A novel information security system is developed, employing quadrature multiplexing, to be implemented using optoelectronic devices. Two information signals are encrypted using the same code but two orthogonal functions, and then they are multiplexed together in the same domain. With orthogonal functions having zero cross-correlation between them, the encrypted information remains unaffected. Each encryption and multiplexing process can accommodate two information signals for a single code and hence for a single storage cell or transmission channel. The same process can be performed in multiple steps to increase the multiplexing capability of the system. The proposed security system can enhance the storage capacity and/or maximize the utilization of the available transmission bandwidth. For decryption, the composite encoded signal is correlated using the appropriate code and function. Computer simulation results show that the proposed security system is capable of dealing with both binary and gray-level images with the encrypted images remaining secure.