Tamper-evident log
A tamper-evident log is an append-only audit store whose entries are cryptographically chained so any later mutation, deletion, or reordering is detectable on read. Each entry includes a hash of the prior entry, a timestamp from a trusted source, and a signature from the writer's key. A reader who replays the chain can detect a single altered byte. Tamper-evident is the weaker, achievable cousin of tamper-proof; the log can still be modified by an attacker who controls the writer, but the modification leaves visible evidence the regulator and the forensic investigator can find.
Why tamper-evident is the regulator's bar for AI audit
EU AI Act Article 12 requires automatic recording of events over the lifetime of the system, with logs covering period of use, input data, and identification of natural persons involved. Fannie Mae Lender Letter LL-2026-04 (effective August 6, 2026) requires the lender to produce per-decision evidence on demand. NIST AI RMF Pillar 3 calls the same primitive action lineage. All three obligations assume the log is trustworthy at audit time. An application-managed log that the application can rewrite collapses under the question "how do you know this record was not edited after the decision was made?" A tamper-evident log answers the question with a chain hash and a signature.
How the chain is constructed
Each log entry serializes the per-decision record (subject, route, classification verdict, policy version, outcome, latency, destination). The writer computes a SHA-256 hash over the prior entry's hash plus the current entry's bytes. The writer signs the new hash with a key stored in an HSM or KMS service the application cannot access. The audit record's signature, the chain hash, and the trusted timestamp travel with the entry. The forensic investigator replays the chain from any anchor point (a periodic checkpoint published to an external store, an inclusion proof to a notarization service) and detects any divergence from the original chain.
Related reading
- AI Audit Logs: The Format Spec That Survives EU AI Act, DORA, and Fannie Mae Review
AI audit logs that survive regulatory review carry a specific set of fields the EU AI Act Article 12, DORA Article 19, Fannie Mae LL-2026-04, NIST AI RMF, and HIPAA all expect on the same record. The fields cover identity, decision provenance, model identity, policy state, and integrity metadata. The format has to support per-record retrieval and per-series replay. The write path has to sit outside the application so the application cannot modify the record. This piece walks through the field-level format specification, the integrity model, the storage characteristics, and the deployment pattern that produces records the regulator and the customer auditor will accept.
- Tamper-Evident Audit Logs for AI: What Cryptographic Integrity Brings to Compliance Records
Tamper-evident audit logs make any post-hoc modification of a record detectable through cryptographic integrity. For AI compliance records, the property closes the self-attestation gap that application-controlled logs cannot. The technique combines per-record signing, hash chaining, and external anchoring. EU AI Act Article 12, Fannie Mae LL-2026-04, HIPAA, DORA, and NIST AI RMF all expect records that an auditor can rely on as evidence. Application logs that the application can modify do not meet that standard. This piece walks through the cryptographic mechanisms, the operational characteristics, and the architectural placement.
- Signed Audit Logs for AI Requests: Per-Decision Signing and What Regulators Will Accept
A signed audit log binds a cryptographic signature to each record at the moment the record is committed. For AI requests, the signature ties the record to the inspection layer that produced it and lets a verifier confirm authenticity without trusting the storage layer. The technique is the cryptographic foundation under tamper-evident audit trails the EU AI Act Article 12, Fannie Mae LL-2026-04, HIPAA, DORA, and NIST AI agent identity framework all expect. This piece walks through the signing schemes, the key management, and the verification flow that auditors and regulators will accept.