docs: function _jws_getHashAlgFromParsedHead(head) {
  	var sigAlg = head["alg"];
  	var hashAlg = "";
  
  	if (sigAlg != "RS256" && sigAlg != "RS512" &&
  	    sigAlg != "PS256" && sigAlg != "PS512")
  	    throw "JWS signature algorithm not supported: " + sigAlg;
  	if (sigAlg.substr(2) == "256") hashAlg = "sha256";
  	if (sigAlg.substr(2) == "512") hashAlg = "sha512";
  	return hashAlg;
      }