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Top1. Introduction
The digital communication has become challenge to transmit the data securely. Digital images are used in many applications such as healthcare, biometric, military applications, police identification etc. Healthcare data is an important digital data when it comes to storing sensitive personal data. These personal health data images can easily be accessed by unauthorized person or hacker and can be transmitted over a communication network. Health care data encryption is required in the form of data security whereby electronic medical records (EHR) should be disguised so that unauthorized users may not be able to read or make sense of them. Personal health information (PHI) including medical diagnoses, surgeries and other sensitive health data needs to be secured to guard against malicious motives as well as confidentiality breaches that can result in huge fines. Therefore, optical image encryption can be used as secure method for communication. Many new optical image encryption (Matoba et al., 2009; Alfalou & Brosseau, 2009; Millan & Perez-Cabre, 2011) (Javidi et al, 2016) schemes for reliable and secure transmission in this direction have been proposed. Double Random Phase Encoding (DRPE) is one of the well-known and successful classical optical image encryption technique proposed by Refregier and Javidi (Refregier & Javidi, 1995) so far. In DRPE, aim is to encode the plain image by converting it into stationary white noise called cipher image by applying two random phase diffusers (masks), one in input plane and another in Fourier plane in 4-f system. DRPE with Fourier suffers from many attacks and other security issues. Therefore DRPE was further performed with different transformations such as Fractional Fourier transform (Unnikrishnan et al., 2000; Liu et al., 2014; Zhou et al., 2010; Singh, 2016), Fresnel transform (Matoba & Javidi, 1999; Situ & Zhang, 2004; Singh H et al., 2015), Gyrator transforms (Rodrigo et al., 2007; Singh et al., 2014; Singh et al., 2015), Cosine transform (Wu et al., 2010), Hartley transform Chen & Zhao, 2006), Arnold transform (Abuturab, 2013) and Mellin transform (Zhon et al., 2011; Zhou et al., 2015; Vashisth et al., 2014) to enhance the security. Due to its symmetric and linear nature this technique brings fatal damage to the reliability as it is vulnerable to many attacks such as chosen plain image attack, chosen cipher image attacks, known plain image attack and cipher image only attack (Kumar et al., 2016; Peng X et al., 2006; Carnicer et al., 2005) and suffered from various practical problems. To overcome these issues several other optical image encryption techniques based on DRPE concepts have also been proposed in the literature. They are digital holography (Nishchal et al., 2004), multiplexing (Situ & Zhang, 2005; Matoba & Javidi, 2004; Chen & Zhao, 2006), polarized light (Gopinathanet al., 2006), and interferometery (Li, 2009; Masajada & Dubik, 2001; Vyas & Senthilkumaran, 2007). To enhance more security and to meet requirements of latest applications, DRPE with Fast Walsh hadamard transform (FWHT) has been introduced (Zheng & Huang; Fino & Algazi, 1976). FWHT has been used in the cryptosystem as it protects from the quantization error and image can be perfectly reconstructed. This scheme also includes a strong key as according to the Kerckhoffs's principle, security is based on the strong key i.e. assuming that encryption algorithm is public and is known to the attacker but the resistance of the algorithm should be based on secrecy of the key. Finding out the key should be very difficult so therefore new spiral phase mask has been used here for encryption process which increases the key space and makes it robust to many standard attacks.