  This paper focuses on secure backscatter transmission in the presence of a
passive multi-antenna eavesdropper through a symbiotic radio (SR) network.
Specifically, a single-antenna backscatter device (BD) aims to transmit
confidential information to a primary receiver (PR) by using a multi-antenna
primary transmitter's (PT) signal, where the received symbols are jointly
decoded at the PR. Our objective is to achieve confidential communications for
BD while ensuring that the primary system's quality of service (QoS)
requirements are met. We propose an alternating optimisation algorithm that
maximises the achievable secrecy rate of BD by jointly optimising primary
transmit beamforming and power sharing between information and artificial noise
(AN) signals. Numerical results verify our analytical claims on the optimality
of the proposed solution and the proposed methodology's underlying low
complexity. Additionally, our simulations provide nontrivial design insights
into the critical system parameters and quantify the achievable gains over the
relevant benchmark schemes.
