ABSTRACT
Optical cryptography essentially refers to encoding any information using optical means. The encryption and decryption procedure or either of them is accomplished using the principles of optics. Electromagnetic fields offer various degrees of freedom such as the amplitude, phase, wavelength and state of polarization which can be used as parameters to encrypt information. Moreover, use of optical field for encryption enables faster, parallel processing which is an advantage 124over non-optical methods. The first optical encryption technique to be reported was the double random phase encoding (DRPE) [1]. The DRPE involved a simple 4f optical set-up to encrypt an image. The principle being simple and efficient, DRPE saw many variants that aimed at strengthening the security by different means such as using different optical transform instead of the conventional Fourier transform [2,3]. The fractional Fourier transform (FRT) was proposed as a better option to encode plaintexts as it offers a larger key space in the form of fractional order of the system. On similar lines other transforms such as the Fresnel, gyrator, cosine and wavelet transforms were introduced as a platform for encryption [4–11]. Though the DRPE technique offered various advantages, the encrypted information in DRPE was complex and it required a holographic set-up to record the phase of the ciphertext. To overcome this obstacle, other methods such as the diffractive imaging based encryption [12–16] or polarization based encoding [17–19] were introduced, which used intensity information as ciphertexts.
