code: func New(login string, expires time.Time, secret []byte) string {
  	if login == "" {
  		return ""
  	}
  	llen := len(login)
  	b := make([]byte, llen+4+32)
  	// Put expiration time.
  	binary.BigEndian.PutUint32(b, uint32(expires.Unix()))
  	// Put login.
  	copy(b[4:], []byte(login))
  	// Calculate and put signature.
  	sig := getSignature([]byte(b[:4+llen]), secret)
  	copy(b[4+llen:], sig)
  	// Base64-encode.
  	return base64.URLEncoding.EncodeToString(b)
  }
docs: // New returns a signed authentication cookie for the given login,
  // expiration time, and secret key.
  // If the login is empty, the function returns an empty string.