Measurement of Encryption Quality of Bitmap Images with RC6, and two modified version Block Cipher

With the fast evolution of digital data exchange, security information becomes much important in data storage and transmission. Due to the increasing use of images in industrial process, it is essential to protect the confidential image data from unauthorized access. In this paper, RC6 with two modified version 512 bit RC6 and 640 bit RC6-Cascade encryption algorithms will be analyzed to investigate the encryption efficiency for them to digital images and providing a new mathematical measure for encryption efficiency. Detailed results in terms of security analysis and implementation are given. Comparative study with three versions of RC6 encryption algorithms is shown the superiority of the modified algorithms. Three measuring quality factors will be considered to evaluate and compare between the three encryption algorithms RC6, 512 RC6, and 640 RC6-Cascaded. These measuring factors are the maximum deviation, the correlation coefficient and irregular deviation.

In this paper some of related work will be presented.In [2] investigate the implementation and application of the RC5 block cipher algorithm for digital images and provide testing, verification, and encryption efficiency of the RC5 block cipher for digital images.In [3] a block-based transformation algorithm is proposed for image security using a combination of image transformation and encryption techniques.This algorithm will be used as a pre-encryption transform to confuse the relationship between the original images and the generated$ ones.The generated images are then fed to the Blowfish encryption algorithm.Correlation, histogram, and entropy have been used to measure the security level of the images.While in [4] encryption quality for bitmap images encrypted with rijndael and KAMKAR block ciphers will be measured.
In this paper, different Bitmap images are encrypted with RC6, 512 RC6 and 640 RC6-Cascade.The quality of the encrypted images are tested with visual inspection and evaluated with different quality of measuring algorithms.

RC6 Block Cipher Algorithm
This algorithm depends mainly on the use of four working registers, each of size 32 bits.So, it handles 128 bits input/output blocks.Its parameterized family is: (w) word size in bits, (r) non-negative number of rounds, and (b) the length of encryption/decryption key in bytes.RC6 has six primitive operations, which are (+,−,<<<,>>>,×,⊕).The use of multiplication greatly increases the diffusion achieved per round, allowing for greater security, fewer rounds, and increases throughput.RC6 uses an expanded key table, S[0, . . ., t − 1], consisting of key t = 2r + 4 w-bit words figure (1) shows the encryption with RC6.All details of RC6 are described in [5].

512 Bit RC6 Block Cipher
This algorithm differs from the RC6 of 128 bits which could be used to encrypt and decrypt 512 bits block size.The structure of 512 bit RC6 is a Feistel network.It consists of splitting the plaintext into two 256 bit halves as shown in figure (2).Feistel ciphers are a special class of iterated block ciphers, where the ciphertext is calculated from the plaintext by repeated application of the same transformation or round function.The round function is applied to one half using a subkey and the output of F function is XORed with the other half.The two halves are then swapped.Each round follows the same pattern except for the last round where there is no swapping.

Measurement of Encryption Quality of Bitmap Images with RC6, and two modified version Block Cipher
function of Feistel network where the word is doubled to 64 bit instead of 32 bits in the 128 bit RC6 previous algorithm.All the operation of 512 RC6 is on 64 bits as shown in [6].

RC6-Cascade Block cipher
RC6-Cascade is 640-bit RC6-like block cipher.The plaintext is 640 bit which is divided into five parts p1, p2, p3, p4 and p5 each of which is 128 bit.The F-function in RC6-Cascade will be used cascaded design instead of rounds as it is shown in Figure (3).The output is 640 bit c1, c2, c3, c4, and c5 where each of is128 bit [7].

The F-function
The F-function of RC6-Cascaded uses two rounds of pervious RC6 algorithm in a Feistel network.The input of each function is two plaintexts of 128 bits and four subkey Si,Si+1,Si+2,Si+3 as shown in figure (4).The Feistel network consists of dividing the input into two halves, and applying a non-linear function only to the right half.The result is added into the left half and subsequently left and right half are swapped.Ciphers following this approach are called Feistel ciphers.The output of one nonlinear function is input directly to the next one, which increases the propagation of local changes.The non-linear function of RC6-Cascaded is the pervious RC6 algorithm [7].

The Maximum Deviation Measuring Factor
• The maximum deviation measures the quality of encryption in terms of how it maximizes the deviation between the original and the encrypted images.The steps of this measure will be done as follows: Count the number of pixels of each grayscale value in the range from 0 to 255 and present the results graphically (in the form of curves) for both original and encrypted images (i.e.; get their histogram distributions).
Where i h is the amplitude of the absolute difference curve at value i.Of course, the higher the value of D, the more the encrypted image is deviated from the original image.

The Correlation Coefficient Measuring Factor
Correlation is a measure of the relationship between two variables.If the two variables are the image and its encryption, then they are in perfect correlation (i.e.; the correlation coefficient equals one) if they are highly dependent (identical).In this case the encrypted image is the same as the original image and the encryption process failed in hiding the details of the original image.If the correlation coefficient equals zero, then the original image and its encryption are totally different, the encrypted image has no features and highly independent on the original x and y are gray-scale pixel values of the original and encrypted images.

The Irregular Deviation Measuring Factor
This quality measuring factor is based on how much the deviation caused by encryption (on the encrypted image) is irregular.It gives an attention to each individual pixel value and the deviation caused at every location of the input image before getting the histogram.This method can be summarized in the following steps: The lower the ID value, the better the encryption algorithm.

Measurement of Encryption Quality of Bitmap Images with RC6, and two modified version Block Cipher
In this paper, three different BMP images are evaluated.These images are Nike.bmp(Figure 5) as an example of an image containing very large areas of a single color and it is an example of a binary image, Lena.bmp (Figure 6) as it is the reference image used in image processing research (it does not contain many high frequency components), and peppers.bmp(Figure 7) as an example of an image containing many high frequency components.
The three images are encrypted using RC6, 512 RC6, and 640 RC6-Cascaded.The results of the three measuring factors are given in table (1 2) and (3),which show the histograms for images and each encryption method, we see that the Irregular Deviation (ID) did not give any misleading results and it can be used alone to test the quality of encryption in the field of image encryption.So, if the Irregular Deviation (ID) agrees with other measuring factor, it will be good judging, otherwise the final decision on measuring the quality of the encryption algorithms will be based on the Irregular Deviation (ID) which is based on each pixel value. Nike ), where (D) indicates the Maximum Deviation measure, (C.C.) indicates the Correlation Coefficient measure, and (ID) indicates the Irregular Deviation measure.With the measure of the Maximum Deviation (D) the greater is the better; with the Correlation Coefficient (C.C) the closer to zero is the better, while with the Irregular Deviation (ID) the smaller is the better.Based on the figures (1),(

Tech. Journal, Vol.28, No.17, 2010 Measurement of Encryption Quality of Bitmap Images with RC6, and two modified version Block Cipher
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