  We consider the generation of a secret key (SK) by the inputs and the output
of a secure multipleaccess channel (MAC) that additionally have access to a
noiseless public communication channel. Under specific restrictions on the
protocols, we derive various upper bounds on the rate of such SKs.
Specifically, if the public communication consists of only the feedback from
the output terminal, then the rate of SKs that can be generated is bounded
above by the maximum symmetric rate $R_f^\ast$ in the capacity region of the
MAC with feedback. On the other hand, if the public communication is allowed
only before and after the transmission over the MAC, then the rate of SKs is
bounded above by the maximum symmetric rate $R^\ast$ in the capacity region of
the MAC without feedback. Furthermore, for a symmetric MAC, we present a scheme
that generates an SK of rate $R_f^\ast$, improving the best previously known
achievable rate $R^\ast$. An application of our results establishes the SK
capacity for adder MAC, without any restriction on the protocols.
