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We propose an improvement method for an asymmetric cryptosystem based on spherical wave illumination. Compared with the phase-truncated Fourier transform-based cryptosystem and the reported improving methods, the encryption process uses a spherical wave to illuminate the encryption system, rather than a uniform plane wave. As a result, the proposed method can avoid various types of the currently existing attacks and maintain the asymmetric characteristic of the cryptosystem. Moreover, due to only changing the illuminating mode, the proposed method can be easily implemented in optics compared with the reported improving methods. Simulation results are presented to demonstrate the feasibility and the security performance of the proposed method.