• A watermark is a form, image or text that is impressed onto paper, which provides evidence of its authenticity.

• A distinguishing mark impressed on paper during manufacture; visible when paper is held up to the light (g.g bill)

  

Digital Watermarking

• Allows users to embed SPECIAL PATTERN or SOME DATA into digital contents without changing its perceptual quality.

• When data is embedded, it is not written at header part but embedded directly into digital media itself by changing media contents data

• Watermarking is a key process for the protection of copyright ownership of electronic data.

• Digital watermarking is the process of embedding information into a digital signal in a way that is difficulty to remove.

• The signal may be audio, pictures or video, for example. If the signal is copied, then the information is also carried in the copy. A signal may carry several different watermarks at the same time.

General Procedure for Digital Watermarking:

Cryptography vs. Watermarking

• cryptography is the most common method of protecting digital content and is one of the best developed science.

• However, encryption cannot help the seller monitor how a legitimate customer handles the content after decryption.

• Digital watermarking can protect content even after it is decrypted.

  

Importance of Digital watermarking:

• As seen above in fig, Alice creates an original image and watermarks it before passing it to Bob. If Bob claims the image and sells copies to other people Alice can extract her watermark from the image proving her copyright to it.

• The caveat here is that Alice will only be able to prove her copyright of the image if Bob hasn't managed to modify the image such that the watermark is damaged enough to be undetectable or added his own watermark such that it is impossible to discover which watermark was embedded first.

Visible Watermarking

• Visible watermark is a translucent overlaid into an image and is visible to the viewer. Visible watermarking is used to indicate ownership and for copyright protection.

  

Invisible Watermarking

• Invisible watermark is embedded into the data in such a way that the changes made to the pixel values are perceptually not noticed. Invisible watermark is used as evidence of ownership and to detect misappropriated images.

  

Dual watermarking

• Dual watermark is the combination of visible and invisible watermark. An invisible watermark is used as a backup for the visible watermark.

Digital watermarking life cycle phases

A Watermarking system is usually divided into three distinct steps.

$\hspace{1.5cm}$- Embedding

$\hspace{1.5cm}$- Attack

$\hspace{1.5cm}$- Detection

Embedding

• In embedding, an algorithm accepts the host and the data to be embedded, and produces a watermarked signal.

  

• Inputs to the scheme are the watermark, the cover data and an optional public or secret key. The output are watermarked data. The key is used to enforce security.

Attacks

• Thw watermarked digital signal is transimitted or stored, usually transmitted to another person. If this person makes a modification, this is called an attack.

• Few possible attacks:

1. Robustness attacks: Which are intended to remove the watermark such as JPEG compression, cropping, etc.

2. Presentation attacks: Under watermark detection failure they come into play. Geometric transformation, rotation, scaling, translation, change aspect ratio, affine transformation etc.

3. Counterfeiting attacks: Rendering the original image, generate fake original.

Extraction/Detection

• Detection (often called extraction) is an algorithm which is applied to the attacked signal to attempt to extract the watermark from it.. if the signal was unmodified during transmission, then the watermark still is present and it may be extracted.

  

• Inputs to the scheme are the watermarked data, the secret or public key and depending on the method, the original data and/or the original watermark. The output is the recovered watermarked W or some kind of confidence measure indication how likely it is for the given watermark at the input to be present in the data under inspection.

Fragile/Semifragile/Robust

• A watermark is called fragile if it fails to be detected after the slightest modification.

• A watermark is called simi-fragile if it resists begining transformations but fails detection after malignanat transformations.

• A watermark is called robust if ti resists a designted class of tranformations.

Text/Image/Audio watermarking

• Text/Image/Audio watermarking refers to embedding watermarks in an text/image/audio in order to protect the image from illegal copying and identify manipulation.

Video watermarking

• Video watermarking refers to embedding watermarks in a video sequence in order to protect the video from illegal copying and identify manipulation

• Algorithm for video watermarking

1. DFT

2. DCT

3. DWT

• Watermarking in I-frame.

Discrete cosine transformation

• DCT convert images from spatial-domain to frequency-domain to decor-relate pixels.

• The discrete cosine transform (DCT) helps to separate the image into parts of differing importance (with respect to the image's visual quality). The DCT is similar to the discrete fourier transform.

• Remember that JPEG breaks an image into 8x8 units so for DCT n=8

$\hspace{1.5cm}$e.g

$\hspace{1.5cm}$256 gray-scale image each pixel is stored as a value between O - 255

$\hspace{1.5cm}$O = black pixel

$\hspace{1.5cm}$255 = white pixel

$\hspace{1.5cm}$value between are shades of gray

Watermark in Video(DCT)

Types of watermark

1. Robust watermarking: A robust watermark is a watermark that is difficult to remove from the object in which it is embedded.

2. Fragile watermark: A fragile watermark is destroyed if anybody attempts to tamper with the object in which it is embedded.

3. Visible watermarking: A visible watermark is immediately perceptible and clearly identifies the cover object as copyright-protected material, much like the copyright symbols

4. Invisible watermarking: An invisible watermark is not normally perceptible, but can still be used by the rightful owner as evidence of data authenticity in a court of law.