Many watermarks have been proposed so far. Below are examples of spatial and frequency domain watermarks. Digital watermarking assumes inserting of a digital watermark at the source (e.g., camera) and verifying the marks integrity at the detection side. Digital watermarks are mostly imperceptible; they are inseparable from the digital media they are embedded in, and they undergo the same transformations as the digital media itself. A major drawback of approaches based on watermarks is that the watermarks must be inserted either at the time of recording the image, or later by a person authorized to do so. This limitation requires specially equipped cameras or subsequent processing of the original image. Furthermore, some watermarks may degrade the image quality. It also requires hiding the watermark key from the device owner. Digital watermarks are classified as visible or invisible. The visible group is perceptible to the human eye. In the case of the latter group, the existence can only be determined using a detection algorithm. In addition, watermarks can also be designed to be fragile or robust. Fragile watermarks become corrupted when any part of the image is modified. Thus, the most fundamental property of invisible fragile watermarks is the test of image authenticity and tamper detection. Robust watermarks are not affected by common image- manipulation procedures. Therefore, they are a proper way of ownership protection.
Image Authentication using Digital Watermarks
Image Authentication using Digital Signatures
The digital signature approach offers an interesting alternative to classical watermarking techniques and is based on the idea of traditional paper- based signing by transforming it into the digital world. It consists mainly of extracting unique features from the image at the source side and encoding these features to form digital signatures. Afterwards signatures are used to verify the image integrity by signature comparison at the detection side. If any changes are made to the image after it was signed, they automatically invalidate the signature. Signatures as well as watermarks provide, among other qualities, protection from tampering, copyright infringement and illegal distribution. There are many methods to generate the signatures, for example, based on image histograms, colors, geometric information, frequency information, etc. The major drawbacks of digital signatures are similar to watermarks drawbacks. The main disadvantage is the need for a fully secure and trustworthy source. We need a common algorithm on both source and detection sides. The image alone is not self- sufficient to perform the authentication process. Because of this, the benefits of the active approach are significantly reduced.
Some Examples of Historical Image Forgeries
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Creating image forgeries has a long history. Here, are some examples of earlier image forgeries. Here, a photo manipulation with Stalin is shown (1930):
This image shows a historical manipulation with a photo of Mao Tse- tung (1936):
This image shows an earlier photo manipulation with a photo of Adolf Hitler (1937). Here, Joseph Goebbels has been removed from the original photo.
Another example of earlier image forgeries. In the summer of 1968 Fidel Castro approves of the Soviet intervention in Czechoslovakia. Carlos Franqui (the man in the middle) cuts off relations with Castro and goes into exile in Italy. His face was then removed from the photograph.