Improved Image Steganography Using LSB Insertion - An Overview

Aparna S Shinija B

Information Technology Information TechnologyGovt. Engg. College Govt. Engg. College

Trivandrum Trivandrum

Abstract

Steganography is a technique used for transmissionof secret message. Here the existence of message isunknown. The secret message can be embedded withina digital medium like audio, video, image or text.

Image steganography uses image as the cover ob-ject. The most commonly used Image Steganographytechnique is LSB Insertion. This paper intends to givean overview about how to enhance the security of thetechniques by using a suitable cryptographic algorithm,i.e. , how cryptography can be used to supplementsteganography.

1 Introduction

In todays world, communication is one of the basicneeds.There are different techniques used for transmis-sion of secret message. Steganography and cryptogra-phy are two common methods used for the transmis-sion of digital data.

1.1 Cryptography

Cryptography (or cryptology; from Greek krypts,hidden, secret; and graphein, writing, or logia,study, respectively[1]) is the practise and study oftechniques for secure communication in the presenceof third parties (called adversaries).[2]

In cryptography, the secret essage is encrypted withor without using a key, i.e ,the message is convertedfrom a readable form to scrambled one.

1.2 Steganography

Steganography is the art or practice of conceal-ing a file, message, image, or video within another file,message, image, or video. The word steganography

combines the Ancient Greek words steganos, mean-ing covered, concealed, or protected, and grapheinmeaning writing. Here the existence of mesage isunknown. The object in which the secret message ishidden is called the cover object and the object ob-tained as the output of steganography is called stegoobject.In image steganography, image is used as themedium for the transmission of secret message.

2 An Overview of LSB Based ImageSteganography

LSB Insertion is one among the most commonmethod used in Image Steganography.To give anoverview on LSB Based Image Steganography, termsand concepts are explained first.

2.1 Image

To a computer, an image is a collection of numbersthat constitute different light intensities in differentareas of the image [3]. This numeric representationforms a grid and the individual points are referred toas pixels. Most images on the Internet consists of arectangular map of the images pixels (represented asbits) where each pixel is located and its colour [4].These pixels are displayed horizontally row by row.

The number of bits in a colour scheme, called the bitdepth, refers to the number of bits used for each pixel[5]. The smallest bit depth in current colour schemes is8, meaning that there are 8 bits used to describe thecolour of each pixel [5]. Monochrome and greyscaleimages use 8 bits for each pixel and are able to dis-play 256 different colours or shades of grey. Digitalcolour images are typically stored in 24-bit files anduse the RGB colour model, also known as true colour[5]. All colour variations for the pixels of a 24-bit im-age are derived from three primary colours: red, green

and blue, and each primary colour is represented by 8bits [3]. Thus in one given pixel, there can be 256 dif-ferent quantities of red, green and blue, adding up tomore than 16-million combinations, resulting in morethan 16-million colours [5]. Not surprisingly the largeramount of colours that can be displayed, the larger thefile size [4].

2.2 Image Domain

Image steganography techniques can be dividedinto two groups: those in the Image Domain and thosein the Transform Domain . Image also known as spa-tial domain techniques embed messages in the inten-sity of the pixels directly, while for transform alsoknown as frequency domain, images are first trans-formed and then the message is embedded in the im-age .

2.2.1 LSB Insertion

It is one among the most common method used forhiding secret message in cover image.Here, the LeastSignificant Bit (LSB) of some or all the byte inside animage is changed into the bit of the secret message.Suppose, we are using a 24 bit image. Then 3 bits canbe stored in each pixel. For eg, consider 3 pixel of a24 bit image as follows:

10111000 10111000 1011101111110000 10111001 1011010101101101 00010101 11010101

Now, let the secret message be 00001011. When thissecret message is embedded into the LSB of the image,resulting gris is as follows:

10111000 10111000 1011101011110000 10111001 1011010001101101 00010101 11010101

Even though the secret message is embedded in thefirst 8 bits, onlythe underlined 2 bits have changed.So,on an average only half of the bits may undergochange.Also, changing the LSB of the image resultsonly in small change in intensity of the colour whichcannot be perceived by human eyes.

