Data, usually fairly short in length, which describes a file and which is calculated from the contents of the file. It has the property that if the file is altered and the data recomputed there is a very high probability that the new data is different from its previous value. It is similar to a message digest. File signatures are used to check whether a file has been altered by, say, a virus or some malicious intruder. Periodically, special-purpose software processes sensitive files in a system checking the value of each file's signature against the previous value. If it has changed then these files are flagged as having a high probability of having been changed.