Trying to do HMAC SHA256 for SwitchBot API -- need help!

I am trying to do a HMAC SHA256 digest in order to get access to the SwitchBot API, but there is not enough libs available to pull this off…Or, I am not looking in the right place.

Here is the doc for the SwitchBot API. I am trying to adapt it to Skylark and the skimp stdlibs. I need some help, please.