The authors propose an image encryption process based on chaos that uses block scrambling to reduce the correlation among the neighboring pixels and random order substitution for slightly changing the value of the pixel. The chaotic sequence for encrypting the image is generated by using two 3D logistic maps called enhanced logistic map and intertwining logistic map; the cos function helps in reducing linearity. The entire encryption process is composed of scrambling, image rotation, and random order substitution. Scrambling is used for permuting the pixels in the image so that we can reduce the correlation among the neighboring pixels, and this is followed by image rotation which can ensure that shuffling of pixels is done to the remaining pixels in the image, and at last the authors use random order substitution where they bring the small change in the pixel value. The proposed method has the capability of encrypting digital colored images into cipher form with high security, which allows only authorized ones who hold the correct secret key to decrypt the images back to original form.
TopIntroduction
As a result of fast growth in computer network technologies, transmission of digital data over the network channels has rapidly increased, and it has become a convenient way to communicate in daily life. Most of the communication over the networks has been carried out in the form of images, as it conveys the information in a pictorial form to understand easily. However, it also gives a chance for the intruders to attack the communication channels and steal the information. The methods in digital image processing always focus on the advancement of visualizing the information and processing of bulky image information for storage and transmission. In a case where image has to be sent over the network, security is the primary issue which needs to be addressed.
Among various techniques like data hiding, watermarking, image encryption, steganography etc., which ensure image security when transmitted over the internet, image encryption is the most popular one. Image encryption encrypts the original image into unrecognized one (M. Yang et al., 2004), where original image is known as plaintext image and the encrypted image is called as cipher text image (Stinson, 2006).
One of the old traditional ways of encrypting the digital image is using a block cipher or stream cipher, which extracts the bulky plaintext image’s data as a stream of one dimensional binary bit and then encrypting these bits. The well-known algorithms of this type of encrypting standards are Digital Encryption Standard (PUB, 1977), Advanced Encryption Standard (Rijmen & Daemen, 2001), Twofish Cipher (Schneier et al., 1999) and Blowfish Cipher (Schneier, 1993). However, using the above mentioned one dimensional algorithms are not efficient for encrypting the two dimensional image data, due to high correlation among the image pixels. Following sections gives a brief discussion about image encryption and chaos based image encryption.
Image Encryption
Encryption is basically changing the original data called as plaintext into an unreadable format called cipher text such that only authorized people can decrypt that back to plaintext and access the original data. Generally, the encryption standard uses the encryption key which is produced by the algorithm for encrypting the data or the information. In some cases, it is viable to decrypt the information without having the key by the intruders. However, for a well-organized encryption standard that uses a standard algorithm to generate the key, it is not possible to decrypt the same easily. Earlier encryption standards were often used to encrypt military information. However, the usage and exchange of information has changed from heavy or bulky devices systems to handheld devices in the last few decades. Hence, in this modern era, with the availability of internet in every part of the world, many new encryptions algorithms have developed. The modern encryption standards use the concepts of public key and symmetric key. Figure 1 shows the basic encryption approach.
Figure 1. (M. Yang et al., 2004) Basically, the entire encryption process is based on the keys that are used while encrypting the information. The most important and commonly used keys in cryptography are symmetric key and public key (which is also known as asymmetric key). In the symmetric key encryption process the key should be the same for both encryption as well as decryption side. Since the originator of the communication will share the key with the other party, it ensures that the communication between two parties is secured. Figure 2 shows the symmetric key encryption mechanism.
Figure 2. (Stinson, 2006) In the asymmetric key encryption process, the key is published so that it can be used by anyone to encrypt the information. On the other hand, only the receiver will have the access only to the decryption key, which allows him to read the sent information after decryption. Hence, in this way it ensures that the communication between two parties is secured. The most popular public key encryption schemes are Diffe-Hellman key exchange and RSA(Rivest-Shamir-Adleman). Figure 3 shows the asymmetric key encryption approach.
Figure 3. Asymmetric Key Encryption