code: function rstr_hmac(key, data) {
          var bkey, ipad, opad, hash, i;
  
          key = (utf8) ? utf8Encode(key) : key;
          data = (utf8) ? utf8Encode(data) : data;
          bkey = rstr2binl(key);
          if (bkey.length > 16) {
            bkey = binl(bkey, key.length * 8);
          }
  
          ipad = Array(16), opad = Array(16);
          for (i = 0; i < 16; i += 1) {
            ipad[i] = bkey[i] ^ 0x36363636;
            opad[i] = bkey[i] ^ 0x5C5C5C5C;
          }
          hash = binl(ipad.concat(rstr2binl(data)), 512 + data.length * 8);
          return binl2rstr(binl(opad.concat(hash), 512 + 128));
        }
docs: Calculate the HMAC-MD5, of a key and some data (raw strings)