code: function core_hmac_sha256(key, data)
  {
      var bkey = str2binb(key);
      if(bkey.length > 16) bkey = core_sha256(bkey, key.length * chrsz);
  
      var ipad = Array(16), opad = Array(16);
      for(var i = 0; i < 16; i++) {
          ipad[i] = bkey[i] ^ 0x36363636;
          opad[i] = bkey[i] ^ 0x5C5C5C5C;
      }
      var hash = core_sha256(ipad.concat(str2binb(data)), 512 + data.length * chrsz);
      return core_sha256(opad.concat(hash), 512 + 256);
  }
docs: /*
  Calculate the HMAC-SHA256 of a key and some data