Photo Image Files

Photo Image Files

1. Photo images. Your source or the "anonymous" publisher of a leaked document online may use a Scanner, but they may, nowadays use a Digital Camera.

   There is often camera make / model identifying metadata embedded in the raw digital images taken by many types of Digital Camera. These may be used as "evidence" if your Digital Camera is seized during a "leak inquiry" investigation.

   There is even facility for Global Positioning Satellite latitude and longitude data (likely to become increasingly common with mobile phone camera pictures) to be stored within this metadata, and camera specific serial numbers, which, if cross referenced with purchase or repair or warranty registration records, may provide clues or evidence as to the identity of your confidential source.

   See this report in The Times (19th July 2007) about the embedded EXIF data which reveals the Camera Model and Serial Number, which may be of use to copyright lawyers, in tracing the photographer who allegedly leaked images onto the internet, of the then as yet unpublished popular novel Harry Potter and the Deathly Hallows, the final book in the immensely popular and profitable series by J.K.Rowling.

2. There is an excellent free Perl module and a Windows executable command line tool called ExifTool, which displays, and can selectively edit, most of this metadata which is encoded according to the industry standard Exchangeable File Image Format (Exif)

3. You can examine (but not change or delete) such photo image metadata via this website, which is powered by the ExifTool perl script software: Jeffrey's Exif Viewer

4. Standard image editing software such as Adobe PhotoShop can preserve the original metadata, which is useful for keen photographers, but not so good for preserving the anonymity of your anonymous sources.

   Very often using File Save As within the image editor, and saving to a different filename from the original automatically digital camera name and numbered images, reduces the amount of metadata to an acceptable level.

5. Another freely available command line tool, for both Windows and Linux, (which does not require Perl to be installed) is jhead. Whilst not as comprehensive in displaying all the EXIF data, as ExifTool (which also now has a Windows executable binary version), and restricted only to .jpg files (the most common digital camera output), it does provide the ability to edit comments (e.g. to put in your own copyright notice) and to delete all the potentially whistleblower source betraying EXIF data.

6. You may wish to blank out or censor items in .jpg or .gif or .bmp graphics image.

   Again, there is a temptation by the uninitiated to use, say, a PhotoShop pixellation or motion blur special effect filter. Remember, that these standard filters effects can often be reversed. e.g. as Interpol has shown with the enhanced version of the reversible PhotoShop Twirl plug-in effect used to try to identify a suspected child rapist

   Since Digital Camera images and Scans of documents are usually much too large for web pages, you might want to reduce the number of colours and probably the size of the images, before publishing them as thumbnails and even as larger images on a blog or website.

   Remember to apply your PhotoShop pixellation etc. after reducing the image size and number of colours, i.e. after you have thrown away some of the identifying data, so as to reduce the chances of the filter effects being reversed.

7. The jhead documentation and program options remind us that many digital cameras embed a small, up to 10Kb thumbnail image in the file, used by the camera display itself, or external software, to show for thumbnail gallery views of a set of photos.

   If you are digitally manipulating the main image e.g. to pixellate out a face or a location specific sign, a vehicle number plate, or to redact an email address or telephone number etc., then the thumbnail also might need to be re-generated from the modified main image using jhead, or else the thumbnail should be deleted.

8. EXIF metadatais not the only way of forensically linking a digital image from a whistleblower source to other digital images which may be more easily traced to the source camera or scanner.

   Cameras or Scanners introduce potentially characteristic non-random background noise into the images which they produce, as a combination of individual wear and tear patterns and the variations within the manufacturing tolerances, and small errors, such as faulty pixels on the Charge Coupled Device electronic chip, of any particular device.

   Professor Jessica Fridrich, of the Thomas J. Watson (founder of IBM) School of Engineering and Applied Science at Binhampton Uniiversity in the State of New York, has published Camera Identification From Printed Images (.pdf) academic research and software which can statistically compare such background noise patterns (Photo-Response Non-Uniformity) , and match a series of digital photos together as having been made by the same digital camera or mobile phone camera or scanning device. If some of the photos are easily identifiable, due to their content or metadata, e.g. on a public photo sharing website like Flickr.com, where family album or holiday snaps might betray the identity of the whistleblower, if he or she uses the same equipment for their confidential or leaked photos.

   See the step by step guide and comments - Avoiding Camera Noise Signatures