Ehsan Ardestanizadeh, Massimo Franceschetti, Tara Javidi, and Young-Han Kim
IEEE Transactions on Information Theory, vol. 55, no. 12, pp. 5353–5361, December 2009.
Preliminary results appeared in Proceedings of IEEE International Symposium on Information Theory, pp. 101–105, Toronto, Canada, July 2008.
This paper studies the problem of secure communication over a wiretap
channel with a secure feedback link of rate
, where
is the channel input, and
and
are channel outputs observed
by the legitimate receiver and the eavesdropper, respectively. It is
shown that the secrecy capacity, the maximum data rate of reliable
communication while the intended message is not revealed to the
eavesdropper, is upper bounded as
The proof of the bound crucially depends on a recursive argument which
is used to obtain the single-letter characterization. This upper bound
is shown to be tight for the class of physically degraded wiretap
channels. A capacity-achieving coding scheme is presented for this
case, in which the receiver securely feeds back fresh randomness with
rate , generated emph{independent} of the received channel
output symbols. The transmitter then uses this shared randomness as a
secret key on top of Wyner's coding scheme for wiretap channels
without feedback. Hence, when a feedback link is available, the
receiver should allocate all resources to convey a new key rather than
sending back the channel output.