The filtered hologram (Oi) can be securely encoded because the phase distribution ofFourier-transformed image (Fi) is subjected to phase modulation by applying RPM in the Fourier domain using the DRPE. We can finally obtain four phase-encoded images byapplying this procedure to the other holograms. The pixel in the 2nd, 3rd, and 4th phaseencoded images that is coincident with a pixel in the 1st phase-encoded image is expanded and then phase encoded with the 2nd, 3rd, and 4th rows of the same Hadamard matrix, respectively.The four holograms are phase encoded by applying the four BPMs, which are generated from four rows of the 4-order Hadamard matrices, respectively, and are called ‘a set of four BPMs’. These four phase-encoded images are superimposed to get a single complex multiplexed image, as shown in Fig. 4(e).As described above, similarly to the process of storing four encrypted holograms as a single multiplexed image, the n encrypted holograms can be stored as a single multiplexed image by using ‘a set of n BPMs’, which is generated from n-order Hadamard matrices. The multiplexed image can be expressed aswhere (x, y) are coordinates of the (x, y)th pixel in the phase-encoded image, HT is the transpose of the Hadamard matrix H, ei(x, y) and ρi(x, y) are the amplitude and phase information of the (x, y)th pixel in the ith encrypted hologram, n is 2k, and k is the integer.The restoration is the reverse of the encryption and multiplexing process, as shown in Fig.5. The BPM used for phase encoding an expanded encrypted hologram in the multiplexing process is applied to the multiplexed image to obtain the expanded encrypted hologram.Then, we convert n segments in the expanded hologram into one pixel to recover the original encrypted hologram. According to Eq. (10), the recovered encrypted hologram is expressed asThe recovered encrypted hologram is phase modulated by applying the complex conjugateof the RPM2 in the Fourier domain, inverse Fourier transformed, and then phase modulated by applying the complex conjugate of the RPM1 to retrieve finally the desired hologram,which can be represented bywhere * is the complex conjugate operator.When storing n digitally encrypted holograms as a single multiplexed image on a pagebasis, ‘a set of n BPMs’ can be used as a multiplexing code. By doing so, it is possible not only to efficiently store and manage many encrypted holograms but also to search for.