Eavesdropping in quantum cryptography with mixed states

We discuss the six-state quantum cryptographic protocol in the presence of noise, and compare the mutual information between sender/receiver and sender/Eve with the case of pure states. We show that for the case of mixed states the mutual information in both cases (sender/receiver and sender/Eve) is less than for pure states, and depends on the noise parameter. We find the crossing point of the two mutual information curves as a function of the amount of noise, and investigate the question whether the optimal eavesdropping strategy for pure states is the same as for mixed states.