In computing, a file signature is data used to identify or verify the contents of a file. In particular, it may refer to:

• File magic number: bytes within a file used to identify the format of the file; generally a short sequence of bytes (most are 2-4 bytes long) placed at the beginning of the file; see list of file signatures
• File checksum or more generally the result of a hash function over the file contents: data used to verify the integrity of the file contents, generally against transmission errors or malicious attacks. The signature can be included at the end of the file or in a separate file.