Proof of Trustworthiness (PoT) NFT

Minting

A user calls NFT_mintPoTNFT() after accumulating sufficient behavior data. The circuit:

1. Derives the user’s Veil ID from their local secret key.
2. Triggers a lazy epoch update to advance the protocol clock.
3. Recomputes and persists the user’s credit score from their current accumulators.
4. Validates the score commitment against the on-chain Merkle tree.
5. Determines the trust tier (Unranked → Platinum) based on repayment ratio vs. config thresholds.
6. Mints a shielded token using the domain separator "veil:protocol:nft".
7. Stores NFT metadata in the public nftRegistry under the user’s Veil ID, including:
   • Token URI (IPFS link resolving to tier-appropriate image)
   • Token ID (from tokenIssueCounter)
   • Tier snapshot
   • Mint timestamp
   • Expiry epoch (epochLastUpdateTimeStamp + EPOCH_DURATION)
   • Credit score commitment hash
   • Ownership commitment (persistentCommit(veilId, ownershipSecret)

The shielded token itself serves as the bearer credential; the registry entry enables on-chain verification without revealing user identity.

Verification

Issuers call NFT_verifyPoTNFT(issuerPk, veilId, challenge, challengeExpiresAt, ownershipSecret) to validate a user’s trust status. The circuit:

1. Confirms the calling issuer is registered and approved.
2. Checks the user has an NFT in nftRegistry.
3. Computes a challenge hash (domain-tagged, per-user, per-issuer) and asserts it has not been used before (replay protection).
4. Verifies the ownership commitment: persistentCommit(veilId, ownershipSecret) == nftMetadata.ownershipCommitment.
5. Validates the stored credit score commitment against the on-chain Merkle tree.
6. Checks the challenge has not expired (challengeExpiresAt >= currentTime).
7. Auto-revokes the NFT if nftMetadata.expiresAtEpoch < currentTime and returns false.
8. Returns true if the NFT is valid, active, and not revoked.

The verification result is a boolean: the protocol learns only whether the user passes (no score value, no identity).

Revocation

NFTs can be revoked by:

Expiry: auto-revoked during any verification call if the current time exceeds expiresAtEpoch. Admin action: revokePoTNFT(veilId, adminPk) allows an authorized admin to revoke a specific user’s NFT (for example, in response to detected misconduct outside the protocol’s event system).

Revoked NFTs cause verifyPoTNFT to return false.

Time-Bound Validity and Renewal

PoT NFTs expire after nftEpochValidity epochs (default: 12 epochs ≈ 60 days). This is intentional: requiring periodic renewal ensures the trust signal reflects ongoing good behavior, not just a historical snapshot.

Renewal flow (NFT_renewPoTNFT):

1. User provides their existing shielded PoT token.
2. The circuit verifies the token color matches the Veil domain separator.
3. Recomputes the credit score and determines the new tier.
4. Burns the old token (sendImmediateShielded to the burn address).
5. Mints a fresh token with updated metadata and a new expiry.

If a user does not renew before expiry, their NFT is automatically revoked during the next verification call.