  In this paper, we present a signaling design for secure integrated sensing
and communication (ISAC) systems comprising a dual-functional multi-input
multi-output (MIMO) base station (BS) that simultaneously communicates with
multiple users while detecting targets present in their vicinity, which are
regarded as potential eavesdroppers. In particular, assuming that the
distribution of each parameter to be estimated is known \textit{a priori}, we
focus on optimizing the targets' sensing performance. To this end, we derive
and minimize the Bayesian Cram\'er-Rao bound (BCRB), while ensuring certain
communication quality of service (QoS) by exploiting constructive interference
(CI). The latter scheme enforces that the received signals at the eavesdropping
targets fall into the destructive region of the signal constellation, to
deteriorate their decoding probability, thus enhancing the ISAC's system
physical-layer security (PLS) capability. To tackle the nonconvexity of the
formulated problem, a tailored successive convex approximation method is
proposed for its efficient solution. Our extensive numerical results verify the
effectiveness of the proposed secure ISAC design showing that the proposed
algorithm outperforms block-level precoding techniques.
