# Proof of Integrity

The integrity proof links a unique physical coin to a deterministic registry token ID. This is a **verifiable commitment**, not a zero-knowledge proof. See [ZK Roadmap](/numex-docs/zero-knowledge-proofs/zk-roadmap.md) for planned privacy features.

## Fingerprint

A fingerprint is a human-readable identity string for the physical asset:

```txt
Numex Coin | {grading_company} {cert_number_or_internal_uid} | {year} {mint_mark} {denomination} {series} | {grade} | {vault_item_uid}
```

## Proof Mapping

```mermaid
flowchart LR
  A["Human-readable fingerprint"] --> C["keccak256"]
  B["Salt"] --> C
  C --> D["Integrity proof: 0x...64 hex chars"]
  D --> E["Token ID: uint256(proof)"]
  E --> F["NumexRegistryV3 ERC-721 token"]
```

Current formula (on-chain, keccak256):

```solidity
bytes32 proof = keccak256(abi.encodePacked(fingerprint, salt));
uint256 tokenId = uint256(proof);
```

The TypeScript generation script uses `solidityPackedKeccak256` from ethers to produce contract-compatible hashes. See [Hash Conventions](/numex-docs/technology/hash-conventions.md) for why this differs from the SHA-256 used in reveal proofs.

## Ownership Assignment

```mermaid
flowchart TD
  A["Marketplace purchase"] --> C["Ownership assigned"]
  B["Pack reveal"] --> C
  C --> D["Integrity proof generated"]
  D --> E["Token ID derived"]
  E --> F["Chain anchor queued"]
```

A physical coin cannot be owned by two users simultaneously. Both marketplace purchase and pack reveal check that the coin is unowned before assigning custody.

## Relationship to ZK Proofs

The integrity proof is a **verifiable commitment**, not a zero-knowledge proof. It proves a specific physical coin maps to a specific token ID, but the fingerprint and salt are known to the verifier.

The [ZK Ownership Proof](/numex-docs/zero-knowledge-proofs/zk-ownership-proof.md) adds a privacy layer on top: it proves a user owns *some* asset in the registry without revealing *which* asset. The integrity proof links physical coins to on-chain tokens; the ZK proof enables private ownership claims over those tokens.

## Demo Caveat

The demo backfill uses deterministic salts so the proof mapping can be repeatedly verified. Production salts must be random, server-generated, protected, and revealed only when the product design intentionally discloses them.


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