The Internet has become a popular communication network where the distribution of multimedia content, confidential data such as military information, financial documents, etc. has become a common practice. But, over Internet the information is viewed to many users. Hence, now the security of visual information has become more and more important in many real applications. To fulfill such an increasing demand of security, many security providing tools are there in this scenario and Visual cryptography is one of them. This was introduced by Naor and Shamir in 1979 to provide confidentiality and security when secret visual information is transmitted through unsecured communication channels. Using secret sharing concepts, the encryption procedure encrypts a secret image into number of shares (printed on transparencies) which can be distributed among group members or transmitted or distributed over number of an untrusted communication channels such that only stacking of these shares can reveal the information otherwise not. Then the idea of secret sharing was also separately proposed by Adi Shamir and G. Blakley in 1979. In 1983 another method of secret sharing was proposed by Asmuth and Bloom. Shamir’s scheme is based on Polynomial Interpolation. Blakley secret sharing is based on hyper plane geometry. Asmuth-Bloom secret sharing scheme is based on Chinese Remainder theorem. Then there are many methods to protect sensitive data in image and Steganography is also one of them. This method hides a secret message in a innocent cover medium which could be a digital image,video, audio; etc. but, the weakness of this technique is that an entire secret data is kept in a single cover medium and if this cover medium get lost or corrupted then that hidden data may also get lost or corrupted. In 2004, Lin and Tsai proposed a method that used Steganography for generation of meaningful shares with secret image sharing. This scheme also used polynomial-based secret sharing approach proposed by Shamir which leads to high computational complexity. As the decryption process is done by human visual system, secret information can be retrieved by anyone if the person gets at least k number of shares. So that simple visual cryptography became very insecure 1.
In this paper we are using symmetric algorithm for image encryption. In Symmetric algorithm sender and receiver ends use a same cryptographic key (symmetric key) for data transformation. Symmetric key cryptography is also known as shared key cryptography. A cryptographic key is a special kind of information that helps the sender to convert original data into encrypted form and at other end it helps the receiver to access the encrypted data. At sender side the message or data is converted into a special data format called cipher text using various encryption algorithm and secret key. The receiver side performs the same operation but in reverse order. It takes cipher text as input and then converts it into the original message. There are various algorithms for image encryption such as chaotic, blowfish, AES algorithms etc 2.We are also using the concept of Binary Tree Data Structure. A binary tree is a tree data structure in which each node has at most two children, which are referred to as the left child and the right child.A binary tree has the benefits of both an ordered array and a linked list as search is as quick as in a sorted array and insertion or deletion operation are as fast as in linked list.
The basic terms used in any cryptosystem are as folows3:-
1) $Plain Text$- The original message that is easily readable by humans. It is a term used in cryptography that refers to a message before encryption or after decryption. For example, A is a person wishes to send “Hello Friend how are you” message to the person B. Here “Hello Friend how are you” is a plain text message.
2) $Cipher Text$- In cryptography, cipher text is data that has been encrypted. This text is unreadable until it has been
converted into plain text with a key. For example, “phqgiumealy” is a cipher text produced for “abcdefghijk”.
3) $Encryption$- It is security tool for computer network. It is process of converting information (known as plain text ) using an algorithm to make it unreadable (known as cipher text) to anyone except those processing special knowledge, usually referred to as a key. It is the most efficient method to achieve data security. Encryption can protect confidentiality of message. For data encryption, a secret key is used. Encrypted data is called as cipher text and decrypted data is called as plain text.
4) $Decryption$- It is process of taking encoded or encrypted text and converting it back into original text. Decryption is used for un-encrypting the data with keys or algorithm. Cryptography uses the decryption technique at the receiver side to obtain the original message from non-readable message (Cipher Text). The decryption process requires two things- a Decryption algorithm and a key. A Decryption algorithm indicates the technique that has been used in Decryption. Usually, the encryption and decryption algorithm are same.
5) $Key$- A key is a numeric or alpha numeric text or may be a special symbol. The Key is used at the time of encryption work on the plain text and at the time of decryption work on the cipher text. The selection of key plays vital rolein cryptography process as the security of encryption algorithm fully depends on it. For example, if A uses a key of 2 to encrypt the Plain Text “University” then Cipher Text produced will be “wpkxgtkva”.