introduction: science of writing hidden messages in such a

introduction:in the present world of communication, computer andinternet are the major media that connects different parts of the world as oneglobal virtual world in this modern era. so we can easily exchange lots ofinformation  within seconds of time , butthe confidential data that needs to be transferred should be kept confidential.so by using steganography and cryptography we can secure the confidential data. but individually  steganography and cryptography providesconfidentiality  to the data but theysome vulnerability so by combining steganography with cryptography we have moresecurity.steganography is the art and science of writinghidden messages in such a way that no one, except the sender and receiver,suspects the existence of the message, a form of security through hiding themessage.ie., Steganography is concealed writing and is the scientific approachof inserting the secret data within a cover media such that the unauthorizedviewers do not get an idea of any information hidden in it.Cryptography is also known as the science of secretwriting.

Cryptography is the study of mathematical techniques related toaspects of information security such as confidentiality, entity authentication,integrity of data and message origin authentication The goal of cryptography isto make data unreadable by a third party. Cryptography algorithms are divided into symmetric(secret-key) and asymmetric (public-key) network security protocols. Symmetricalgorithms are used to encrypt and decrypt original messages (plaintext) byusing the same key.

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While Asymmetric algorithms uses public-key cryptosystem toexchange key and then use faster secret key algorithms to ensureconfidentiality of stream data. In Public-key encryption algorithms, there is apair of keys, one key is known to the public, and is used to encryptinformation to be sent to a receiver who owns the corresponding private key.The private and public keys are both different and need for key exchange.Individually both steganography and cryptography provides security to the data butthey have some vulnerability.so by combination of  steganography and cryptography we have moreconfidentiality and security.There are different techniques available in steganographyand cryptography  so can we havedifferent combinations of steganography.

In this paper we compare different techniquesavailable in steganography based on their security. And on the improved LSBbased  steganalysis combined with  RSA algorithm of cryptography.Steganography Techniques:Steganographyis the art of hiding the existence of data in another transmission medium toachieve secret communication.In steganography the secrete data is embedded intothe cover image in such way that only cover image  is visible which is sent from transmitter toreceiver without scrambling. It does not replace cryptography but it can beused to improve the security of cryptography . COMPARISIONOF DIFFERENT TECHNIQUES IN STEGANOGRAPHY:1)lsb-steganography:In LeastSignificant Bit (LSB) steganography 8 embed the text message in leastsignificant bits of digital picture. In which data is embedded by replacing theLSB of cover carrier with the data to be send.ie first read the cover image andtext message which is to be hidden in the cover image, then convert textmessage in binary.

Calculate LSB of each pixels of cover image. Replace LSB ofcover image with each bit of secret message one by one so we get an image inwhich data is hidden.2)dct–steganography:Thehidden message is converted into binary stream of “1” and “0” are insert theinto the DCT domain of the cover image. The color-based transformation convertsthe image (cover image) into 8×8 blocks of pixels.

8 Next, take largerpositive coefficients need to embed in the cover image in the low-mid frequencyrange. DCT can divide the image into high, middle and low frequency components.As the high frequency coefficients are vulnerable and less robust on thequality of image.

The main issue of this work is robustness against with highquality of image, thus the low and mid frequency coefficients are the mostappropriate. The selected coefficients ci are modified by the corresponding bitin the message stream. This K quantity represents the persistence factor. Assoon as the ith term of message bit s(i) is “1”, the coefficient of the imageis added with a quantity K; otherwise the same quantity is subtracted from it.3)DWT-SteganographyAdiscrete wavelet transform (DWT) is any wavelet transform 6 for which thewavelets are discretely sampled. This is one of the frequency domains in whichsteganography can be implemented.

DCT is calculated on blocks of independentpixels, a coding error causes discontinuity between blocks resulting inannoying blocking artifact. This drawback of DCT is eliminated using DWTbecause DWT applies on entire image. DWT offers better energy compaction thanDCT without any blocking artifact. DWT splits component into numerous frequencybands called sub bands known asLL –Horizontally and vertically low passLH –Horizontally low pass and vertically high passHL -Horizontally high pass and vertically low passHH -Horizontally and vertically high passSinceHuman eyes are much more sensitive to the low frequency part (LL sub band) wecan hide secret message in other three parts without making any alteration inLL sub band.

As other three subbands are high frequency sub-band they containinsignificant data. Hiding secret data in these subbands doesn?t degrade imagequality that much. S.

NO Technique MSE PSNR 1 LSB     2 DCT       CRYPTOGRAPHYTECHNIQUES:Cryptographyis the study of sending messages in cipher form so that receiver only  can able to read the message. It is performedby converting messages into  encrypted byusing symmetric key algorithm and asymmetric key algorithm. In symmetric keyalgorithm a single key is used for both encryption and decryption. Inasymmetric key algorithm  we use publickey for encryption purpose and private key for decryption purpose.RSA algorithm:RSA wasfirst described in 1977 by Ron Rivest, Adi Shamir and Leonard Adleman of theMassachusetts Institute of Technology. RSA is one of the earliest public-keycryptosystems and it is widely used for securing data transmission.

  Here encryption key is public and decryptionkey is private, it kept secret. RSA is based on factorizing two large primenumbers. The public and the private key-generation algorithm is the mostcomplex part of RSA cryptography. We can consider two  prime numbers, x and y,. A modulus iscalculated by multiplying x and y. This number is used by both the public andprivate keys and provides the link between them. Its length is called the keylength.