  We investigate the effects of user cooperation on the secrecy of broadcast
channels by considering a cooperative relay broadcast channel. We show that
user cooperation can increase the achievable secrecy region. We propose an
achievable scheme that combines Marton's coding scheme for broadcast channels
and Cover and El Gamal's compress-and-forward scheme for relay channels. We
derive outer bounds for the rate-equivocation region using auxiliary random
variables for single-letterization. Finally, we consider a Gaussian channel and
show that both users can have positive secrecy rates, which is not possible for
scalar Gaussian broadcast channels without cooperation.
