The Fingerprint of Authenticity
To prove Fixity, we use a Checksum—an algorithmic "fingerprint" generated from the file's contents. If a single bit flips (due to cosmic rays or Bit Rot), the checksum changes completely. Regular "Fixity Checks" are the rounds a digital custodian walks to ensure the prisoners (files) haven't escaped or died.
Field Notes
The Chain of Custody: In Digital Forensics, Fixity is what proves evidence hasn't been tampered with. In Archaeobytology, it proves that the history hasn't been rewritten. A file without a Fixity history is just a rumor; a file with verified checksums is a fact.
Self-Healing Archives: Advanced file systems (like ZFS) do automatic Fixity checks. If they detect a bad sector (a checksum mismatch), they automatically repair the file from a redundant copy. This is the goal of Sustainable Preservation.