In many previous works, a single-pixel imaging (SPI) system has been constructed as an optical image encryption system. Unauthorized users are not able to reconstruct the plaintext image from the ciphertext intensity sequence without knowing the illumination pattern key. However, the cryptanalysis of encrypted SPI has been seldom investigated in the past. In this work, we propose a known-plaintext attack scheme and a ciphertext-only attack scheme for an encrypted SPI system for the first time. The known-plaintext attack is implemented by interchanging the roles of illumination patterns and object images in the SPI model. The ciphertext-only attack is implemented based on the statistical features of single-pixel intensity values under certain circumstances. The two schemes can crack encrypted SPI systems and successfully recover the key containing correct illumination patterns.
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