Security aspects of the authentication used in quantum cryptography

Unconditionally secure message authentication is an important part of Quantum Cryptography (QC). We analyze security effects of using a key obtained from QC in later rounds of QC. It has been determined earlier that partial knowledge of the key in itself does not incur a security problem. However, by accessing the quantum channel used in QC, the attacker can change the message to be authenticated. This, together with partial knowledge of the key does incur a security weakness of the authentication. We suggest a simple solution to this problem, and stress usage of this or an equivalent extra security measure in QC.