# https://developers.facebook.com/docs/instagram-platform/webhooks/examples#message
expectedSignature = split(to_string!(.headers."x-hub-signature-256"), "=")[1]
actualSignature = encode_base16(hmac(to_string!(.body), to_string!(.secrets.SANDBOX_CLIENT_SECRET)))
if actualSignature != expectedSignature {
  null
} else {
  body = parse_json!(.body)
  messagingEntry = body.entry[0].messaging[0]
  message = messagingEntry.message
  identifier = if to_bool!(message.is_echo) {
    null
  } else {
    messagingEntry.sender.id
  }
  identifier
}