  In many previous works, a single-pixel imaging (SPI) system is 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, little cryptanalysis about
encrypted SPI has been investigated in the past. In this work, we propose a
known-plaintext attack scheme and a ciphertext-only attack scheme to 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. The two schemes can
crack encrypted SPI systems and successfully recover the key containing correct
illumination patterns.
