Every Contact Leaves a Trace

This phrase, adapted from pioneering forensic investigator Edmond Locard, is not only one of my chapter titles in Mechanisms, but also one of the central dictums of the book, for I claim this holds true as much (or more) in digital environments as the physical world. So I was intrigued to read an account of software developed to detect digital image tampering in the current issue of the Chronicle of Higher Education (June 6, 2008):

Mr. Farid, of Dartmouth, has developed software tools that can automatically check for image tampering. The software looks for patterns in the digital code underlying an image. When files are opened and altered in Photoshop, for instance, codes are added that Mr. Farid's software can detect. Likewise, when scientists copy and paste parts of images in software programs, their actions leave a digital mark. (A10)

In the book, I described the kind of "marks" Farid's software detects as an artifact of what I termed formal materiality. I wrote:

An image file is typically thought of as consisting of nothing but information about the image itself—the composition of its pixilated bitmap, essentially. In fact, however, the image can carry metadata (documentation as to how it was created, embedded as plain text in the “header” of the file), as well as more colorful freight, such as a steganographic image or a digital watermark. This content will only become visible when the data object is subjected to the appropriate formal processes, which is to say when the appropriate software environment is invoked—anything from the “Show Header” function of an off-the-shelf image viewer to a 128-bit encryption key. (12-13)

The kind of software the described in the Chronicle's article contributes to our understanding of digital objects as mechanisms, that is as artifacts with a recoverable past--as opposed to black boxes or inscrutable blobs of code.