Steganography: A domain beyond cryptography

Popularity of the Internet in the last decade gave a new life to the field of

information hiding. Extensive use of digital media like text, images, audio and

video on the Internet generated a requirement for providing traffic security. It

was found that merely concealing the contents of the message (cryptography) was

not adequate for transmitting confidential information over public networks.

What was required was an additional layer in security that conceals the very

fact that communication has taken place. This is when information hiding

techniques started to gain significance.

Information hiding started its development as a separate discipline under

information security with steganography as one of its main branches. It borrowed

ideas from the similar, more established disciplines like cryptology and

low-probability-of-intercept communication. With the progress in signal

processing and multimedia technologies in addition to growing security

requirements over the Internet, steganography made a noticeable impact within a

short time.

In the mid 90s research was initiated in the field of digital steganography

to provide a solution for securing multimedia communications. Steganography

deals with a host of techniques that conceals the existence of a hidden

communication. The secret message to be transmitted is camouflaged in a carrier

such that its detection becomes difficult. Information related to the sender and

the receiver of the message can also be hidden in this way. A number of cover

types are available for hiding data that include text, network packets, images,

audio and video. The secret data can be a simple textual message, an image or a

voice recording. Different techniques are available for modifying or

manipulating the cover to embed secret messages and can be broadly classified as

substitution, transform-domain and spread-spectrum techniques.

Substitution techniques

Substitution techniques replace the less significant parts of the cover media

with message bits to be embedded. They allow higher data capacity and low

computational complexity compared to other techniques. Moreover these techniques

can be used to hide not only text messages but also images and audio. The

disadvantage with substitution techniques is that they are not robust to attacks

and cannot sustain high levels of noise.

Transform-domain techniques

Using transform-domain techniques it is possible to embed a secret message in

different frequency bands of the cover. Popular transform-domain techniques are

Discrete Fourier Transforms (DFT), Discrete Cosine Transforms (DCT) and Discrete

Wavelet Transforms (DWT). These techniques are comparatively robust to attacks.

The disadvantages however is by embedding in these high frequencies creates less

impact on the perceivability of the media but provide low robustness to

different attacks. In contrast, embedding in the lower frequencies helps to

withstand many attacks but creates perceptible impact on the media. It is also a

relatively more expensive technique.

Spread Spectrum Steganography

Spread Spectrum techniques enable communication that cannot be jammed or

intercepted by tuning in to a single or a particular set of frequencies. The

frequency band that a signal occupies is spread many times so as to make

detection and removal difficult. Direct sequence (DS) and frequency hopping (FH)

spread spectrum techniques are used for information hiding. Using spread

spectrum methods the secret message is scattered throughout the cover, which has

a higher bandwidth. Hiding in images, audio and video using spread spectrum

techniques are quite robust to attacks.

In today’s networked world, steganography finds a number of important

applications, here’re a few potential applications: -

• Network intrusions and hacking of Websites have caused huge losses to
  
  enterprises. Steganography can be a useful technique in protecting your
  
  valuable data resources.
• Automatic audit of technical data, art, music or videos on the Internet
  
  can be done with steganographic techniques. Even radio programs and
  
  advertisements on the Internet can be subjected to such audits with these
  
  techniques.
• Healthcare IT is catching up like wild fire. Steganographic techniques can
  
  be put to advantage here through medical imaging applications. For instance
  
  the patient name and details can be easily embedded in the scanned image
  
  without loss of quality and can be extracted later for medical verification
  
  from the network.
• Video conferencing systems are being used commonly today in enterprises.
  
  Steganographic techniques can be put to good use here. For instance
  
  information that is transmitted including the sender and receiver details
  
  can be hidden using steganography thus protecting the transmission from
  
  being snooped.

Though information hiding techniques like steganography complemented by

techniques such cryptography can provide foolproof security, there is room for

improvement in terms of robustness and flexibility of use.