A Study On Steganography Techniques

The quick development in correspondence advances and the expanded accessibility of people in general systems (Internet) encouraged information exchange. Anyway the general population correspondence channels are defenseless against security assaults that may prompt unapproved access to some data. Encryption has been utilized to continue and keep these assaults. In any case, when the data is unscrambled it will be presented to the assailants again and it won’t have any security insurance. Steganography is the investigation of implanting the mystery messages inside other medium documents (content, sound, picture, and video) in a way that shrouds the presence of the mystery message by any means. Steganography applies an implanting procedure in which the repetitive bits of the medium are supplanted by the bits of the mystery message. Image steganography is the field of steganography in which the medium that used to convey the mystery information is a computerized picture. Picture steganography is a vital zone of research in the ongoing years. This article surveys the steganography in light of advanced pictures; representation of the idea and the regular methodologies are examined.

Introduction

Nowadays internet offers more convenience in transmitting large amounts of information in many parts of the world. However, the safety and security of long distance communication remains a problem. In order to solve this problem by using steganography schemes. The process of hiding secret data in the cover image while hiding its own existence is referred to as steganography. Audio, video, text, images etc can be the secret data. The confidential data is converted into binary form and then it is embedded into the cover image This output image is called as stegoimage. If the cover image is an image file, then the technology is called as image steganography. Securing multimedia data needs blocking unauthorized people from access, distortion, destruction, detection or change of data during its trasfer. There are two basic methods for data security that is encryption and steganography. The encryption method protects data by converting it to an unclear form that cannot be understand by hackers. A data hiding system consists of two main processes;the embedding process, which embeds the secret file inside the stego file, and the extracting process which retrieves the embedded secret file. Some data hiding schemes use lossy compression, to allow for higher hiding capacity at the expense of loosing bits of the secret file. Image steganography indicates sending the secret data in the cover image while hiding the very existence of it self. Before embedding the secret data in the cover image, it is encrypted in order to obtain betterlevel of security. Steganalysis is concerned with field of defeating steganographic techniques.

1. Steganography

Image Steganography requires following elements to carry out the work:

• Cover medium : It is an image that holds secret message.
• Secret message: it is message to be transmitted. It can be plain or encrypted text, images or any other data.
• Stego-key: it is key used to hide the message. The well- known technique namely LSB embedding process.

n order to hide information in the cover image, some particular bits in the cover image are replaced by the bits of the secret data. Steganographic techniques have gained a great importance in a certain number of application areas. Steganography is sometimes used when encryption is prohibited. Or, more commonly, steganography is used to supplement encryption. By using steganography, an encrypted le can still hide information, even if the encrypted le is decrypted; the hidden message may not be visible. Several approaches have been proposed to hide information. LSB is mainly based on the modication of the least signicant bit of each pixel in the image.

2. Factors Affecting a Steganographic Method

The effectiveness of any steganographic method can be determined by comparing stego-image with the cover Image. There are some factors that determines the efficiency of a technique. These factors are:

1. Robustness: Robustness refers to the ability of embedded data to remain intact if the stego-image undergoes transformations, such as linear and non-linear ltering, sharpening or blurring, addition of random noise, rotations and scaling, cropping or decimation, lossy compression.
2. Imperceptibility: The imperceptibility means invisibility of a steganographic algorithm. Because it is the first and foremost requirement, since the strength of steganography lies in its ability to be unnoticed by the human eye.
3. Payload Capacity: It refers to the amount of secret information that can be hidden in the cover source. Watermarking usually embed only a small amount of copyright information, whereas, steganography focus at hidden communication and therefore have sufficient embedding capacity.
4. PSNR (Peak Signal to Noise Ratio): It is dened as the ratio between the maximum possible power of a signal and the power of corrupting noise that affects the delity of its representation. This ratio measures the quality between the original and a compressed image. The higher value of PSNR represents the better quality of the compressed image.
5. MSE (Mean Square Error): It is dened as the average squared difference between a reference image and a distorted image. The smaller the MSE, the more efficient the image steganography technique. MSE is computed pixel-by-pixel by adding up the squared differences of all the pixels and dividing by the total pixel count.
6. SNR (Signal to Noise Ratio): It is the ratio between the signal power and the noise power. It compares the level of a desired signal to the level of background noise.

