Wednesday, July 26, 2006

Metadata in JPEG files

Digital cameras and image manipulation programs add hidden data to JPEG files. These data should be removed before publishing them on the Internet.The JPEG file format allows it to embed additional information called "metadata" in the file header. (Other image file formats can contain metadata, too.) The purpose of these metadata is to provide additional and useful information along with the picture. Image manipulation programs and especially digital cameras take advantage of this feature.
Metadata can be embedded in different ways. A common way is to store them according to the Exif specification, which has been created by the Japan Electronic Industry Development Association (JEIDA). Other popular specifications are the IPTC headers defined by the International Press Telecommunications Council (IPTC) and XMP developed by Adobe Systems. More detailed information about these metadata formats as well as descriptions of other metadata formats can be found on ExifTool's Tag Names page.
Among other things, the metadata section of a file can contain information about:
make and model of the digital camera
time and date the picture was taken
distance the camera was focused at
location information (GPS) where the picture was taken
small preview image (thumbnail) of the picture
firmware version, serial numbers, name and version of image manipulation program, etc. ...
Several software tools support the removal of metadata. A recommendable one is jhead or a more popular one IrfanView.
Many users may also not know that digital cameras leave an individual fingerprint in each picture. This allows to reliably link pictures to the camera with which they were taken--in much the same way that forensic examiners can link bullets to the gun that fired them.
Something that should be distinguished from digital fingerprints is digital watermarking. Among other things, digital watermarking is used to prevent--or at least expose--picture altering. Digital cameras equipped with digital watermarking technology append an extra stream of identifying data to each picture, which is usually invisible. If the picture is changed, these data and therefore the digital watermark are corrupted.
Cameras with watermarking technology are mainly purchased by professionals who need to prove that the pictures they have taken are unaltered. Just like digital fingerprints, digital watermarks could make it possible to determine if a picture was taken by a certain camera.
Although software tools that can remove digital watermarks do not seem to exist, digital watermarks are not really a problem. Simple countermeasures are to use only digital cameras without watermarking technology or with the option to disable watermarking.


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