A hash chain can make edits to an application audit log detectable: each entry’s SHA-256 digest incorporates the preceding entry’s digest, so changing, reordering, or removing a record breaks verification. The small TypeScript library chainlog described here adds that check to an existing application and offers in-memory, JSONL-file, and SQLite storage examples. It does not make logs impossible to rewrite, and it cannot prove that the recorded events are true.
How a hash chain makes log changes detectable
In a conventional log, records are separate entries. A hash chain links them: each record’s digest includes its index, timestamp, data, and the previous record’s digest. The construction described for chainlog is:
hash(entry) = SHA256(index + timestamp + data + prevHash)
The first entry has no earlier record to link to; each later entry depends on the one before it. Verification walks the records and checks their digests and links. If an earlier entry is altered, later links no longer match. Reordering or removing an entry similarly causes the chain check to fail. The article describing the library presents this as a way to detect those changes in an application log. Read the chainlog article on DEV Community.
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What chainlog adds to an application
Chainlog is presented as a small TypeScript library that wraps an application’s audit-logging workflow with a hash-chain integrity check. The article describes three storage options:
- In-memory: useful for a demonstration or short-lived data, but it does not preserve the chain across process restarts.
- JSONL file: stores entries as line-delimited JSON, making the log file-based rather than a replacement database.
- SQLite: stores entries in a local database.
The article describes a small storage interface so the chain logic can work with different stores. It does not establish current repository releases, maintenance status, independent security review, or production use, so treat the listed support as what the article documents rather than a current project-health assessment. Postgres, MySQL, MongoDB, Python, PHP, and an external anchoring helper are described there as future plans, not as verified current support.
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Tamper-evident is not tamper-proof
The crucial limitation is that a hash chain only detects changes relative to a trusted expected chain head. Someone who can rewrite the entire log and recompute every digest can produce a replacement chain that verifies internally. As the article puts it, “chainlog is tamper-evident, not tamper-proof.”
To detect a complete rewrite, preserve or publish the expected final digest—the chain head—somewhere outside the log, then verify the log against that independently held value. The article points to an expected-head check, expressed as verify(expectedHead). The anchor matters only if an attacker cannot quietly change both the log and the separately stored expected head.
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Even a successful check establishes integrity against a particular expected head, not the truth or completeness of the events. A valid chain cannot tell you whether an application omitted an event before recording it, whether a timestamp is accurate, or whether the underlying event actually occurred.
How the approaches differ
| Approach | What it provides | What changes for the operator | Important qualification |
|---|---|---|---|
| Hash-chain library | Links application audit entries and checks whether the chain is consistent; chainlog’s article describes memory, JSONL-file, and SQLite stores. | Can be integrated with an existing application and its logging flow. | A local chain needs a trusted expected head held outside the log to detect a full rewrite. The article does not establish writer-concurrency guarantees. |
| Transparency log | Trillian describes append-only logs built around Merkle trees, inclusion and consistency proofs, and signed tree heads. Google Trillian project | Offers a proof-oriented model for checking log contents and consistency, with additional operational complexity. | The project page says Trillian is in maintenance mode and recommends Tessera for new log operators. Trillian maintenance notice |
| Verifiable database | immudb documents structured audit events in its cryptographically verifiable key-value store. immudb structured audit events documentation | Changes the storage and operational dependency: the application uses a verifiable database rather than simply adding a check around an existing log. | Assess how its retention, querying, and verification model fit the application; it is not interchangeable with a local hash-chain library. |
These options solve related but distinct problems. A hash-chain library is the narrowest change when the goal is to check an application log. A transparency log is built around externally checkable proofs. A verifiable database changes where and how data is stored.
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When a local hash chain is a reasonable fit
A library such as the one described may suit an application that already writes audit events and wants a straightforward way to detect accidental or unauthorized edits, provided the operator can manage the store and protect an external copy of the expected chain head. It is not, by itself, a complete audit architecture or a guarantee against a privileged attacker.
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- Decide who can write the log and who can change the independently stored chain head.
- Choose a store based on the application’s persistence needs; the article describes memory, JSONL, and SQLite examples.
- Define when and how verification runs, and what the application does when a check fails.
- Keep a trustworthy anchor outside the log if detecting full-log replacement is part of the threat model.
- Consider a proof-based transparency log or a verifiable database if the required assurance and operating model exceed a local chain’s scope.
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