The last of the five ACI Core properties, and the one that separates a crafted, witnessed claim from one merely generated: external anchoring. A claim earns trust not by sounding right but by being tied to something outside itself that a stranger can check. Here are two claims — one anchored to a witness, one floating free. Challenge them, tamper them, try to forge an anchor. Only the tethered one survives.
ACI · a CORE property made hands-on — external anchoring, the differentiating axiom
The two claims · challenge, tamper, forge
Two layers, honestly
Real
The binding is genuine: the anchor is sha256(claim │ witness), and “challenge” recomputes it live and compares. Tamper the claim or the witness and the recomputation no longer matches — the anchor catches it. A forged anchor (a witness the claim was never bound to) fails the same check. This is really how a hash-anchored provenance works.
Figure
The tethered star is the image. And the honest limit: this witness is local — a value you can recompute in the page. True external anchoring binds to an authority outside the artifact and its author — an RFC-3161 timestamp, a git commit, a URL a third party controls — which a single self-contained file can only simulate. The mechanism shown is exact; the “externality” is stood in for. The point still holds: anchored-and-re-verifiable beats a confident assertion, every time.