  We study quantum secure direct communication by using a general preshared
quantum state and a generalization of dense coding. In this scenario, Alice is
allowed to apply a unitary on the preshared state to encode her message, and
the set of allowed unitaries forms a group. To decode the message, Bob is
allowed to apply a measurement across his own system and the system he
receives. In the worst scenario, we guarantee that Eve obtains no information
for the message even when Eve access the joint system between the system that
she intercepts and her original system of the preshared state. For a practical
application, we propose a concrete protocol and derive an upper bound of
information leakage in the finite-length setting. We also discuss how to apply
our scenario to the case with discrete Weyl-Heisenberg representation when the
preshared state is unknown.
