Arbitrary jamming can preclude secure communication

We investigate the effect of certain active attacks on the secrecy capacity of wiretap channels by considering arbitrarily varying wiretap channels. We establish a lower bound for the secrecy capacity with randomized coding of a class of such channels and an upper bound for that of all such channels. We show that if the arbitrarily varying wiretap channel possesses a bad ¿averaged¿ state, namely one in which the legitimate receiver is degraded with respect to the eavesdropper, then secure communication is not possible.