  In conventional cryptography, information-theoretically secure message
authentication can be achieved by means of universal hash functions, and
requires that the two legitimate users share a random secret key, which is
twice as long as the message. We address the question as of whether quantum
resources can offer any advantage over classical unconditionally secure message
authentication codes. It is shown that passive prepare-and-measure quantum
message-authentication schemes cannot do better than their classical
counterparts. Subsequently we present an interactive entanglement-assisted
scheme, which ideally allows for the authentication of classical messages with
a classical key, which is as long as the message.
