Optical Cryptosystems. Naveen K. Nishchal

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СКАЧАТЬ to attacks. The image encryption algorithms can be classified into frequency-domain and spatial-domain algorithms. Both are able to protect the data/image with a high level of security. Their output encrypted images are either texture-like or noise-like images. From a security point of view, it is an obvious visual sign indicating the presence of an encrypted image that may contain some important information. It is apprehended that this will attract people’s attention and can result in a significantly large number of attacks and analysis. The solution has been reported in the form that the original image is transformed into visually meaningful encrypted images. This is because people generally consider these images as normal images rather than encrypted ones.

      Securing data/image is important in all the domains including medical diagnosis. There is a fear that patients’ computed tomography (CT) and medical resonance imaging (MRI) scan results can easily be changed by hackers, thereby deceiving radiologists and artificial intelligence algorithms that diagnose malignant tumors. The hackers could access to add or remove medical conditions from the scans for the purpose of insurance fraud, ransom, and even homicide. A large number of techniques have been proposed in literature to date, each have an edge over the other, to catch up to the ever-growing need of security. The focus has been devising a mechanism for image encryption that should have the following characteristics.

Low correlation	The value of correlation between the original and the encrypted image should be as low as possible. Ideally its value should be zero.
Large key space	The key size should be very large since the more the key space, the higher the brute force search time would be.
Key sensitivity	The image encryption algorithm should have high key sensitivity. In other words, a slight change in the key value should change the encrypted image significantly.
Entropy	It is a measure of the degree of randomness or disorder. As the level of disorder rises, the entropy rises, and events become less predictable. The minimum entropy value should be zero and it happens when the image pixel value is constant in any location. The maximum value of entropy for an image depends on the number of gray scales. For an image with 256 gray scales, the maximum entropy is log 2(256) = 8. The maximum value happens when all bins of the histogram have the same constant value, or, image intensity is uniformly distributed in [0,255].
Low time complexity	Usually, an encryption algorithm with high computational time is not recommended for practical applications. Therefore, an image encryption algorithm should have low time complexity.

      The technology for information security using digital methods is being enhanced by applying more powerful algorithms. Longer key lengths are chosen such that current computers using the best cipher-cracking algorithms would require an unreasonable amount of time to break the key. When encryption key length becomes longer, the processing speed of digital techniques goes down. In order to counter the processing speed and security problem, in 1995 a new technology was proposed that used physical keys employing the principles of classical optics. Owing to the speed of light, it is envisaged that data can be secured at unparalleled speed along with parallel processing. Additionally, optics offers several degrees of freedom that could help encode information more securely [8–14]. Also, there is a natural match between optical processing for optical communications.

      With the belief that cryptology based on the optics principle would provide a more complex environment and would be more resistant as compared to purely digital techniques, developing optical cryptosystems have gained much emphasis [13, 14]. Since 1995, a large number of research articles have appeared with so many different techniques. These topics are discussed in detail in the following chapters.

References

[1] A Al Falou 2018 Advanced Secure Image Processing for Communications (Bristol: IOP Publishing)

      [2] S Ramakrishnan 2018 Cryptographic and Information Security Approaches for Images and Videos (Boca Raton, FL: CRC Press)

[3] J R Vacca 2017 Computer and Information Security Handbook 3rd edn (Amsterdam: Elsevier)

[4] Pfleeger C P, Pfleeger S L and Margulies J 2018 Security in Computing 5th edn (Noida: Pearson India)

[5] Beckett B 1988 Introduction to Cryptology (Oxford: Blackwell)

[6] Stallings W 2000 Cryptography and Network Security; Principles and Practice 2nd edn (Hoboken, NJ: Prentice Hall)

[7] van Tilborg H C A 2000 Fundamentals of Cryptology (Boston, MA: Kluwer)

[8] T J Naughton and J Sheridan 2005 Optics in information systems SPIE Int. Tech. Gr. Newsletter 16 1–12

      [9] B Javidi 2005 Optical and Digital Techniques for Information Security (Berlin: Springer)

      [10] B Javidi 2006 Optical Imaging Sensors and Systems for Homeland Security Applications (New York: Springer)

      [11] Alfalou A and Brosseau C 2009 Optical image compression and encryption methods Adv. Opt. Photon 1 589–636

      [12] Chen W, Javidi B and Chen X 2014 Advances in optical security systems Adv. Opt. Photon 6 120–55

[13] Javidi B et al 2016 Roadmap on optical security J. Opt. 18 083001

[14] Muniraj I and Sheridan J T 2019 Optical Encryption and Decryption (Bellingham, WA: SPIE Press)

      IOP Publishing

      Optical Cryptosystems

      Naveen K Nishchal

      Chapter 2

      Optical techniques of image encryption: symmetric cryptosystems

      2.1 Introduction

      In an encryption system, an input data/image is encoded in such a fashion that only the application of the correct key would reveal the original information. The security of digital methods is being enhanced by using more powerful algorithms. Longer key lengths are chosen such that even advanced computers would require an unreasonable amount of time to break the key. Therefore, digital techniques are falling short of expectation due to the fact that when the encryption key length becomes longer, the processing speed goes down. It is primarily because digital techniques process data serially and in one dimension. Optical processing is inherently two-dimensional (2D) and does parallel processing. Every pixel of the 2D image can be both relayed and processed at the same time. So, when a large volume of data/information is to be processed, parallel processing offers enormous advantages. In addition to the fast speed, optical technology offers several advantages such as high space-bandwidth product, СКАЧАТЬ