# https://developers.facebook.com/docs/messenger-platform/reference/webhook-events#format
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]
  messagingEntry.sender.id
}