26-06-2012, 04:26 PM
SEMINAR ON DIGITAL IMAGE ENCRYPTION
[attachment=25632]
INTRODUCTION
Nowadays, information security is becoming more important in data storage and transmission. Images are widely used in several processes. Therefore, the protection of image data from unauthorized access is important. Image encryption plays a significant role in the field of information hiding. Image hiding or encrypting methods and algorithms range from simple spatial domain methods to more complicated and reliable frequency domain ones. It is argued that the encryption algorithms, which have been originally developed for text data, are not suitable for securing many real-time multimedia applications because of large data sizes. Software implementations of ciphers are usually too slow to process image and video data in commercial systems. Hardware implementations, on the other hand, add more cost to service providers and consumer electronics device manufacturers. A major recent trend is to minimize the computational requirements for secure multimedia distribution by “selective encryption” where only parts of the data are encrypted. A classification of the proposed schemes from the open literature is given in Table 1. There are two levels of security for digital image encryption: low level and high-level security encryption. In low-level security encryption, the encrypted image has degraded visual quality compared to that of the original one, but the content of the image is still visible and understandable to the viewers.
DISCRETE COSINE TRANSFORM
The DCT is a mathematical transformation that takes a signal and transforms it from spatial domain into frequency domain. Many digital image and video compression schemes use a block-based DCT, because this algorithm minimizes the amount of data needed to recreate a digitized image. In particular, JPEG and MPEG use the DCT to concentrate image information by removing spatial data redundancies in two-dimensional images. In the standard JPEG encoding, the representation of the colors in the image is converted from RGB to YCbCr, then the image is decomposed in 8×8 blocks, these blocks are transformed from the spatial to the frequency domain by the DCT. Then, each DCT coefficient is divided by its corresponding constant in a standard quantization table and rounded down to the nearest integer. After this step, the DCT quantized coefficients are scanned in a predefined zigzag order to be used in the final step, the lossless compression as illustrated in fig. 2.1. In each block the 64 DCT coefficients are set up from the lowest upper left corner) to the highest frequencies (lower right corner). The most important visual characteristics of the image are placed in the low frequencies while the details are situated in the higher frequencies. The HVS (Human Visual System) is most sensitive to lower frequencies than to higher ones.
The proposed method
The proposed method based on the idea of decomposing the image into 8x8 blocks, these blocks are transformed from the spatial domain to frequency domain by the DCT. Then, the DCT coefficients correlated to the higher frequencies of the image block are encrypted using Non-Linear Shift Back Register (stream cipher). The concept behind encrypting only some selective DCT coefficients (the coefficients [0,0], [0,1], [1,0], [2,0], [1,1], [0,2]) based on the fact that the image details are situated in the higher frequencies, while the human eye is most sensitive to lower frequencies than to higher frequencies. Figure 3.1, shows the general block diagram of the proposed method of image encryption. The proposed algorithm is lossless; hence the images used in such applications are of highly important information, and loosing any amount of information is not allowed. In the followings, we illustrate the encryption, decryption and shuffling of the images.
Discussions
In the proposed algorithm the images is highly encrypted since the encryption is done by encrypting the basic frequencies of the image, and the operation is done not by encrypting all the bytes in the image but only special frequencies in which when it return to its corresponding pixel values all the bytes of the image will be affected. Note that the image size will increase in width, since for each byte needs an extra bit for the sign of the pixel value. This will lead to a bigger size of the encrypted image that the plain original image.
CONCLUSION
The proposed encryption method uses the Selective Encryption approach where the DC coefficients and some selective AC coefficients are encrypted, hence the DC coefficients carry important visual information, and it's difficult to predict the selective AC coefficients, this give a high level of security in comparison with methods mentioned above. The algorithm will not encrypt bit by bit the whole image but only selective DCT coefficients will be encrypted, and extra security has been added to the resulted encrypted blocks by using Block Shuffling method depending on two prime numbers, where these two primes will generate sequences or row and column numbers to be used in shuffling. The algorithm considered as a fast image encryption algorithm, due to the selective encryption of certain portion of the image (the DC and some AC coefficients).
