code: func decode(value []byte) ([]byte, error) {
  	decoded := make([]byte, base64.URLEncoding.DecodedLen(len(value)))
  	b, err := base64.URLEncoding.Decode(decoded, value)
  	if err != nil {
  		return nil, cookieError{cause: err, typ: decodeError, msg: "base64 decode failed"}
  	}
  	return decoded[:b], nil
  }
docs: // decode decodes a cookie using base64.