3 Enhancing security of steganographyusing cryptographic algorithm

Inorder to enhance the security of steganography,cryptographic algorithms can be used, i.e. , steganog-raphy is not a replacement for cryptography, but cryp-tography can be used to supplement steganography.

First, the secret message is converted to binaryform. Then the message is encrypted using a suit-able cryptographic algorithm, before embedding intothe image.In this paper, we use RSA Algorithm to en-crypt the message which will be explained in the nextsection.A model of the steganographic process withcryptography is shown in fig 1.

Figure 1: Steganography combined with Cryptogra-phy

3.1 RSA Algorithm

3.1.1 Introduction

RSA is one of the first practicable public-key cryp-tosystems and is widely used for secure data trans-mission. In such a cryptosystem, the encryption keyis public and differs from the decryption key which iskept secret. In RSA, this asymmetry is based on thepractical difficulty of factoring the product of two largeprime numbers, the factoring problem. RSA stands forRon Rivest, Adi Shamir and Leonard Adleman, whofirst publicly described the algorithm in 1977. CliffordCocks, an English mathematician, had developed anequivalent system in 1973, but it was not declassifieduntil 1997 [6]Suppose, Alice wish to send a secret message toBob.Alice will first encrypt the secret message usingBobs public key and send it to Bob.Bob can decryptthe message using his private key.

3.1.2 Operation

RSA Algorithm involves three steps: key generation,encryption and decryption.

Key GenerationRSA involves two type of keys: public key andprivate key. Public key is made public.The se-cret message encrypted by a public key can bedecrypted only using the corresponding privatekey.The keys are generated as follows:

1. Choose two very large random prime num-bers ,say p and q

2. Compute the product n=p.q

3. Compute f(n)=(p-1)(q-1)

4. Choose a random number e such that1ef(n) and GCD(e,f(n))=1(e,n) is the public key.

5. Compute d such that (d*e) mod f(n)=1(d,n) is the private key.

EncryptionEncryption is done using the public key (e,n).Suppose the secret message be M.ThenC=Me mod n is the cipher text where M n

DecryptionDecryption is done using the private key(d,n).Suppose the cipher text is C.ThenM=Cd mod n is the plain text.

3.2 Encoding

The embedding algorithm is as follows:

1. Choose the cover image and secret message

2. Encrypt the secret message using RSA Algorithm

3. Embed the encrypted secret message in the coverimage using LSB Insertion to obtain stego image

4. Send the stego image to the receiver

3.3 Decoding

The decoding algorithm is as follows:

1. Receive the stego image

2. Retrieve the cipher text from the stego image

3. Apply RSA decryption to decrypt the retrieveddata

Figure 2: Embedding secret data into cover image

Figure 3: Retreiving secret data from cover image

4 Summary and Conclusions

The security of image steganography system is en-hanced and iproved using a cryptographic algorithmlike RSA Algorithm.Here, even if a third person comesto know about the existence of secret data in an im-age and retrieve the seret data, he will get only theencrypted message which is meaningless, thereby en-hancing security of image steganography.So steganog-raphy is not intended to replace cryptography,butrather to supplement it.

References

[1] Liddell and Scotts Greek-English Lexicon, OxfordUniversity Press , 1984.

[2] Rivest,Ronald L(1990), Cryptology In J.VanLeeuwen Handbook of Theoritial Computer Sci-ence, 1. Elsevier.

[3] Johnson N.F.,Jajodia S. Exploring Steganog-raphy: Seeing the Unseen Computer Journal,February 1998

[4] Reference guide:Graphics Tech-nical Options and Decisionshttp://www.devx.com/projectcool/Article/19997

[5] Owens M. A discussion of covert channels andsteganography SANS Institute, 2002

[6] Smart, Nigel Dr Clifford Cocks CB Bristol Uni-versity, 2008.