A crypto system can be used to encrypt messages sent between two communicating parties so that an eavesdropper who overhears the encrypted messages will not be able to decode them. The paper mainly concentrates on the method in which the substitution technique of steganography can been used to hide data in a 24-bit bitmap file. Popular audio hiding techniques based on methods of steganography is also discussed here.
Information hiding started its development as a separate sub discipline of information security with steganography 1 and watermarking as its main branches. It borrowed ideas from the more established sister disciplines, namely, cryptography 2 . A crypto system can be used to encrypt messages sent between two communicating parties so that an eavesdropper who overhears the encrypted messages will not be able to decode them.
The paper mainly concentrates on the method in which the substitution technique of steganography can been used to hide data in a 24-bit bitmap file and try to focus on the popular audio hiding techniques based on methods of steganography.
The following section tries to develop a sample code to show the effect of hiding a secret message in an image large enough to cover the message.
Cover Media : A 24-bit BMP file ( File name: C).
Secret Message : Can be text, audio, video or any other file format (File name: M).
Start. 2. The RGB information is read from the cover file C into bytes r, g and b.
Two bits from the LSB of r is replaced by the first two bits of the first character in the secret message file M. 4. Three bits from the LSB of g is replaced by the next three bits of the first character in M. 5. Three bits from the LSB of b is replaced by the next three bits of the first character in M. In this way one byte of M gets covered by the r, g, b bytes of C. 6. The next r, g and b bytes are chosen from the cover file C after leaving a fixed amount of space which equals (cover file size)/size of (r+g+b)*(message file size). 7. Steps 3 to 6 are repeated throughout the file C until the entire message gets embedded.
The information about the message file size and the extension of the message file to be formed after decryption at the receiver end also remains hidden in the encrypted file. The encrypted file also bears information whether it is a genuine encryption or not.
The decryption is done in just a process similar to that of encryption.
Encrypted Message or the Stego Media : A 24-bit bitmap file (File name: E).
- Start. 2. At first the file E is tested to know whether it is a genuine encryption or not. 3. Next while reading the header of E, the size of the message file is obtained from the reserved bits and the pixel spacing is calculated using the same formula as in the case of encryption. 4. The extension information of the message file is read from the header of E and accordingly an empty file is built with the same extension that serves as the destination. 5. Next the r, g, and b bytes are read from E at equal pixel spacing and the message bits are extracted and stored sequentially in the destination file created above. 6.
Step 5 is continued until the end of file of E is encountered.
Let us consider the case of encrypting the character ‘a’.
The binary form is :
Let the r, g ,b values be as shown :
As an integral backbone of voice communication, the internet provides various audio applications like voice query, voice activated websites, etc. On the internet, the windows audio visual (WAV), audio interchange file format (AIFF) and motion picture experts group layer III (MP3) files support a data rate varying from 8 kbps to 44.1 kbps. Audio traffic on the internet system is increasing rapidly, that’s why it is obvious to choose an audio file as a cover media.
Audio hiding techniques rely on the weakness of the human auditory system ( HAS ). Small variation in the amplitude level, phase, spectral magnitude and some other parameters can be done without substantially degrading the signal. LSB insertion can be easily done to generate high embedding capacity in audio signals. The noise introduced due to embedding can be adjusted using adaptive data attenuation. 3
There are various techniques of audio encryption with phase coding and echo hiding as its main sub categories.
Small modification in the phase of the signal is done in such a way that the relative phase among segment is preserved. As for example, the cover signal has already been transmitted to the receiver and now the same signal with some delay in the phase is being transmitted as given in Figure 1. Here the receiver has to follow the two incoming signals to recover the original message that is being embedded into the phase of the cover signal.
Here an echo is introduced in a signal by manipulating the parameters (initial amplitude, decay and offset) such that the echo is not audible. For a discrete signal f(t), an echo f(t-dt), with some delay can be introduced to produce the stego signal s(t)=f(t) + f(t-dt). Figure 2 represent the same.
By two procedures it can be explained how the echo is embedded and how it is extracted from cover signal so as to recover the data. Firstly, it can be explained by means of convolution theory. As for example, a non return to zero (NRZ) cover signal has been transmitted to the receiver and now the cover signal with some delay (echo) is again transmitted. Now the convolut
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