Research On Chaos-based Medical Image Encryption Algorithms

With the dramatic developments of medical imaging and computer network technologies,digital medical image has occupied more and more important role in the medical diagnosis.The contemporary medical information system has converted the patient stream to information stream,which brings great convenience for the communication and storage of the patients' information inside and accross the hospital,whreas the security threats emerge at the same time.As there is a great deal of private information of the patients,it is of great necessity to build a suite of security guarantee mechanism from the legal as well as ethical perspectives.Therefore,how to guarantee the security of the paitients' privancy becomes a critical issue of digital medical care field.In the present medical image information systems,the security is mainly ensured using traditioanal encryption algorithms,such as DES,AES,IDEA,etc.However,different from conventional texual plaintexts,digital medical images are always with high resolution,high precision,large volumn,and with high redundancy.Hence,traditional block ciphers are not suitable for medical image encryption;especially the operation time cannot satisfy the demand of real-time high-volumn medical image transmisson.The achivements of chaos theory have paved a noval perspective for medical image protection.Chaos is deterministic non-linear system,from which the resultant sequences are pseudo-random,they possess infinite period and behave like Gaussian white noise.Besides,as the chaotic system is high sensitive to the control parameter and initial condition,the produced sequence is un-predictable and can provide huge key space.Meanwhile,chaos-based image cryptosystems are easy to be software or hardware implemented,and the encryption time own substantional superiority than traditional block ciphers,which renders it a suitable candinate for high-volumn image encryption.In 1998,the US researcher Fridrich proposed the classical permutation-diffusion architecture for image encryption,it subsequently drawn world-wide concern and promote the develoment of chaos-based encryption area.The present research is conducted in the following aspects.(1)Throughout in-depth analysis of permutation-diffusion architecture,we found that the permutation procedure which merely shuffles the pixels' location without changing their values has no diffusion effect,such a drawback makes the time-consuming diffusion procedure have to perform multiple rounds to achieve the satisfactory security.Hence,we propose an improved permutation approach,IPA.As a kind of permutation method,IPA can bring about better permutation effect with more encryption efficiency,in comparison with the traditional permutaiton ciphers,such as Cat map or Baker map,etc.In the train of the permutation performance,IPA can also brigh the pixel value modification and difference spreading effects.Such diffusion contribution can dramatically reduce the required diffusion rounds which are the most time-consuming part of the whole cryptosystem.Therefore,the required encryption rounds decrease and the encryption efficiency is promoted.Based on the IPA achievement,two complete cryptosystems are then built,whose superior security performance is subsequently experimentally verified.(2)Modern medical images are always with large volumn,which bring about severe difficulties during their transmission or storage.Combined with the security requirement,establishing a combined medical image encryption and compression scheme is of great importance from the theoretical and practical perspectives.A combined encyption and compression scheme based on chaos and compressive sensing is proposed in the present paper.The measurement matrix is generated using chaotic system;such a design make the compressive sensing procedure simutaneously owns both the compression and encryption attributions.In combination with the permutation-diffusion procedure,a complete encryption and compression system is established.Experiment results validate that is has satisfactory image reconstruction and security performance at the decoder end within a large compression area.(3)Accompanied with the achievements of cryptanalysis,many chaos-based image ciphers have been found insecure and been successfully broken.We cryptanalyze a recently proposed medical image cipher[Comput Biol Med,2013,43(8):1000-1010],investigate and verify a chosen-plaintext attack which can precisely recover the diffuison key stream elements and the permutation matrix of the original encryption system.Except for the security defect,there is high implementation redundancy.The massive bit-level permutation operations have no security contribution whereas reduce the encryption efficiency of the cryptosystem.We improve the system from the efficiency and the security perspectives,and then the experimental resut is given out for validation.Finally,we conclude the paper and prospect the future research of chaos-based medical image encryption.