Materials & methods

1. LSB based steganography method

An image or a picture is a set of numbers that represent the different light in-tensities in the image. This digital representation forms a grid or a rectangular array of points called pixels. The Color image is made of colored pixels. Eachpixel rep- resents the light intensity at a single point in the image. These pixels are displayed horizontally row by row The color images use usually 24-bits for color representation where the RGB model (red, green, blue) is used to represent colors. Each pixel holds three numbers corresponding to thered, green, and blue levels in the image location. The red, thegreen and the blue colors are the primary colors for mixing light and any other color can be derived from these primary colors. Each primary color is represented by 8 bits. Each color pixel can be stored in 24 bits of memory. Indeed, for an 8-bit image, the least signicant bit ie the bit number 8 of each byte of the image is changed to the secret message bit. For a 24-bit image or RGB image model, the colors of each pixel (red, green and blue) are replaced by the secret information. LSB is effective on BMP images because BMP compression is lossless. However, to hide the secret message inside a BMP le image with LSB, we need a great image to use as a cover.

The main steps in LSB technique can be given as follows:

1. Browse all pixels of the cover image where each pixel iscomposed of three RGB components (red, green, blue).
2. Find the value of the color of each pixel between (0-255)and convert it to binary.
3. Convert the message to be hidden (the secret information) in ASCII and binary.
4. For each bit of the secret message, replace the last bit ofthe color of each pixel color by this bit.
5. Hide three bits of the message in each pixel and save thenew color.
6. Save all changes in the image and get the new image.
1. The data encoding: To encode a hidden message in an image with LSB technique, we give the following steps:
1. Extract the pixels of the cover image.
2. Extract the message to be hidden.
3. Extract the key to be used in the encoding method.
4. Start with the first pixel and pick characters of the key and place it in first component of pixel.
5. Place some terminating symbol to indicate end of the key.
6. Insert characters of the message file in each first component of next pixels by replacing it.
7. Repeat Step 6 until all the characters have been embedded.
8. Place again some terminating symbol to indicate end of data.
9. Get the Stego image.
2. The extraction of the hidden image: The extraction process permits to find the hidden bits of the secret message in the least significant bit of the pixels within a cover image. The receiver can extract the hidden message by using the stego- key. The process of message extraction from the Stego image is given as follows:
1. Input: Stego image, stego-key.
2. First we extract the pixels of the Stego image.
3. From each pixel, we extract the three colors RGB (red, green, blue).
4. Convert each color in binary.
5. Recover the 8 th bit of each color and always keep the bit order.
6. If the number of recovered bits equals to 8 then convert to decimal and get the corresponding character in ASCII code.
7. Follow step 5 and 6 until the key indicates the end of the message.
8. Reconstruct the secret message. give the binary representation of the secret message as 11111010000. When applying the LSB technique we will integrate the secret image in the least signicant bits of this part of the image.
3. LSB-S Steganography Method Blend with Cryptography

A new method of image steganography, here gray images blend with cryptography. By this method, the message is first encrypted using Vernam cipher algorithm and then the message is embedded inside an image using the new image steganography method i. e. LSB with Shifting (LSB-S).

In LSB-S method take four LSB of the pixel and performed circular Left shift operation and XOR operation. The combinations of Cryptography and steganography method enhance the security of information.

