Ciphertext-only attack on the JTC-based cryptosystem with sizes and locations of the plaintext and the key code unknown.

We demonstrate an effective ciphertext-only attack (COA) method on a joint transform correlator-based cryptosystem with sizes and locations of the plaintext and the key code unknown. In this paper, we first obtain the signal domain support by using the method, which is based on the geometry of the autocorrelation support of the object. And then, the hybrid input-output algorithm is used to retrieve the plaintext. Compared with the work on COA recently reported by Zhang et al., our proposed method needs fewer resources and much fewer iteration times for binary image to retrieve the plaintext. Simulation results are provided to demonstrate the validity of the presented method.