  A simple four node network in which cooperation improves the
information-theoretic secrecy is studied. The channel consists of two senders,
a receiver, and an eavesdropper. One or both senders transmit confidential
messages to the receiver, while the eavesdropper tries to decode the
transmitted message. The main result is the derivation of a newly achievable
rate-equivocation region that is shown to be larger than a rate-equivocation
region derived by Lai and El Gamal for the relay-eavesdropper channel. When the
rate of the helping interferer is zero, the new rate-equivocation region
reduces to the capacity-equivocation region over the wire-tap channel, hence,
the new achievability scheme can be seen as a generalization of a coding scheme
proposed by Csiszar and Korner. This result can naturally be combined with a
rate-equivocation region given by Tang et al. (for the interference assisted
secret communication), yielding an even larger achievable rate-equivocation
region.
