The main drawback of quantum key distribution is that it usually relies on having an authenticated classical channel of communications. In modern cryptography, having an authenticated classical channel means that one has either already exchanged a symmetric key of sufficient length or public keys of sufficient security level. With such information already available, one can achieve authenticated and secure communications without using QKD, such as by using the Galois/Counter Mode of the Advanced Encryption Standard. Thus QKD does the work of a Stream Cipher at many times the cost. Noted security expert Bruce Schneier remarked that quantum key distribution is "as useless as it is expensive".