Secure authentication of classical messages with decoherence-free states

We present an efficient scheme for secure authentication of classical messages with decoherence-free states and hash functions. We also analyze the security of this scheme and show it is not possible to forge valid message authentication codes. In addition, the lengths of the authentication key and message authentication codes are invariant no mater how long the length of the message to be authenticated is. Moreover, although this scheme uses entangled states for encoding, only single-particle product measurements are required.