Checksum
A value computed from a file's data used to verify its integrity — recomputing it later detects any change or corruption. The basis of fixity checking in digital preservation.
A checksum (or hash) is a value mathematically computed from a file’s contents — using algorithms like MD5 or SHA-256 — that acts as a digital fingerprint. Recompute it later and compare: if the checksum matches, the file is unchanged; if it differs, the file has been altered or corrupted.
Checksums are the mechanism behind fixity checking in digital preservation. A repository records a file’s checksum at ingest and periodically re-verifies it; a mismatch flags corruption (“bit rot”) so a clean copy can be restored from redundant storage. Checksums also support chain of custody and authenticity — letting an archive prove a record is bit-for-bit identical to what was preserved. They are recorded as part of preservation metadata and are a routine, automated safeguard in any serious preservation program.