Eavesdropping in quantum cryptography with six mixed states

For the case of the six state protocol in the presence of white noise (six mixed states) we express the optimal mutual information for both cases Alice/Bob and Alice/Eve in terms of the noise parameter and the quantum bit error rate. In comparison to the pure state case the crossing point of the two mutual information curves moves to a higher quantum bit error rate. We conclude that the six state protocol with mixed states is more robust against eavesdropping than for pure states