAC).
The key used to create the HMAC is a shared secret, and you verify it by running the algorithm yourself with the payload and the key to re-create the HMAC.
This verifies that the sender of the message knows the secret key.
The secret key used in the algorithm depends on which kind of request is being sent, and you can find your keys in the Kindly web interface.
If you suspect that the key might have been compromised, you can revoke it and issue a new key in the web interface.
The HMAC is created with the HMAC_SHA256 algorithm, then encoded in base 64.
The HMAC is attached to the request in the Kindly-HMAC header. You should also read the value of the Kindly-HMAC-algorithm request header and verify that it matches HMAC-SHA-256 (base64 encoded). If it becomes necessary to change the algorithm in the future, this value will change to let you know.
Verifying a HMAC-SHA256-value should be quite simple by using a crypto library in your preferred language.
The hashing algorithm works on bytes, so we make sure to do any necessary conversions from unicode str to bytes.
Depending on your setup you may need to rewind the request body stream after consuming it (see code comment).