Checksum

Data does not always stay the same. A file written to disk correctly can be read back wrong much later. A packet can arrive with one bit changed. A copy sent from one machine to another can differ from the original.

The hard part is that nothing warns you. The damaged copy is still a file of the right size, and the machine holding it reports no error. This is called silent data corruption. The data is wrong, and the system believes it is fine.

A checksum is a small value calculated from the data. You store it next to the data, calculate it again later, and compare the two values