1. Insertion Algorithm:
1. Take a gray scale image i. e. I
2. Take the message and convert it into cipher text using vernam cipher and store it in the form of bit in M.
3. Perform the followings. Extract the four LSB Bits in a Temporary matrix Tl of pixel and perform I-Bit circular left shift over these bit and again store them in Tl. Shift on next element of Tl by increasing 1 in present location. Get the LSB of first element in T1 from and perform XOR with first message bit from message matrix. Decrease the length of message by 1.
4. Store the result obtained previous step in I at first pixels LSB and Shift on next element of I by increasing 1 in present location.
5. Repeat step 3 till length of M=O;
6. Stop
2. Retrieval Algorithm:
1. Take Stego-Image and extract all the pixel i. e. embedded pixels.
2. Perform I-Bit circular left shift operation over 4-LSB bits of these pixels and get the rotated pixel. Store these shafted pixels in a matrix T2.
3. Perform XOR of 1. LSB bit of first embedded pixel of Stego image with LSB bit of corresponding shifted pixel from T2.
4. Repeat step 3 for whole message, as receiver knows the message length.
5. After XOR operation, the corresponding message bit will be got. Which is the required encrypted message Convert this message into character form.
6. Apply Vernam cipher decoding algorithm to decrypt the received message.
7. Finally the secret message will be received.
2. Hash Based Least Signicant Bit (2-3-3)

Hash based LSB 2-3-3 Technique is used in spatial domain. An application of the algorithm is illustrated with Color Image file as a cover medium. The results obtained are good and encouraging, compared with based LSB technique. The proposed technique takes eight bits of secret data at a time and put them in LSB of RGB (Red, Green and Blue) pixel value of the cover image in 2, 3, 3 order respectively. Such that out of eight (08) bits of message five (05) bits are inserted in R and G pixel and remaining three (03) bits are inserted in B pixel. The proposed 2-3-3 algorithm, for encoding and decodingare given in this section.

1. Algorithm of Encoding:
1. Input cover image file.
2. Read secret image file.
3. Take 4 LSB bits of each RGB pixels of the cover image.
4. Obtain the position for inserting the secret data into cover image using hash function.
5. Embed the eight bits of the secret image into 4 bits of LSB of RGB pixels of the cover image in the order of 2, 3, 3 respectively using the position obtained from step 4.
6. Repeat steps 3 to 5 until all pixels of secret image are embedded in cover image.
2. Algorithm of Decoding:
1. Input stego image file.
2. Take 4 LSB bits of each RGB pixels of the stego image.
3. Obtain the position of embedded bits of the secret data using hash function.
4. Retrieve the bits using these positions in the order of 2, 3, 3 respectively, using the position obtained from step3.
5. Reconstruct the secret information.
6. Repeat steps 3 to 5 until all pixels of secret image embedded are retrieved.
3. Image Steganography Using Neural Networks and Visual Cryptography

Information Security has been a basic region of worry as the correspondence medium is generally utilized over web. Image steganography demonstrates sending the mystery information in the cover image while concealing the specific presence of it self. Before embed-ding the secret data in the cover image, it is encrypted in order to obtain better level of security. Cryptography involves a process called encryption and is not concerned with hiding the secret data in the cover image. Visual cryptography is a renowned technique to protect data which is image based. During encryption, the image is split into n number of shares. During decryption, these shares are stacked together to retrieve original image. Hence specific decryption algorithm is not necessary and just human visual system would suffice. In order to enhance security mechanism, visual cryptography and image steganography are used together. Neural networks are concerned with identifying the best locations in host image in order to embed the secret data thus improving the image quality.

Conclusion

In this paper different steganographic techniques were studied. Each of these techniques tries to satisfy the three most important factors of steganographic design. They are imperceptibility, capacity, and robustness. Multimedia files are usually larger than text and still image documents, hence require higher embedding space within cover files. Least Significant Bit Substitution (LSB )is the most commonly used stenographic technique. The least significant bit (LSB) technique is a suitable choice for high capacity hiding. There have been several alternatives of the LSB method regarding the number of least significant bits per pixel or element of a pixel that can be replaced, such as 1-bit, 2-bit, 3-bit, and 4-bit LSB. As many new application areas are identified like internet banking, mobile communication security, cloud security etc. , the insight into the steganographic principles will definitely guide us to identify new areas and to improve its applications in the already existing application areas also.