Public records and timestamping: proving that a document has not changed

Integrity is a public-sector problem

Public administrations hold records that people rely on for years or decades: company registers, court decisions, building permits, official journals, statistics. When a dispute arises, someone may need to prove that a document is exactly what was issued on a particular date. Traditionally that trust came from physical archives, seals and the authority of the institution. As records became digital, it became easier to change them invisibly — and harder to prove that nobody had.

How hash anchoring works

A hash function turns any document into a short fingerprint. Changing even a single character produces a completely different fingerprint, and it is not feasible to find a different document with the same one. That makes fingerprints useful as evidence: if you have the original document and a trustworthy record of its fingerprint from a certain date, you can show that the document existed then and has not changed since.

Anchoring means writing such fingerprints to a ledger that many independent parties replicate. Rather than one entry per document, systems usually combine many fingerprints into a tree structure and record only the top value, called the root. A short proof can later show that a particular document was included in that batch. Thousands of records can be anchored with one ledger entry, and nothing about their content becomes public.

Timestamping before blockchains

The idea is older than blockchains. In 1991 Stuart Haber and W. Scott Stornetta published a method for timestamping digital documents by linking their fingerprints into a chain, so that backdating one would require altering all later entries. Their work is cited in the Bitcoin white paper.

The Internet Engineering Task Force standardised a protocol for trusted timestamp authorities in RFC 3161 in 2001. In the European Union, the eIDAS Regulation recognises qualified electronic time stamps, which enjoy a legal presumption that the date and time they indicate are accurate and that the data has not been altered since. For many legal purposes, this is the benchmark any ledger-based approach is compared against.

Choosing where to anchor

A public ledger with many independent participants is hard for any single party to rewrite, which makes it attractive for evidence. But public sector bodies must also consider longevity — will the ledger still be running and readable in thirty years? — and cryptographic ageing: hash functions considered secure today may weaken, so long-term archives need a plan to re-anchor records with stronger algorithms. A permissioned ledger run by several public institutions offers more control, but its credibility depends on how independent those institutions are from each other.

What anchoring cannot do

An anchored fingerprint proves that a document existed at a certain time and has not changed. It does not prove that the document is true, that its author had authority to issue it, or that the person presenting it is entitled to rely on it. Those questions still depend on signatures, institutional rules and law.

Questions to ask before anchoring records

• What exactly is being proved? Existence at a point in time and integrity since then — not authorship or truth. Users of the records need to understand the difference.
• Is personal data protected? Fingerprints of documents that contain personal data can still raise data protection questions, especially if the documents are short or predictable. Adding random values (salting) and batching reduce the risk.
• How will verification work in twenty years? Keep the original files, the proofs and a description of the method together, so that someone without today's software can still check them.
• Is there a plan for ageing algorithms? Archives should be able to re-anchor records with stronger hash functions before older ones become unsafe.
• How does it fit the legal framework? Where qualified timestamps carry a legal presumption, a ledger anchor may need to be supported by expert evidence if it is challenged in court.

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