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(IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 4, July 2010 Secured Communication through Hybrid Crypto- Steganography A. Joseph Raphael Dr. V.Sundaram Research Scholar – Karpagm University, Coimbatore, Head and Director, Department of Computer Applications India and Lecturer in Information Technology, Karpagam College of Engineering Ibra College of Technology, Sultanate of Oman Coimbatore, India raphaelaj@gmail.com dr.vsundaram@gmail.com Abstract-In this paper we present a hybrid technology of messages can be hidden in the cover image which can be mixing cryptography and steganography to send secret selected from seasonal greetings. messages. This method has got the advantages of both the methods, and even if one fails the other comes to the rescue. The power of a cryptographic / steganographic system In cryptography we have used the RSA method for should depend only on a small part of information namely encryption and decryption of the original message and the key to uncover the cipher text/material. further the LSB (Least Significant Bit) method is used to hide the encrypted message in the cover image and send to We employ RSA public key cryptography for the the recipient. The original message is retrieved by the encryption and decryption of the original message reverse process, first by collecting the LSB of the pixels and followed by steganography using LSB. In both methods then by RSA decryption. Since the private key for RSA the encryption and decryption depend only on a small key. method is very difficult to find, this method we suggested is a strong encryption method and the messages can be A digital key is a set of bits that are employed to communicated in much secured way in an insecure channel. encrypt and decrypt the messages. A public key Keywords-stegano object; cryptosystem cryptography uses two different keys. 1) A public key to encrypt the original message. 2) A private key to decrypt the cipher text and expose the original message. I. INTRODUCTION Public and Private keys are generated in pairs so that The importance of secret communication by only a specific pair of keys can perform encryption and governments and private organizations has increased a lot. decryption. The public key is made known to everyone By the advent of e-commerce and the increasing trade, whereas the matching private key is kept as a secret by the administration, terrorism, security of nations, secret owner. During mid 1970s cryptogists introduces the communications have taken the top most priority in the concept of public key cryptosystems. In this system communication sector. everyone can have a public key for encryption; however the decryption keys are kept secret only by the intended Steganography is derived from the Greek words stegos, recipient who can decrypt it. meaning roof or covered and graphia which means writing, is the art and science of hiding or embedding a In 1976 Ronald Rivest, AdlShamir, and Leonard secret message over a piece of information such as an Adleman introduced a public key cryptosystem known as image, audio or video and sent them over an insecure RSA system [1]. This system is based on the modular channel to the recipient so that no one can detect or exponentiation modulo, the product of the large primes. decode the secret message. The encryption key consists of a modulus n = pq where p and q are large primes more than 200 digits each and an Steganography is very closely related to Cryptography, exponent e that is relatively prime to ( p − 1)(q − 1) . The both are used to maintain the data in a confidential manner. The main difference between the two is that with two large primes p and q can be found quietly on a Cryptography the message is scrambled and anybody can computer using probabilistic primality tests. The product see that both parties are communicating in secret. of the primes n = pq with about 400 digits cannot be Steganography on the other hand, hides the existence of a factored in a reasonable length of time and hence the secret message and in the best case nobody can see that decryption of the cipher text cannot be done quickly both parties are communicating in secret. In some without a separate decryption key. situations, sending an encrypted message will arouse Here, we describe the process how the original suspicion while an "invisible" message will not do so. message is encrypted, masked and sent, and on the other Both sciences can be combined to produce better end how it is received, unmasked and decrypted into protection of the message. In this case, when the original message, and the same is presented in the steganography fails and the message can be detected, it is schematic diagram below: still of no use as it is encrypted using cryptography techniques. Moreover, to avoid unnecessary suspicious the 45 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 4, July 2010 II. AFFINE TRANSFORMATION using the RSA rule or formula (block ) mod n to get E a set of encrypted integers [1]. This encryption is done by using the following modular exponentiation SOURCE Original Message Secured Domain Encryption using Encrypted message is Crypto/Stegno Compressed file is