To hide the information, several techniques are used. The information hiding is classified into three. They are:
Steganography literally means covered writing. In this technique, the message is secured by hiding the message with another object. It can only seen by the intended receiver. It is invisible to everyone other than intended receiver.
To transmit the message over public network, the message (plain text) is first converted into coded message (cipher text) using an encryption algorithm. At the receiver, the cipher text is decrypted to produce the plain text.
During manufacturing, a distinguishing mark is impressed on the paper. It is visible when facing to light. It is an identifying image or pattern in paper which can view by transmitting light through it. It appears as various shades of lightness or darkness. It is used in government documents currency etc. to protect from counterfeiting. Counterfeit produces the same replica of the original product. There are two ways for producing watermark. They are:-
- Dandy Roll process
- Cylinder mould process
Dandy Roll process
In this process, the water coated metal stamp is impressed on the paper during manufacturing. It is a light roller covered by a material to form a pattern. If the line pattern is distinct and parallel which contain a watermark, then the paper is called as laid paper. If the line pattern is mesh which does not contain watermark, then the paper is called as wove paper. This method is called as line drawing watermarks.
Cylinder mould process
This watermark is much clear and detailed than Dandy Roll process. This method is used for watermarking currency, passports etc. to protect from counterfeit. It produces shaded watermark. After dry, the paper is again rolled to produce the watermark with specified thickness and varied density.
The identifying code is encoded into digitalized music, video, etc. are called digital watermark.
It is an extension of watermarking in the digital form. There are lots of developments occurred in the field of internet and computer technology. Multimedia data such as images, audio, video is distributed over the internet. Anyone can download this data and there is a possibility of claim for the ownership. To avoid such situations digital watermarking can be used. It prevents illegal copying modifying or redistribution of data. It ensures ownership rights, authorized access, reproduction or redistribution of content etc. A pattern of bits are inserted into the digital image, music, video etc. which identifies the copyright information of files. The main aim of digital watermark is to provide copyright protection for the files digital format. The printed watermark is visible while the digital watermark is invisible. The bits used for representing watermark is scattered throughout the file. So it is not easily alterable or identified. The information is embedded in the digital signal which cannot be removed. The signal may be audio, video or images. When someone copies this signal, the information also carried in that signal. A signal is possible to carry several watermarks simultaneously.
- Robustness: - Image processing does not alter the watermark.
- Invisibility: - The digital watermark is invisible.
- Security: - An unauthorized person cannot access the watermark. So it is highly secure.
Digital Watermarking lifecycle phases
The information embedded in a signal is called digital watermark. The phrase digital watermark is the difference between watermarked signal and cover signal. The signal which embedded the watermark is called host signal.
Watermarking system is divided into three distinct steps such as
Algorithm accepts the host and the data to be embedded which produces a watermarked signal. This watermarked signal either be stored or transmitted to the receiver.
Any modification in the watermarked signal is known as attack. It does not mean that removing watermark. It may lead to disable the readability. To create and apply watermark, either image processing or transforms may be used. The same techniques can be used to disable or overwrite watermarks. If multiple watermarks are present on the image then it is difficult to find which on is the valid watermark. The modification will not be malicious when the term attack arises from copyright protection application, where third parties may attempt to remove the digital watermark through modification.
Some of the possible attacks are:-
- Robustness attack: - By using compression techniques or cropping the watermark can be removed.
- Presentation attacks: - This type of attack occur when the watermark detection failure.
- Counterfeiting attacks:- The original image may rendered or generate the fake original etc
It is an algorithm applied to the attacked signal for extracting watermark from the signal. During transmission if the signal was unmodified, then the watermark is still present and can be extracted.
Types of watermark
- Robust watermark: - In this watermark, it is difficult to remove from the signal in which it is embedded.
- Fragile watermark: - If anyone tried to tamper the object in which it is embedded, then the fragile watermark will get destroyed.
- Visible watermark: - This type of watermark is immediately perceptible and identifies the cover object as copyright protected material.
- Invisible watermark: - It is not normally perceptible. But it can still be used by rightful owner as evidence of data authenticity.
There are two types such as:
- Spatial domain watermarking
- Frequency domain watermarking
Spatial domain techniques: - They are two types such as least significant bit and SSM modulation.
- Least Significant Bit: - Each pixel in the image can be represented by 8 bit. The watermark is added at the Least Significant Bit (LSB) of the sequence. It is an easier technique. It does not provide any distortion on the image. The disadvantage of this technique is that the hidden information can be easily destroyed by simply altering the LSB. So it is not much secure against attacks.
- SSM modulation: - In this technique, a pseudo noise signal is linearly combined with the image which is modulated by the embedded watermark.
Frequency domain techniques: - Frequency domain techniques are commonly used than spatial domain techniques. The watermark is embedded in the spectral coefficients of image. Most frequency domain techniques use spread spectrum techniques. Similar to spread spectrum communication, the watermark is embedding in the whole image. The watermark is embedded in the whole frequency band. To destroy this watermark, the attacker has to add noise with large amplitude. It will affect the quality of watermark. So the attack is not possible.
- Discrete Cosine Transformation (DCT):- This technique is more secure than spatial domain techniques. It is strong against simple image processing like contrast or brightness adjustment, blurring etc but it is weak against geometric attacks such as rotating, cropping etc. This technique is computationally expensive.
The below figure shows the embedding of watermark using discrete cosine transformation.
The host image is transformed using discrete cosine transform. The watermark is embedded into the mid band of transformed image. Thus it produces the watermarked image. The block diagram of extraction or detection of watermark is shown below.
The image to be examined is taken and takes its discrete cosine transform. Then the watermark is extracted from the image. After that the extracted watermark is compared with the original watermark to know whether it is original or not.
- Discrete Wavelet Transformation (DWT):- This technique is commonly used in signal processing applications. In real life, all signals have time varying nature. So this technique can be used in variety of applications. In wavelet, the energy is concentrated on time. So it is easier to analyze the transient time varying signals.
- Security: - For data security, the watermark can be used for certification, authentication and conditional access. The official documents such as passports, identity cards are watermark to prevent counterfeit. It is also possible to detect the alteration in the message. Also in case of watermarking the confidential data on CD-ROM, anyone except user cannot able to access the data without having the required watermark.
- Copyright protection: - Without losing the quality, the copyright protection is inserts the copyright information into the digital object.
- Copy protection: - It prevents the reproduction of videos, audio etc due to copyright reasons.
- Broadcast monitoring: - The news in television contains watermarked videos.