Sandy Harris wrote: > There are ways to watermark that are quite hard to detect or remove. > Quoting text I wrote most of at : > http://en.citizendium.org/wiki/Steganography#Digital_watermarking > > Encryption is often used as one of the steps in steganographic hiding > of information. Consider an image file with 10 megapixels, each 8 bits, > in which you want to hide a message of a few megabits. The simplest way > to do it is to just put the message in the least significant bits of > each pixel. However, that has several > disadvantages: it can be detected by an enemy who checks those bits > (since they will be non-random), an enemy who finds it can read it > (since it is not encrypted), and the message can be removed from the > image by overwriting those bits. > > If you encrypt the message before inserting it in the image then — > since the output of any good cipher is apparently random — it generally > becomes very difficult for any enemy who does not have the encryption > key to detect the message, or to read it if he does detect it. Note, > however, that if the low-order bits are initially non-random then > replacing them with random material is easily detected; this might > occur for example with a low-cost camera that puts real data only in > the high six bits of an 8-bit pixel. Generally symmetric key > cryptography is used in such applications. For example, when a media > company embeds a watermark in a video as part of a DRM system, it is > often encrypted so only that company, or agents to whom it has provided > the key, can recover it. ... > > Often some sort of transform is applied to the cover file before the > steganographic data is added. Many different transforms can be used; > among the commonest are the Fourier transform for sound data and > discrete cosine transform for images or video. The sequence is then: > > apply the transform to the input data > embed the encrypted message > apply the inverse transform to produce the output data > > Choice of transform is a rather complex question. ... > /end quote > > That sort of trickery can give a watermark that is quite difficult to > detect or remove, but easily proven in a court case.
Very interesting and it hadn't occurred to me that such encryption could be used as an indelible watermark. For those hosting a continuous stream of high quality images which are offered for sale as prints, this could well be worth inclusion. Malcolm -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