hidden in an image RSA Algorithm compressed Object using LSB method Insecure RSA Key Channel Domain Secured Domain Decryption using UnZipping to get Collect and Check the size of files Reception of RSA Algorithm encrypted message retrieved from the object and the sizes Crypto/Stegno embedded in the object Object Original Message TARGET algorithm. At first the original message (OM) consisting of a sequence of letters is converted into another sequence of Procedure for modular exponentiation letters using a ceaser cipher or by a more general affine transformation defined by the bijection f. b:integer, n = ( a k −1 a k − 2 ....a1 a 0 ) , f : c = f ( p ) = ( ap + b) mod 26 m: positive integers x:=1 Here p is the rank of a particular letter in OM which is power:=b mod m converted to the number c. In the encrypted version of the for i = 0 to k-1 message, the letter represented by p in OM is replaced begin with the letter represented by c. if a i =1 then x:=(x.power) mod m power := (power.power) mod m III. RSA ENCRYPTION end The new message obtained by the affine transformation { x equals b n mod m } is encrypted by the RSA method as follows: 4. These encrypted blocks are converted into a sequence 1. Each letter in the message is represented as its ASCII of 16 bit binary numbers that is split into two 8 bit code number and all such ASCII code is converted numbers. into an equivalent binary number using 8 bits. Usually the encrypted message is directly inserted into 2. Then each pair of characters are grouped into blocks an image using LSB method, which generally requires a by taking two adjacent 8 bits side by side as one 16 lot of space to hide a relatively few bits of information bit number. If odd numbers of letters are present the which is also one of the disadvantages of LSB method. binary number corresponding to the last character is To overcome this drawback, the encrypted message is first padded with zeros in the beginning to have 16 bits. compressed before it is embedded so that a large amount 3. Each of the message block is represented as an of information can then be hidden. Using the Least equivalent decimal number set that will be encrypted significant bit insertion method of Steganography the 46 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 4, July 2010 binary equivalent of the compressed file is encoded into of the bits would have to be changed in an LSB encoding the Least significant bit of each byte as each pixel is scheme. represented by 3 bytes of a 24 bit image. The outcome of the above stage produces a Crypto/Stegno Object which consist of hidden message in compressed form, within a cover image. Also, the size of IV. USE OF DIGITAL IMAGES the encrypted message, the size of the compressed file and Most popular file formats being BMP (Bitmaps) GIF the size of the crypto/stegno object itself can be embedded (Graphics Interchange Format) and JPEG (Joint in the same cover image after embedding the bits of the Photographic experts Group). Of these formats the first original compressed file. All the above said stages are in two provide a loseless message transfer where the last the Secured Domain, from where the Crypto/Stegno method provides a lossy transfer. Loseless files won’t Object is passed to an Insecured Channel Domain to the shatter the image much if the pixel intensities are altered, recipient target. Once the crypto/stegno object is arrived at whereas the Lossy files tend to shatter heavily while the the recipient target, the following embedded informations pixel values are altered. Hence the lossy images are are retrieved : 1) all the LSBs of the pixels are collected to preferred much for data transfer. form the compressed file 2) size of the encrypted message and 3) size of the crypto/stegno object. To ensure that, the Images are classified as 8 bit or 24 bit images based full and correct message is reached at the recipient target, upon the number of bits they use to represent a color. The the following comparisons are made between 1) the size image files supporting larger intensities are preferred for of the crypto/stegno object received at the recipient target covert transmission and are stored in variety of file and the size of the crypto/stegno object which is formats. Image file is a large array of pixel intensities embedded in the crypto/stegno object and sent 2) the size (Color Values). These pixel values form the raster data of of the compressed file obtained from the object and the the image and could be used to impose the message. Each size of the compressed file which is embedded in the one of these pixels has its own color, and it is represented object and dispatched 3) the size of the encrypted internally as separate quantities of red, green and blue. message after unzipping and the size of the encrypted Each of these color levels may range between 0 (none of message which is embedded in the object. If any the color) and 255 (a full amount of the color). A pixel mismatch occurs in any one of the sizes then it is with an RGB value of (0,0,0) is black, and one with a understood that the crypto/stegno object is subjected to value of (255,255,255) is white. For a 24 bit image this is natural attack while passing over an