Optics Communications 417 (2018) 30–36 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom Optical image security using Stokes polarimetry of spatially variant polarized beam Areeba Fatima, Naveen K. Nishchal * Department of Physics, Indian Institute of Technology Patna, Bihta, Patna-801 106, Bihar, India article info Keywords: Optical security and encryption Phase retrieval Polarization Fourier optics and signal processing abstract We propose a novel security scheme that uses vector beam characterized by the spatially variant polarization distribution. A vector beam is so generated that its helical components carry tailored phases corresponding to the image/images that is/are to be encrypted. The tailoring of phase has been done by employing the modified Gerchberg–Saxton algorithm for phase retrieval. Stokes parameters for the final vector beam is evaluated and is used to construct the ciphertext and one of the keys. The advantage of the proposed scheme is that it generates real ciphertext and keys which are easier to transmit and store than complex quantities. Moreover, the known plaintext attack is not applicable to this system. As a proof-of-concept, simulation results have been presented for securing single and double gray-scale images. 1. Introduction Study of optical methods to secure information has found widespread interest among the researchers. The double random phase encoding (DRPE) scheme gave a convenient method to optically encrypt in- formation to stationary white noise [1]. Since then, various other features of light have been explored for establishing a robust optical cryptosystem. Optical transformations such as the Fourier transform, gyrator transform, Fresnel transform, fractional Fourier transform etc. were used as tools to perform the encryption and decryption [2–4]. As vulnerabilities of the DRPE system to various attacks were established due to its inherent linearity [5–7], newer optical cryptosystems were introduced which were asymmetric systems. These asymmetric systems included phase truncation based Fourier transform (PTFT) techniques or the equal modulus decomposition techniques (EMD) [8–15]. Though these asymmetric cryptosystems provided resistance against the known- plaintext attack, yet it was soon found that they were vulnerable to the specific attack [16,17]. Hence, it is always an effort to find various schemes that gives a strong optical cryptosystem. Moreover, various studies have been reported that propose double image or multiple image encryption [18–22]. Apart from robustness, the research trends also show studies that try to find newer aspects of light that could be used in encoding information. In this regard, polarization is another aspect of light that has been extensively used to construct efficient cryptosystems and authentication schemes [23–28]. Studies using the Stokes formalism to encrypt information have been widely reported as it uses the intensity information and, therefore, it is easier to implement [23]. * Corresponding author. E-mail address: nkn@iitp.ac.in (N.K. Nishchal). As compared to the mainstream polarization techniques, the use of spatially variant polarization (or vector beams) is a lesser studied tool for optical encryption [29–31]. A geometrical phase generated by space variant polarization condition due to a subwavelength grating has been employed to encrypt information [29]. In another work, Maluenda et al. developed a polarimetric measurement based encryption and verification scheme that used non uniform state of polarization (SOP) distribution [31]. The encryption setup was based on the Mach–Zehnder interferometer and it processed the transverse components of the beams in the two arms of the interferometer. In general, vector beams are characterized by spatially varying SOP across the optical beam cross-section [32]. This property of vector beams has led to new features that enhance the range of performances of optical systems. For example, the vector beams can give tight focal spots which in turn have enabled high resolution imaging, optical trapping and plas- monic focusing. Vector beams have found applications in various other fields like atmospheric sensing and singular optics, to name a few [32]. Several methods have been proposed to generate different vector beams with non-uniform polarization. These methods involve interferometric arrangement or optical set-ups using liquid crystal displays [33–35]. In one of the works, a Mach–Zehnder interferometer set up was used to generate optical beam with non-uniform state of polarization [33]. Recently, a method to generate vector beams with tailored phase and polarization has been proposed [34,36]. The incident beam which would give the desired phase and polarization was determined using iterative Gerchberg–Saxton (GS) algorithm [37]. https://doi.org/10.1016/j.optcom.2018.02.030 Received 26 December 2017; Received in revised form 22 January 2018; Accepted 12 February 2018 0030-4018/© 2018 Elsevier B.V. All rights reserved.