  In this work, a class of information theoretic secrecy problems is addressed
where the eavesdropper channel states are completely unknown to the legitimate
parties. In particular, MIMO wiretap channel models are considered where the
channel of the eavesdropper is arbitrarily varying over time. Assuming that the
number of antennas of the eavesdropper is limited, the secrecy rate of the MIMO
wiretap channel in the sense of strong secrecy is derived, and shown to match
with the converse in secure degrees of freedom. It is proved that there exists
a universal coding scheme that secures the confidential message against any
sequence of channel states experienced by the eavesdropper. This yields the
conclusion that secure communication is possible regardless of the location or
channel states of (potentially infinite number of) eavesdroppers. Additionally,
it is observed that, the present setting renders the secrecy capacity problems
for multi-terminal wiretap-type channels more tractable as compared the case
with full or partial knowledge of eavesdropper channel states. To demonstrate
this observation, secure degrees of freedom regions are derived for the
Gaussian MIMO multiple access wiretap channel (MIMO MAC-WT) and the Gaussian
MIMO broadcast wiretap channel (MIMO BC-WT) where the transmitter(s) and the
intended receiver(s) have the same number of antennas.
