fn := func(accessKey, secretKey, audience string) (s string, err error) {
		k := cacheKey{accessKey: accessKey, secretKey: secretKey, audience: audience}

		var ok bool
		s, ok = cacheLRU.Get(k)
		if !ok {
			s, err = authenticateNode(accessKey, secretKey, audience)
			if err != nil {
				return "", err
			}
			cacheLRU.Add(k, s)
		}
		return s, nil
	}