Hybrid Cipher for Secure Multimedia by using AES and RC4 Chain

— With development, technology, computer science , computer networks and transmission of multimedia between two or more parts, a security of multimedia becomes an essential issue since most of the systems became easy to attack. In this newspaper, we suggest a model to Hybrid Cipher for Secure Multimedia by using AES and RC4 Chain. The analysis and evaluate the performance of this model is measured by testing several parameters. Show The resulting multimedia is found to be more distorted in hybrid Cipher.

In recent years, with the development of digital communications and digital transmission of multimedia.Become many of us in connection with the internet and intranet networks and transmits digital multimedia without think about the safety of digital multimedia.We share a much of our own data and secrets.Today in telecommunications networks has become necessary to protect multimedia because this multimedia are either files or medical pictures of these needs to be high confidentiality to prevent a thief from modification.Or the data may be, especially in a company or organization also needs to secure and save this data from unauthorized persons [1].Encryption is the process of converting plain_multimedia to cipher_multimedia in order to be invisible this means to prevent anyone not authorized to recover the original multimedia.Encryption is used mainly to ensure confidentiality.Usually, companies or organizations use encryption before transmission to ensure the confidentiality of information during transmission across networks and be decrypted by the intended person [2].While in the past, encryption indicates to the "encryption and decryption using secret keys".Today there are three kinds of keys: -"symmetric-key and asymmetric-key and hashing".The Symmetric key uses one key secret for "encryption and decryption".In the asymmetric key, there are two keys instead of one where the key public key is used to encrypt the private key is used to decrypt.In hashing, "a fixed-length message digest is created out of a variable-length message".Symmetric encryption includes two classes: -stream ciphers and block ciphers."In a stream-cipher, encryption and decryption are done one symbol (such as a character or a bit) at a time".We have a plain-multimedia stream, a cipher-multimedia stream, and a key stream."In a block cipher, a block of plainmultimedia symbols of size N (N>1) are encrypted together, creating a block of cipher-multimedia of the same size" [3].

2-RELATED WORKS
In 2012 Prabhudesai .And Vijayarajan [8].Developed a novel mix cipher by merging the features of double ciphers called "AES (Advanced Encryption Standard) and Rc4 (also identified as ARC4)".The features of in cooperation ciphers have been calculated and a novel cipher merging the features of mutually the ciphers is created which is added more protected than the basic encryptions.AES features are safety and its confrontation against attacks and the main features of Rc4 is quickest.Then, these features are mixed in a new created code.Thus, it shows to be quicker than the basic AES and protected against greatest attacks.Three grouping ways have been expressed to create a mongrelized cipher and the process along with the strong point and flaws outline.The third cipher is the main cipher that is focused on this newspaper.It is also displayed, that this cipher is impervious against most attacks.This determination ensures the " confidentiality" of the data which is used to encode.
In 2013 Nares and et al. [2].Have emargined a new amalgam cipher by joining the features of 3 ciphers name "AES (Advanced Encryption Standard), Rc4 (identified as ARC4) and Serpent".The features of in cooperation ciphers are calculated, and a novel cipher merging the features of in cooperation the ciphers is produced that is more protected than the plain codes."AES, SERPENT features are its security and its resistance against attacks and the main specific of Rc4 is its speed".Thus, these features are mixed in the new produced code.Thus, it shows to be quicker than the basic AES and protected against greatest attacks.
In 2014 Dilpeet K. and Gurjit S. B. [7] used a combined concept of existing encryption algorithms AES and RC4 along with Hash Function and whitening to obtain a hybrid model which can be used for encrypting various kinds of data.

3-THE PROPOSED SYSTEM
The security of multimedia performs by "using symmetric-key that is both encryption and decryptions use the similar key.
The key must be distributed to both the transmitter and the receiver of the multimedia".When seeing time difficulty, effectiveness, and costs, "symmetric-key" encryption is considered the finest solution, and key sharing remains a problematic when using this method.The AES algorithm is practical with" key length of 128-bits" which is appropriate for the resolution of encoding, multimedia with dissimilar size and handling time resulting in an equitable cost.As shown in fig. 1.The decryption model works in reverse.To products the first block of multi plain , first block of multi cipher is decrypted with the original key after that the result XOR with " an initialization vector (IV)".The second block of multi cipher is decrypted by the key stream after then the result XOR with previous block of multi cipher.Through the decryption process, all blocks will be determined by the" prior block to be decrypted right, "otherwise these blocks will not decrypt right".This can also be measure useful when the multi plan is not recovered in the correct form, it directs that the multi plan has been possibly disclosed or altered.Then, this system can also provide integrity by privacy".

