Anchoring, and what it proves

Anchoring answers one question: did this exact text exist, in this exact form, on this date, and has it been changed since?

It works in three steps. First, the document is hashed — run through a one-way function that turns any input into a short fixed string. Change one character and the string changes completely, so the hash is a fingerprint of that exact text.

Second, many fingerprints are combined into a tree. Each pair is hashed together, then each pair of those, up to a single value at the top called the root. The useful property is that you can prove any one document is inside the tree by showing a handful of intermediate hashes, without revealing the other documents and without anyone having to trust a list.

Third, the root is written into a public blockchain transaction, which carries a timestamp nobody controls. From then on, anyone holding the original text can recompute its fingerprint, walk it up through the proof to the root, read the root off the chain themselves, and compare. The verification does not run on our servers. That is the entire point.

In our own build the leaf is a hash of the canonical form of the document, the tree is built pairwise, and the root goes into a zero-value transaction as calldata. On the test network the estimated cost is 22,775 gas, which at the fee levels we measured came to roughly a ten-millionth of an ETH per anchor ¹². It is cheap enough that batching is about tidiness rather than cost.

What it proves, and what it does not

It proves existence and integrity. This text was there by that block, and it has not been quietly edited since.

It proves nothing about whether the text is correct. An anchored sentence that is wrong is a wrong sentence with a timestamp. On a site about rules and payment terms this is the misreading that matters, because a reader who sees a chain reference tends to hear "checked" when the only claim being made is "unchanged".

It also proves nothing about who wrote the document, unless a signature is added separately, and it says nothing about whether anyone was authorised to publish it.

Status on this site

Where an entry in the Register carries an anchor block, it links to a page that recomputes the proof in the reader's own browser and reads the root from a public network: cryptowhat.net/verify/. Where an entry has no anchor block, it is marked not anchored, and no block is printed rather than an empty one. In our own build the anchoring path runs end to end against a real network's checks, and the shipped default remains the stub until the test-network wallet is funded. An empty anchor field means the anchor has not happened, and it is passed through untouched rather than filled in with something that looks reassuring.