Magic Numbers

           Magic numbers are common in programs across many operating systems. Magic numbers implement strongly typed data and are a form of in-band signaling to the controlling program that reads the data type(s) at program run-time. Many files have such constants that identify the contained data. Detecting such constants in files is a simple and effective way of distinguishing between many file formats and can yield further run-time information.

Example:

GIF image files have the ASCII code for "GIF89a" (47 49 46 38 39 61) or "GIF87a" (47 49 46 38 37 61)

PDF files start with "%PDF" (hex 25 50 44 46).