4-RESULTS AND DISCUSSION
The suggested system is tested on four selected images, four audio files and four text files with different sizes.The proposed system is implemented on a PC with a 2.30 GHz Core i3 CPU and 4 GB of RAM.Visual C#.NET programming language was used to implement the suggested system.In this segment, a number of measures of taken into account; "histogram, correlation coefficient, Number of Pixel Changing Rate (NPCR) and Unified Averaged Changed Intensity (UACI), Execution Time and entropy of information".

a. Histogram
The histogram is a statistical measure used to supply image statistics.It computes the frequency distribution of the elements in each input color image by distributing the amount of pixels to each value [4].

b. Correlation Coefficients
The correlation coefficient is a measure that is can be used to compute the amount of match between two variables.It is a beneficial ration to determine the encryption quality of any encryption system.The encryption system to be, well, if encryption Algorithm hiding all features of a normal image, and encoded image is all random and very uncorrelated.The correlation his values range from -1 to 1, where Values near to 1 indicate that there is a positive "relationship between the variables ,Value near to -1 indicate that there is a negative relationship between the variables, Value near to or equal to 0 suggest there is no relationship between the variables".In this paper correlation value near to zero.It indicates to the efficiency and performance of an image encryption process."Correlation coefficients" can be calculated using equation1 [4,5] "Where P & C are the average of the image and it after one pixel" modification for plain or cipher image which that means "gray-level" values of two neighbouring pixels in the input image, shows the resulting correlation that can be achieved on "horizontal, vertical and diagonal" in the following Table 1 .

c. Attack Resistant
Commonly, the attacker tries to make simple changes in the cipher multimedia such as changing one pixel of the encrypted image or one byte for audio and text, if the attacker is discover any related information about plain multimedia from the cipher multimedia then the algorithm used for encryption is ineffective.Encryption algorithm to be good must be able to resist differential attack.This is required measuring the effect on pixel change or byte change by using two extensive analyses; they are the "number of pixel changing pixel rate (NPCR) and unified averaged changed intensity (UACI)" [5,6].They are computed by equations 2 and 3.

NPCR
where C1 (i, j) & C2( i, j) are the pixel values in the location of the (I,j).The multimedia are tested here where each type of multimedia is encrypted double, the first one is a plain multimedia which is encrypted by the suggested model , the second is the cipher of plain multimedia after modifying one bit from key encryption.Shows the result NPCR and UACI in Table 2

d. Entropy of Information
Entropy is a very important measure for multimedia encryption.The idea of entropy is to compute the degree of the ambiguity between plain multimedia and encrypted multimedia.The best case will be when the probability of each value is identical.The entropy, H (m) can be computed by equation 4 [4,5].
2N−1 i=0 Where:P(mi)is the probability of mi, H (m) =8 is optimal entropy for multimedia (mi) involves of "256 values when there are equal probabilities" for all value.Shows the result in table 3.

e. Execution Time
The execution time for the encryption practice is a very significant measure.Therefore, the suggested encryption model has good execution time for encryption, plain multimedia and decrypted by using the same secret key.Results of testing for different file multimedia [4].Shows the result in Table 4.
Where D means the decryption algorithm.

Figure 3 :
Figure 3: Histograms of Plain and Cipher Images.The histogram measures in Fig 3 explain distributed pixel values and different between origin image and cipher image, where distributing pixel values for image-cipher are equivalent to avoid-attacks from obtain plain image.

Table 1 :
Correlation Coefficients of Two Adjacent Pixels for Plain and Cipher Image. .

Table 3 :
Entropy for Multimedia Before and After Encryption

Table 4 :
Execution Time for Encryption and DecryptionWith the development of digital communications, multimedia encryption plays a important role, Therefor, an efficient encryption system is necessary.The applied algorithm for encryption is "AES with 128-bit key length".AES key size should be 128-bit.Therefore, the suggested model uses the RC4 algorithm to generated key stream used to products a random key of the wanted size and give more security because if the same multi plain block is repeated, this suggested model produces different multi cipher blocks.Evaluate the performance of this model is tested by applied measures such as histogram where noted distributed pixel values for image cipher are equivalent to avoid attacks from obtain plain image, correlation coefficient noted correlation value near to zero It indicates to no relationship between the variables, Attack Resistant(NPCR and UACI) noted all NPCR values are near to optimum values(100%) and UACI values depending on the density of colors ,entropy of information near to optimum values and suitable execution time.This model can be enhanced the speed of encryption process and decrease the cost of multimedia transition by compression of the multimedia before encryption.