Pure Steganography - does not

e-Security | Vol: 36-12014 © CyberSecurity Malaysia 2014 - All Rights Reserved 13

b. Secret Key Steganography - the

sender chooses a cover and embeds the secret message into the cover using a secret key. It is similar to symmetric cipher in cryptography. Figure 2 shows how Secret Key Steganography works.

c. Public Key Steganography - requires

two keys which are private key and public key. The public key is used in the embedding process and stored in a public database while the secret key is used to reconstruct the secret message. There are several approaches to classify the steganography system. Al-Ani et. al., 2010 has classiied steganography into six categories namely substitution systems, transform domains, spread spectrum techniques, statistical methods, distortion methods, and cover generating methods. However, only steganography substitution systems will be discussed further in this article. Substitution System Methods The basic idea of this substitution systems is to encode secret information by substituting insigniicant parts of the cover with secret message bits. The information can be extracted if the receiver has knowledge of the positions where secret information has been embedded. By using this substitution system, the sender assumes that it will not be noticed by a passive attacker since only minor changes and modiications were made during the embedding process. Steganography substitution systems are divided into eight methods Johnson et. al., 2000 shown in Figure 3. Each method will be discussed in the following subsections. Figure 1: Pure Steganography Figure 2: Secret Key Steganography Figure 3 : Steganography Substitution System Methods A. Least signiicant bit substitution Message and cover will be in binary form which only contains ‘0’ and ‘1’ bits. Figure 4 below shows the position of the most significant bit MSB and the least significant bit LSB in a binary representation. Figure 4 : Binary Representation e-Security | Vol: 36-12014 © CyberSecurity Malaysia 2014 - All Rights Reserved 14 The embedding process starts by choosing the cover elements and performing the substitution operation on them. The least signiicant bits of the cover will be exchanged with the bit of message. This method allows more than one message bits to be stored in the two least signiicant bit of the cover. This process is illustrated in Figure 5. In this example, the cover contains all ‘0’ bits, while the bit message contains all ‘1’ bits. In order to extract the secret message, the least signiicant bit of the selected cover- elements will be extracted and lined up. However, the sequence of element indices used in the embedding process must be accessible by the receiver. By using this method, information can be hidden with little impacts to the carriers. Because of the simplicity of applying it into image and audio, this method is now common to steganography. B. Pseudorandom Permutation In this method, a sequence of element indices will be generated by using a pseudorandom generator. Then the secret messages will be stored into the cover according to the bit position determined by the previous generated sequence. Therefore, the secret message bits can be distributed randomly over the whole cover if all cover bits can be accessed in the embedding process. Figure 6 below is an illustration of the pseudorandom permutation method. C. Image Downgrading and Covert Channels Image downgrading is a special case of a substitution system. It could be used to exchange images covertly. They are usually used for “leaking” information. Covert channels in operating systems allow processes to communicate “invisibly” and possibly across different security zones speciied by a security policy. Images act both as secret messages and covers. In this method, a cover image and a secret image have an equal dimension. The sender exchanges the four least signiicant bits of the cover’s grayscale values with the four most signiicant bits of the secret image. The receiver extracts the four least signiicant bits of the secret image. Four bits are suficient to transmit a rough approximation of the secret image since the degradation of the cover is not visually noticeable in many cases. This process is illustrated in Figure 7 below. Figure 6 : Pseudorandom Permutation Figure 7 : Image Downgrading and Covert Channels Figure 5 : LSB Substitution