HashClue Proof of Existence Note (v1, Non-Normative)

Status

Optional. Non-normative. This note does not modify, extend, or reinterpret the HashClue Protocol Specification.

Purpose

This document describes an optional pre-play trust anchor intended to demonstrate that a physical cache exists and that the Cartographer had possession of it prior to the opening of a round. It is designed to increase participant confidence without disclosing location information or weakening anti-compute guarantees.

Scope and limits

This mechanism provides evidence of existence and pre-commitment only. It is not a cryptographic proof of location, immutability, permanence, or fairness beyond what is defined in the protocol. It does not guarantee that a cache was not relocated after creation, nor does it eliminate the need for trust in protocol execution.

Threats addressed

Absent any pre-play evidence, a participant may reasonably claim that:

• the cache was placed only after revenue collection began, or
• the hunt was fabricated or staged retroactively.

This mechanism is intended to reduce those claims by establishing that specific evidence existed prior to the round opening, while keeping the cache location undisclosed.

Pre-play evidence package

Before a round opens, the Cartographer may create a sealed evidence package ("PoE package"). The package is created prior to any clue unlocks or guess submissions for the round.

A minimal PoE package contains:

1. Cache possession evidence (non-locating)

A short photo or video showing the physical cache and an internal or unique marker associated with it. The capture must avoid identifiable surroundings and must not include landmarks, terrain, skyline, vegetation, signage, audio cues, reflections, or other environmental signals that could be used to infer location.

2. Round commitment binding

The same media frame includes:

• the exact byte-for-byte SHA-256 hash commitment published for the round, and
• a one-time proof phrase generated solely for the PoE package.

This binds the evidence package to the specific round commitment without revealing the canonical secret string itself.

3. Package sealing

All PoE materials are bundled into a single archive. A cryptographic hash of that archive ("PoE package hash") is computed and retained. The archive itself is not published by default.

Public timestamp anchoring

Prior to the round opening, the Cartographer publishes a public timestamp anchor containing:

• the PoE package hash,
• the round's published hash commitment, and
• a timestamp.

The purpose of the anchor is to establish that a specific bytestring existed at or before a verifiable point in time. The anchor may be published via any durable, independently verifiable, append-only or widely witnessed medium. The PoE package itself may remain private unless disclosure becomes necessary.

Disclosure policy

By default, only the PoE package hash and timestamp anchor are public. The underlying PoE package is intentionally withheld to avoid accidental location leakage. Disclosure of the package is discretionary and may occur only if the legitimacy of the round is materially challenged.

Revealed and withheld information

Revealed

• That a physical cache existed prior to round opening.
• That the Cartographer possessed the cache prior to round opening.
• That a specific evidence package existed prior to round opening.
• The already-public round hash commitment.

Withheld

• Coordinates or coordinate-adjacent data.
• Media capable of inferring location.
• Any additional clue content beyond protocol-defined releases.
• The canonical secret string.

Anti-compute considerations

This mechanism does not reduce the coordinate search space defined at placement creation time and does not introduce new geographic constraints. No information disclosed through the PoE package hash or timestamp anchor enables compute-only resolution or weakens the anti-compute guarantees defined elsewhere in the protocol.

Operational cautions

Improper handling of media can unintentionally leak information. Implementers should:

• remove all metadata from captured media,
• minimize visual and audio content,
• avoid repeated or unnecessary captures, and
• treat the PoE package as sensitive material.

Summary

The Proof of Existence mechanism is an optional credibility measure. It demonstrates pre-play existence and pre-commitment, and nothing more. It does not replace protocol guarantees, nor does it attempt to eliminate all trust assumptions.