A file signature in computer programming is a unique identification number located at the beginning of a file. This number identifies the type of file and provides information about the data contained in the actual file. This information can be used to determine what type of file is being read when the file extension or user error incorrectly identified the file as the wrong type. The file signature may also contain information that ensures that the original data stored in the file is still intact and has not been modified. The combination of these elements allows a file signature to serve as an important form of verification, especially against computer viruses.
The file signature may contain information that ensures that the original data stored in the file is still intact and has not been modified.
The concept of a file signature arose from the need for a file header, a block of data at the beginning of a file that defines the parameters of how information is stored in the file. Some of the header information is a sequence of bytes that defines the type of file that was originally created. It can be an image file, a specific program document, or even a type of protocol when a file stream is used as a communication method between a client and a server. The file header does not use a defined pattern; rather, it has each different format, which means that a program or operating system needs a database of file signatures to determine the type of an unknown file.
The actual file signature is sometimes called the magic number. In programming, this is a single value in the data field it occupies. When looking at a file header to determine the file type, this means that no two file signatures must be the same, allowing each format type to have a different identifying byte sequence. This can be of particular use when it comes to online file transfer and interpretation, where a file’s identifying extension can be arbitrary and cannot be considered an identifier for a file type.
In addition to the file type, a file signature can also contain information that allows you to check a file for errors so that the data it contains can be confirmed as intact. This is usually achieved through a function known as a checksum. A checksum is a function that uses the integer values of the file data to create mathematical values that can be replicated after a file is transferred or uploaded. In the most basic form, this process consists of adding the values of a series of bytes to the file and then recording the sum, which allows the program to decode the file to perform the same function. If the results are different during decryption, the file may have been corrupted and the data may be invalid or maliciously modified.
