A file signature is a unique identification number at the beginning of a file that identifies the type of file and can be used for verification against viruses. It emerged due to the need for a file header and can also contain information for error checking using a checksum.
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, providing information about the data contained within the file itself. This information can be used to determine what type of file is being read when file extension or user error has misidentified the file as a wrong type. The file signature may also contain information that guarantees that the original data stored in the file is still intact and has not been modified. The combination of these elements allows a file’s signature to serve as an important form of verification, especially against computer viruses.
The concept of a file signature emerged due to the need for a file header, a block of data at the start of a file that defines the parameters of how information is stored in the file. Part of the header information is a sequence of bytes that defines the type of file originally created. It can be an image file, a specific program document, or even a protocol type when a file stream is used as a communication method between a client and a server. The file header does not use a defined standard; instead, it is proprietary to 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 referred to as a magic number. In programming, it is a unique value in the data field it occupies. When examining a file header to determine the file type, this means that no two file signatures should be the same, allowing each format type to have a distinct identifying byte string. This can be particularly useful when dealing with online file transfer and interpretation, where the identifying extension of a file may be arbitrary and cannot be used as an identifier for a file type.
In addition to just the file type, a file signature can also contain information that allows error checking to be performed on a file so that the data it contains can be confirmed as intact. This is often done using a function known as a checksum. A checksum is a function that uses integer values of file data to create mathematical values that can be replicated after a file has been transferred or uploaded. In its most basic form, this process involves adding the values of a series of bytes to the file and then recording the sum, allowing the program that decrypts the file to do the same thing. If the results are different during decryption, the file may have been corrupted and the data may be invalid or may have been modified for malicious purposes.
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