05-07-2012, 10:22 AM
loud sounds
(loud sounds drown out quiet sounds) and this is what must be exploited to encode secretmessages in audio without being detected.There are two concepts to consider before choosing an encoding technique for audio.They are the digital format of the audio and the transmission medium of the audio.There are three main digital audio formats typically in use.They are Sample Quantization, Temporal Sampling Rate and Perceptual Sampling.Sample Quantization which is a 16-bit linear sampling architecture used by popular audioformats such as (.WAV and. AIFF). Temporal Sampling Rate uses selectable frequencies(in the KHz) to sample the audio. Generally, the higher the sampling rate is, the higher the usable data space gets. The last audio format is Perceptual Sampling. This formatchanges the statistics of the audio drastically by encoding only the parts the listener perceives, thus maintaining the sound but changing the signal. This format is used by themost popular digital audio on the Internet today in ISO MPEG (MP3).Transmission medium (path the audio takes from sender to receiver) must also beconsidered when encoding secret messages in audio. W. Bender
[8]
introduces four possible transmission mediums:1) Digital end to end - from machine to machine without modification.2) Increased/decreased resampling - the sample rate is modified but remains digital.3) Analog and resampled - signal is changed to analog and resampled at a different rate.4) Over the air - signal is transmitted into radio frequencies and resampled from amicrophone.We will now look at three of the more popular encoding methods for hiding datainside of audio. They are low-bit encoding, phase-coding and spread spectrum.Low-bit encoding embeds secret data into the least significant bit (LSB) of theaudio file. The channel capacity is 1KB per second per kilohertz (44 kbps for a 44 KHzsampled sequence). This method is easy to incorporate but is very susceptible to data lossdue to channel noise and resampling.Phase coding substitutes the phase of an initial audio segment with a reference phase that represents the hidden data. This can be thought of, as sort of an encryption for the audio signal by using what is known as Discrete Fourier Transform (DFT), which isnothing more than a transformation algorithm for the audio signal.Spread spectrum encodes the audio over almost the entire frequency spectrum. Itthen transmits the audio over different frequencies which will vary depending on whatspread spectrum method is used. Direct Sequence Spread Spectrum (DSSS) is one suchmethod that spreads the signal by multiplying the source signal by some pseudo randomsequence known as a (CHIP). The sampling rate is then used as the chip rate for the audiosignal communication.
Spread spectrum encoding techniques are the most secure means by which to send hiddenmessages in audio, but it can introduce random noise to the audio thus creating thechance of data loss. There are many applications for Steganography, some good and some bad, which brings us to the closing section of our in-depth look at Steganography inwhich we will look at Steganalysis. Steganalysis is the art and science of stopping or detecting the use of all steganographic techniques mentioned earlier. In Steganalysis, thegoal is to be able to compare the cover-object (cover message), the stego-object (thecover message with the hidden data embedded in it) and any possible portions of thestego-key (encryption method) in an effort to intercept, analyze and/or destroy the secretcommunication. As Fabien A.P. Petitcolas
[2]
points out in his book, there are six general protocols used to attack the use of Steganography.1) Stego only attack - only the stego object is available for analysis.2) Known cover attack - the original cover object and the stego object are available for analysis.3) Known message attack - the hidden message is available to compare with the stego-object.4) Chosen stego attack - the stego tool (algorithm) and stego-object are available for analysis.5) Chosen message attack - takes a chosen message and generates a stego object for future analysis.6) Known stego attack - the stego tool (algorithm), the cover message and the stego-objects are available for analysis.This discussion of Steganalysis by showing the reader one example of how someonecould detect the use of steganographic tools that change the least significant bit (LSB) of an image in order to embed secret data inside of it.Generally, bitmap images (.BMP) have known and predictable characteristics. One suchcharacteristic is the probability of near duplicate colors. Bitmap images get their color from a central color table, which by its nature have little, or no near duplicate colors.When hidden data is embedded into the (LSB) of a bitmap image, it increases the number of near duplicate colors dramatically. Generally speaking, any bitmap image with morethan fifty near duplicate colors should raise the suspicion of embedded data being present.
V. Applications for Steganography in an Open Systems Environment
In this section we will look at some of the possible applications for steganography andthen close by pointing out some of the more popular steganographic tools available today.
