A checksum (also called a hash total) is
|“||calculated from the data using a known formula that returns a single-digit value and is stored with the data. At any point the checksum can be recalculated to see if the value has changed.||”|
|“||the information produced by a mathematical technique used to determine whether or not errors have been introduced into a data stream. Usually used for low-bit-rate data transmission.||”|
|“||[a]n algorithmically-computed numeric value for a file or a set of files used to validate the state and content of the file 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 match, the data was almost certainly not altered.||”|
|“||a small numerical value, typically 128 or 256 characters long, representing a digital file of any size. The value is generated using an algorithm of a particular type, such as MD5, SHA-256, or SHA-1. The formula is designed such that even the smallest change in a file, a single bit, will yield a vastly different output, making the change explicit. As a method of detecting errors or changes in the file it represents, checksums are used for monitoring the authenticity and integrity of data.||”|
|“||[a] value that (a) is computed by a function that is dependent on the contents of a data object and (b) is stored or transmitted together with the object, for the purpose of detecting changes in the data.||”|
A checksum is
|“||[a] code added to the contents of a block of data stored on an RFID microchip that can be checked before and after data is transmitted from the tag to the reader to determine whether the data has been corrupted or lost. The cyclic redundancy check is one form of checksum.||”|
"To gain confidence that a data object has not been changed, an entity that later uses the data can independently recompute the checksum value and compare the result with the value that was stored or transmitted with the object.
"Computer systems and networks use checksums (and other mechanisms) to detect accidental changes in data. However, active wiretapping that changes data could also change an accompanying checksum to match the changed data. Thus, some checksum functions by themselves are not good countermeasures for active attacks. To protect against active attacks, the checksum function needs to be well-chosen and the checksum result needs to be cryptographically protected."