  We consider a relay channel where a relay helps the transmission of messages
from one sender to one receiver. The relay is considered not only as a sender
that helps the message transmission but as a wire-tapper who can obtain some
knowledge about the transmitted messages. In this paper we study the coding
problem of the relay channel under the situation that some of transmitted
messages are confidential to the relay. A security of such confidential
messages is measured by the conditional entropy. The rate region is defined by
the set of transmission rates for which messages are reliably transmitted and
the security of confidential messages is larger than a prescribed level. In
this paper we give two definition of the rate region. We first define the rate
region in the case of deterministic encoder and call it the deterministic rate
region. Next, we define the rate region in the case of stochastic encoder and
call it the stochastic rate region. We derive explicit inner and outer bounds
for the above two rate regions and present a class of relay channels where two
bounds match. Furthermore, we show that stochastic encoder can enlarge the rate
region. We also evaluate the deterministic rate region of the Gaussian relay
channel with confidential messages.
