Hrrm, now you're in a situation where you're using part of the unverified payload to look up part of the secret before validating the signature and hence the pa

Hrrm, now you're in a situation where you're using part of the unverified payload to look up part of the secret before validating the signature and hence the payload. This is a neat idea but it introduces a whole class of potential vulnerabilities. (This is similar to the argument against the algorithm being selected from the unverified payload before being used to validate the signature per the JWT specification.)