  For the multiterminal secret key agreement problem, new single-letter lower
bounds are obtained on the public discussion rate required to achieve any given
secret key rate below the secrecy capacity. The results apply to general source
model without helpers or wiretapper's side information but can be strengthened
for hypergraphical sources. In particular, for the pairwise independent
network, the results give rise to a complete characterization of the maximum
secret key rate achievable under a constraint on the total discussion rate.
