Secure Identification for Wiretap Channels; Robustness, Super-Additivity and Continuity

We determine the identification capacity of compound channels in the presence of a wiretapper. It turns out that the secure identification capacity formula fulfills a dichotomy theorem: It is positive and equals the identification capacity of the channel if its message transmission secrecy capacity is positive. Otherwise, the secure identification capacity is zero. Thus, we show in the case that the secure identification capacity is greater than zero we do not pay a price for secure identification, i.e., the secure identification capacity is equal to the identification capacity. This is in strong contrast to the transmission capacity of the compound wiretap channel. We then use this characterization to investigate the analytic behavior of the secure identification capacity. In particular, it is practically relevant to investigate its continuity behavior as a function of the channels. We completely characterize this continuity behavior. We analyze the (dis-) continuity and (super-) additivity of the capacities. In 1998, N. Alon gave a conjecture about maximal violation for the additivity of capacity functions in graphs. We show that this maximal violation as well holds for the secure identification capacity of compound wiretap channels. This is the first example of a capacity function exhibiting this behavior.