A hash chain is a data structure where each record contains the hash value of its predecessor. If an older entry is modified after the fact, all subsequent hash values become invalid — the tampering is immediately visible. This principle also underlies blockchain technology. In an ISMS context, hash chains are particularly useful for tamper-proof audit logs and protocol data. You can use them to prove that log entries have remained unchanged since creation. This is especially valuable for forensic investigations after security incidents.