insecured channel simple because 24 bit images are stored internally as RGB domain and the process has to start from the beginning. triples, and all that needs to be done is to spread the bits and save out the new file. The images are also very large On the other hand, if there is no mismatch, then from as they contain 3 bytes for every pixel (for a 640 x 480 the received crypto/stegno object all the Least Significant image this is 640 x 480 x 3 = 921600 bytes). Bits of the pixels are combined together to form a compressed file. Futher, the compressed file is unzipped When applying LSB techniques to each byte of a 24-bit to get an encrypted message from which the original image, three bits can be encoded into each pixel. (As each message is obtained by undergoing the process of pixel is represented by three bytes.) Any changes in the decryption using RSA Algorithm. pixel bits will be indiscernible to the human eye [2]-[3]. For example, the letter A can be hidden in three pixels. www.stegoarchive.com [4] is a website which have Assume the original three pixels are represented by the many stegno tools to automate the process of changing the three 24-bit words as below LSB to allow for the insertion of some other data on an image and getting back the data and the image separately (00100111 11101001 11001000) at the other end. A few of the more popular applications (00100111 11001000 11101001) are ExStego, Jstego and hide4pgp. S-Tools is another tool that uses a different method for utilizing the LSB theory. (11001000 00100111 11101001) It closely approximates the cover image and that could The binary value for the letter A is 10000011. Inserting possibly cause extreme palette changes in the original the binary value for A into the three pixels, starting from image. the top left byte, would result in (00100111 11101000 11001000) V. THE RSA DECRYPTION (00100110 11001000 11101000) The RSA decryption key D is the inverse E modulo of (11001000 00100111 11101001) ( p − 1)(q − 1) . The emphasized bits are the only bits that are actually ie. ED = 1 mod( p − 1)( q − 1) , this can be found by changed. The main advantage of LSB insertion is that using the euclidean algorithm. Then the decryption is data can be hidden in the least and second to least bits and still to the human eye, the resulting image with embedded done by the rule (block ) D mod n where block is the data will look practically identical to the cover image. cipher text. Notice that only the bolded bits had to be changed in order to create the letter A. On the average only less than 50% The whole strength of the RSA method lies in the fact that even though n = pq is known, the factorization of n is the most difficult problem (as against finding large primes 47 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 4, July 2010 p and q). It is known that even most efficient factorization methods (till recently by 2002) requires billions of years to factor 400 digit integers. Hence when p and q are 200 digits primes, messages encrypted using n = pq as the modulus cannot be found in a reasonable time unless p or q is known. VI. CONCLUSION The RSA-LSB crypto/stegno method suggested has sound mathematical and logical support. The decryption key for the methods depend only on a small part of information namely i. for RSA – the exponent E and its inverse D ii. for LSB, the collection of least significant digits. The method is highly secure and decryption cannot be obtained by illegal persons within a reasonable period of time. For both the methods there are several application/software available and hence execution of the method is readily possible. REFERENCES [1] Kenneth H. Rosen, “Discrete Mathematics and its Applications”, McGraw Hill, Fifth Edition. [2] Neil F.Johnson and Sushil Jajodia, “Exploring Steganography: Seeing the Unseen”, George Mason University, Available online at http://www.jjtc.com/pub/r2026.pdf. [3] J.R. Krenn, “Steganography and steganalysis”, Available at http://www.krenn.nl/univ/cry/steg/article.pdf [4] James C.Judge, “Steganography Past, Present, Future”, Avaiable at http://www.sans.org/reading_room/whitepapers /stenganography/steganography-past-present-future_552 . AUTHORS PROFILE A. Joseph Raphael obtained his Master degree in Computer Science from St. Joseph’s College, Tiruchirapalli and Master of Philosophy in Computer Science from Alagappa University, Karaikudi. Currently, he is a PhD research scholar at Karpagam University, Coimbatore, India and also working as a lecturer in the department of Information Technology, Ibra College of Technology, Sultanate of Oman. Dr. V. Sundaram earned his PhD in mathematics from Madras University. He is a research guide of Anna University, Coimbatore and Karpagam University in the field of computer science and computer applications. He is currently guiding several PhD students in the areas of theoretical computer science, network security, cryptography and data mining. He has published several papers in national and international journals and organized 5 national conferences. He is a life member of ISTE and member of Computer Society of India 48 http://sites.google.com/site/ijcsis/ ISSN 1947-5500

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