A file signature is a unique identification number seen at the beginning of a file. It tells you the file’s type and provides information about the data it contains. A computer uses it to determine how to read it or what application to use to open it. If a user saves the file using the wrong extension, the computer will return an error saying it can’t be opened.

A file signature also ensures the original data stored in the file remains intact and has not been modified. These make a file signature a necessary form of verification, especially against computer viruses, which digital forensics experts typically identify.

Other interesting terms…

Read More about a “File Signature

File signatures are also known as “magic numbers” or “magic bytes.”

Why Did the Concept of a File Signature Emerge?

The file signature was born out of the need for a file header. A file header is a block of data seen at the start of a file that defines how information is stored in it. Part of the header information is a sequence of bytes or numbers that specifies the file’s type. A file can be an image file, a document created by a specific program like Microsoft Word, or a protocol type when a file stream is used to communicate between a client and server.

The file headers of different files do not use a defined standard. Instead, each application has a proprietary format for its file header or signature. That means a program (e.g., Microsoft Word) or an operating system (OS) (e.g., Windows 11) needs a file signature database to determine different file types.

Digital forensics experts need to know the different file signatures so they can install the appropriate programs to open files under investigation. Why is that? Sometimes, users leave out the file extension when they save files. A cybercriminal may, for instance, leave out .asf when he saves an incriminating Windows Media Audio/Video file. But if the digital forensics expert sees the header, which begins with 30 26 B2 75 8E 66 CF 11, he would know he’ll need Windows Media Player to open it.

How Does a File Signature Work?

If you obtain an application from the Internet, it is usually compressed for faster downloading. If its extension is .7z, you’ll need a program like WinZip on your computer to open and install it.

Your computer will identify that the file (whether or not it has .7z in its name) is a 7-Zip compressed file when it reads the file signature 37 7A BC AF 27 1C in its header. You can then choose WinZip from your list of applications to open it.

How Can Digital Forensics Benefit from File Signatures? 

As mentioned earlier, keeping incriminating files is easier if they can’t readily be identified. Digital forensic investigators would need to have a database of file signatures handy to go through the contents of computers and other digital devices they seized as evidence.

If the investigators found several files without extensions in their names, they’d have to open them one by one to see if they’re possibly connected to the crime. But it may be a little faster if they are familiar with file headers and file signatures, especially if they know what types of files they’re looking for. If they’re searching for videos, for instance, they can collate all files with the following headers for deeper scrutiny later on:

SignatureFile TypeApplication Needed
00 00 00 14 66 74 79 703GPP multimedia filesWindows Media Player
00 00 00 20 66 74 79 703GPP2 multimedia filesWindows Media Player
00 00 00 18 66 74 79 70MPEG-4 video filesVLC Media Player
52 49 46 464X movie videoVLC Media Player
30 26 B2 75 8E 66 CF 11Windows Media Audio/Video FileWindows Media Player

There are many others. You may check out a comprehensive database of file signatures on this page. If you want to be thorough, check out the compressed files, too. Because videos often take up huge chunks of memory, they’re stored in compressed formats.

File signatures, as you’ve seen, are critical in that without them, your computer won’t be able to tell what program to use to open them. And they’re instrumental, too, in digital forensics, as investigators can use them to find the files they need for a case, even if these are masked or don’t have extensions in their names.