Double image encryption method using the Arnold transform in the fractional Hartley domain

A new method for double image encryption based on the fractional Hartley transform (FrHT) and the Arnold transform (AT) is proposed in this work. The encryption method encodes the first input image in amplitude and the second input image is encoded in phase, in order to define a complex image. This complex image is successively four times transformed using FrHT and AT, and the resulting complex image represents the encrypted image. The decryption method is the same method as the encryption method applied in the inverse sense. The AT is a process of image shearing and stitching in which pixels of the image are rearranged. This AT is used in the encryption method with the purpose of spreading the information content of the two input images onto the encrypted image and to increase the security of the encrypted image. The fractional orders of the FrHTs and the parameters of the ATs correspond to the keys of the encryption-decryption method. Only when all of those keys are correct in the decryption method, the two original images can be recovered. We present digital results that confirm our approach.