Checksum

A checksum is a mathematical value used to detect errors or changes in digital data content. The checksum is computed by repeatedly applying mathematical calculations based on the byte values of the digital content, according to a known checksum algorithm.

Good checksum algorithms are designed so that:

1. the number of bytes needed to express the checksum is much smaller than the number of bytes needed to express the original content;
2. it would require a very unlikely coincidence for any two distinct blocks of data to produce the same checksum as one another; and
3. most small or easily-predictable changes to a block of content (such as a change to a single letter, inserting additional content or deleting existing content) will cause the altered content to produce a very different checksum from the original content.

Application

The LOCKSS daemon computes SHA-1 checksums for every file preserved in an Archival Unit, so that preservation nodes within the LOCKSS network can monitor the integrity of preserved data and so that they can regularly compare their own preserved content with the preserved copies kept on other preservation nodes in the network.

See also

• Wikipedia:Checksum