Flexible optical encryption with multiple users and multiple security levels

a r t i c l e i n f o Keywords: Optical image encryption Information security Fractional Fourier transform Three-dimensional image processing Digital holography We present a basic optical encryption architecture that admits several cryptography applications based on multiplexing. Users can decrypt different private images from the same encrypted image, a superuser can have a key that decrypts all encrypted images, and multiplexed images can be encrypted with different levels of security. This system is presented in the context of a general framework of optical encryption application development. We illustrate with a real-world three-dimensional scene, captured with digital holography, and encrypted using the fractional Fourier transform, where different users have access to different three-dimensional objects in the scene. Optical encryption has reached a level of maturity recently with the publication of realistic attacks [1–5] that exploit its inherent weakness of linearity. This serious security problem associated with repeated use of the same key can be defended against through the use of modes of encryption [6], which promise to make optical encryption a viable symmetric cryptosystem. The repeated cycle of publication of an attack followed by publication of an appropriate defense is the natural lifecycle of conventional cryptosystems. To encourage widespread use, the field of optical encryption should offer a cohesive and fully-featured suite of practical and unique applications. A single framework for the different applications will allow a single optical hardware arrangement to support this wide range of applications without modification. The aim of this communication is to illustrate one such framework and associated suite of applications. In the sister field of compression, data compression and image compression have distinct uses. Data compression is synonymous with lossless data compression and in image compression some loss and defects can be tolerated for vastly increased compression ratios and computational efficiency. Similarly, data encryption and image encryption have different motivations, and optical-based techniques are particularly suited for the latter. One unique property of image encryption is that it tolerates lossy multiplexing, which mitigates any disadvantages of optical encryption compared to electronic encryp-tion. The suite of encryption applications presented in this study is based on the concept of secure multiplexing [7] of optical images. In the scenario of a single encrypted image shared between multiple users each with their own private unique decryption key, we consider three user cases, where (i) each user decrypts a different private image, (ii) as before, but where …