[attachment=25632]
INTRODUCTION
Nowadays, information security is becoming more important in data storage and transmission. Images are widely used in several processes. Therefore, the protection of image data from unauthorized access is important. Image encryption plays a significant role in the field of information hiding. Image hiding or encrypting methods and algorithms range from simple spatial domain methods to more complicated and reliable frequency domain ones. It is argued that the encryption algorithms, which have been originally developed for text data, are not suitable for securing many real-time multimedia applications because of large data sizes. Software implementations of ciphers are usually too slow to process image and video data in commercial systems. Hardware implementations, on the other hand, add more cost to service providers and consumer electronics device manufacturers. A major recent trend is to minimize the computational requirements for secure multimedia distribution by “selective encryption” where only parts of the data are encrypted. A classification of the proposed schemes from the open literature is given in Table 1. There are two levels of security for digital image encryption: low level and high-level security encryption. In low-level security encryption, the encrypted image has degraded visual quality compared to that of the original one, but the content of the image is still visible and understandable to the viewers.
DISCRETE COSINE TRANSFORM
The DCT is a mathematical transformation that takes a signal and transforms it from spatial domain into frequency domain. Many digital image and video compression schemes use a block-based DCT, because this algorithm minimizes the amount of data needed to recreate a digitized image. In particular, JPEG and MPEG use the DCT to concentrate image information by removing spatial data redundancies in two-dimensional images. In the standard JPEG encoding, the representation of the colors in the image is converted from RGB to YCbCr, then the image is decomposed in 8×8 blocks, these blocks are transformed from the spatial to the frequency domain by the DCT. Then, each DCT coefficient is divided by its corresponding constant in a standard quantization table and rounded down to the nearest integer. After this step, the DCT quantized coefficients are scanned in a predefined zigzag order to be used in the final step, the lossless compression as illustrated in fig. 2.1. In each block the 64 DCT coefficients are set up from the lowest upper left corner) to the highest frequencies (lower right corner). The most important visual characteristics of the image are placed in the low frequencies while the details are situated in the higher frequencies. The HVS (Human Visual System) is most sensitive to lower frequencies than to higher ones.
The proposed method
The proposed method based on the idea of decomposing the image into 8x8 blocks, these blocks are transformed from the spatial domain to frequency domain by the DCT. Then, the DCT coefficients correlated to the higher frequencies of the image block are encrypted using Non-Linear Shift Back Register (stream cipher). The concept behind encrypting only some selective DCT coefficients (the coefficients [0,0], [0,1], [1,0], [2,0], [1,1], [0,2]) based on the fact that the image details are situated in the higher frequencies, while the human eye is most sensitive to lower frequencies than to higher frequencies. Figure 3.1, shows the general block diagram of the proposed method of image encryption. The proposed algorithm is lossless; hence the images used in such applications are of highly important information, and loosing any amount of information is not allowed. In the followings, we illustrate the encryption, decryption and shuffling of the images.
Discussions
In the proposed algorithm the images is highly encrypted since the encryption is done by encrypting the basic frequencies of the image, and the operation is done not by encrypting all the bytes in the image but only special frequencies in which when it return to its corresponding pixel values all the bytes of the image will be affected. Note that the image size will increase in width, since for each byte needs an extra bit for the sign of the pixel value. This will lead to a bigger size of the encrypted image that the plain original image.
CONCLUSION
The proposed encryption method uses the Selective Encryption approach where the DC coefficients and some selective AC coefficients are encrypted, hence the DC coefficients carry important visual information, and it's difficult to predict the selective AC coefficients, this give a high level of security in comparison with methods mentioned above. The algorithm will not encrypt bit by bit the whole image but only selective DCT coefficients will be encrypted, and extra security has been added to the resulted encrypted blocks by using Block Shuffling method depending on two prime numbers, where these two primes will generate sequences or row and column numbers to be used in shuffling. The algorithm considered as a fast image encryption algorithm, due to the selective encryption of certain portion of the image (the DC and some AC coefficients).