The three most popular and researched uses for steganography in an open systemsenvironment are covert channels, embedded data and digital watermarking.Covert channels in TCP/IP involve masking identification information in the TCP/IPheaders to hide the true identity of one or more systems. This can be very useful for anysecure communications needs over open systems such as the Internet when absolutesecrecy is needed for an entire communication process and not just one document asmentioned next. Using containers (cover messages) to embed secret messages into is byfar the most popular use of Steganography today. This method of Steganography is veryuseful when a party must send a top secret, private or highly sensitive document over anopen systems environment such as the Internet. By embedding the hidden data into thecover message and sending it, you can gain a sense of security by the fact that no oneknows you have sent more than a harmless message other than the intended recipients.Although not a pure steganographic technique, digital watermarking is very common intoday's world and does use Steganographic techniques to embed information intodocuments.Digital watermarking is usually used for copy write reasons by companies or entities thatwish to protect their property by either embedding their trademark into their property or by concealing serial numbers/license information in software, etc. Digital watermarkingis very important in the detection and prosecution of software pirates/digital thieves
(loud sounds drown out quiet sounds) and this is what must be exploited to encode secretmessages in audio without being detected.There are two concepts to consider before choosing an encoding technique for audio.They are the digital format of the audio and the transmission medium of the audio.There are three main digital audio formats typically in use.They are Sample Quantization, Temporal Sampling Rate and Perceptual Sampling.Sample Quantization which is a 16-bit linear sampling architecture used by popular audioformats such as (.WAV and. AIFF). Temporal Sampling Rate uses selectable frequencies(in the KHz) to sample the audio. Generally, the higher the sampling rate is, the higher the usable data space gets. The last audio format is Perceptual Sampling. This formatchanges the statistics of the audio drastically by encoding only the parts the listener perceives, thus maintaining the sound but changing the signal. This format is used by themost popular digital audio on the Internet today in ISO MPEG (MP3).Transmission medium (path the audio takes from sender to receiver) must also beconsidered when encoding secret messages in audio. W. Bender
[8]
introduces four possible transmission mediums:1) Digital end to end - from machine to machine without modification.2) Increased/decreased resampling - the sample rate is modified but remains digital.3) Analog and resampled - signal is changed to analog and resampled at a different rate.4) Over the air - signal is transmitted into radio frequencies and resampled from amicrophone.We will now look at three of the more popular encoding methods for hiding datainside of audio. They are low-bit encoding, phase-coding and spread spectrum.Low-bit encoding embeds secret data into the least significant bit (LSB) of theaudio file. The channel capacity is 1KB per second per kilohertz (44 kbps for a 44 KHzsampled sequence). This method is easy to incorporate but is very susceptible to data lossdue to channel noise and resampling.Phase coding substitutes the phase of an initial audio segment with a reference phase that represents the hidden data. This can be thought of, as sort of an encryption for the audio signal by using what is known as Discrete Fourier Transform (DFT), which isnothing more than a transformation algorithm for the audio signal.Spread spectrum encodes the audio over almost the entire frequency spectrum. Itthen transmits the audio over different frequencies which will vary depending on whatspread spectrum method is used. Direct Sequence Spread Spectrum (DSSS) is one suchmethod that spreads the signal by multiplying the source signal by some pseudo randomsequence known as a (CHIP). The sampling rate is then used as the chip rate for the audiosignal communication.
Spread spectrum encoding techniques are the most secure means by which to send hiddenmessages in audio, but it can introduce random noise to the audio thus creating thechance of data loss. There are many applications for Steganography, some good and some bad, which brings us to the closing section of our in-depth look at Steganography inwhich we will look at Steganalysis. Steganalysis is the art and science of stopping or detecting the use of all steganographic techniques mentioned earlier. In Steganalysis, thegoal is to be able to compare the cover-object (cover message), the stego-object (thecover message with the hidden data embedded in it) and any possible portions of thestego-key (encryption method) in an effort to intercept, analyze and/or destroy the secretcommunication. As Fabien A.P. Petitcolas
[2]
points out in his book, there are six general protocols used to attack the use of Steganography.1) Stego only attack - only the stego object is available for analysis.2) Known cover attack - the original cover object and the stego object are available for analysis.3) Known message attack - the hidden message is available to compare with the stego-object.4) Chosen stego attack - the stego tool (algorithm) and stego-object are available for analysis.5) Chosen message attack - takes a chosen message and generates a stego object for future analysis.6) Known stego attack - the stego tool (algorithm), the cover message and the stego-objects are available for analysis.This discussion of Steganalysis by showing the reader one example of how someonecould detect the use of steganographic tools that change the least significant bit (LSB) of an image in order to embed secret data inside of it.Generally, bitmap images (.BMP) have known and predictable characteristics. One suchcharacteristic is the probability of near duplicate colors. Bitmap images get their color from a central color table, which by its nature have little, or no near duplicate colors.When hidden data is embedded into the (LSB) of a bitmap image, it increases the number of near duplicate colors dramatically. Generally speaking, any bitmap image with morethan fifty near duplicate colors should raise the suspicion of embedded data being present.
V. Applications for Steganography in an Open Systems Environment
In this section we will look at some of the possible applications for steganography andthen close by pointing out some of the more popular steganographic tools available today.
The three most popular and researched uses for steganography in an open systemsenvironment are covert channels, embedded data and digital watermarking.Covert channels in TCP/IP involve masking identification information in the TCP/IPheaders to hide the true identity of one or more systems. This can be very useful for anysecure communications needs over open systems such as the Internet when absolutesecrecy is needed for an entire communication process and not just one document asmentioned next. Using containers (cover messages) to embed secret messages into is byfar the most popular use of Steganography today. This method of Steganography is veryuseful when a party must send a top secret, private or highly sensitive document over anopen systems environment such as the Internet. By embedding the hidden data into thecover message and sending it, you can gain a sense of security by the fact that no oneknows you have sent more than a harmless message other than the intended recipients.Although not a pure steganographic technique, digital watermarking is very common intoday's world and does use Steganographic techniques to embed information intodocuments.Digital watermarking is usually used for copy write reasons by companies or entities thatwish to protect their property by either embedding their trademark into their property or by concealing serial numbers/license information in software, etc. Digital watermarkingis very important in the detection and prosecution of software pirates/digital thieves