This chapter introduces the reader to cryptography, steganography, watermarking, and quality parameters of image steganography techniques. Cryptography is a technique for secret communication. Steganography is a technique for secret and unnoticeable communication. The watermarking techniques hide watermarks inside the digital media. There are four types of steganography techniques: (1) image steganography, (2) audio steganography, (3) video steganography, and (4) text steganography. The quality of image steganographic algorithms can be measured by three parameters like (1) hiding capacity, (2) distortion measure, and (3) security check.
TopIntroduction
During the last few decades usage of Internet has grown tremendously. Now-a-days it is the most popular communication media. But the security of data during transit through internet has become a major challenge which needs to be addressed suitably. There are two main methods to address this challenge; (i) cryptography and (ii) steganography (Swain & Lenka, 2012a). In cryptography, the secret message is encrypted to cipher text, which is subsequently communicated to the receiver. Although an intruder cannot understand the communication, but can notice the communication. On the other hand, steganography hides the secret data inside a carrier file such that it looks as if not hiding anything. So, the intruder can neither understand the communication nor suspect the communication. By using cryptography and steganography together, the security becomes two-fold. A good steganographic algorithm should possess higher hiding capacity, lesser distortion and undetectable by steganalytic attacks.
Cryptography
Cryptographic techniques are broadly categorized into two classes, (i) private key or symmetric key cryptography and (ii) public key or asymmetric key cryptography (Swain & Lenka, 2012a). In private key cryptography both sender and receiver share a common secret key. Sender encrypts the secret data using the encryption algorithm and the secret key, the receiver decrypts the cipher text using decryption algorithm and the secret key. The encryption/decryption algorithms are publicly available to the world but not the key. The only way to protect the secrecy is by maintaining the confidentiality of the secret key between the sender and the receiver. If the hacker captures the key the confidentiality will be lost. On the other hand, public key cryptographic techniques use two keys for communication. Suppose Alice is the sender and Bob is the receiver. Alice maintains two keys; (i) the public key, EA and (ii) the private key, DA. Alice shares his public key to everyone including Bob, but keeps his private key very secret. Bob also uses two keys; (i) the public key, EB and (ii) the private key, DB. Bob shares his public key to everyone including Alice, but keeps his private key secret. To send a secret message to Bob, Alice has to encrypt the message by using Bob’s public key. This encrypted message can only be decrypted by Bob’s private/secret key. Similarly, to send a secret message to Alice, Bob has to encrypt the message by using Alice’s public key. This encrypted message can only be decrypted by Alice’s private/secret key. Figure1 (a) represents how Bob sends message and Alice receives using symmetric key cryptography and Figure 1 (b) represents how Bob sends message and Alice receives message using public key cryptography.