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We investigate a relay network where a multi-antenna source can potentially utilize an unauthenticated (untrusted) relay to augment its direct transmission of a confidential message to the destination. Since the relay is untrusted, it is desirable to protect the confidential data from it while simultaneously making use of it to increase the reliability of the transmission. We present a low-complexity scheme denoted as imbalanced beamforming based on linear beamforming and constellation mapping that ensures perfect physical-layer security even while utilizing the untrusted relay. Furthermore, the security of the scheme holds even if the relay adopts the conventional decode-and-forward protocol, unlike prior work. Simulation results show that the proposed imbalanced signaling maintains a constant BER of 0.5 at the eavesdropper at any SNR and number of source antennas, while maintaining or improving the detection performance of the destination compared to not utilizing the relay or existing security methods. [ABSTRACT FROM PUBLISHER]