A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and comparing it with the stored one. If the checksums don't match, the data was almost certainly altered (either intentionally or unintentionally). The procedure that yields the checksum from the data is… (More on Checksum)