ERC-6672 — Multi-Redeemable NFTs

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ERC-6672: Multi-Redeemable NFTs

ERC-6672 generalises ERC-2135 (consumable) to multiple redemption events per NFT, each tracked independently. A single NFT can be redeemed at the bar, then again at the merchandise booth, then again for a backstage pass — each redemption recorded separately by operator + redemptionId. The NFT itself stays in the holder's wallet and is never burned.

Used by:

• Multi-perk membership NFTs — concert ticket → drink + merch + meet-and-greet, each with separate redemption events.
• Achievement / quest NFTs — game NFT records completion of multiple quests, each as a separate redemption.
• Subscription NFTs — monthly redemptions tracked per period.
• Loyalty programs — purchase-discount redemptions per merchant campaign.

Required Interface

interface IERC6672 {
    event Redeem(address indexed _operator, uint256 indexed _tokenId,
                 address redeemer, bytes32 _redemptionId, string _memo);
    event Cancel(address indexed _operator, uint256 indexed _tokenId,
                 bytes32 _redemptionId, string _memo);

    function isRedeemed(address _operator, uint256 _tokenId, bytes32 _redemptionId)
        external view returns (bool);

    function getRedemptionIds(address _operator, uint256 _tokenId)
        external view returns (bytes32[] memory);

    function redeem(uint256 _tokenId, bytes32 _redemptionId, string calldata _memo) external;
    function cancel(uint256 _tokenId, bytes32 _redemptionId, string calldata _memo) external;
}

The _operator is the redemption authority (e.g. a venue's contract); the _redemptionId is a unique identifier for the redemption type (e.g. keccak256("free-drink")). Cancellation lets the operator reverse a redemption (e.g. customer returns the merch).

Neo Equivalent: NEP-11 + Per-(tokenId, operator, redemptionId) State

public static void Redeem(ByteString tokenId, ByteString redemptionId, string memo)
{
    var operatorAddr = Runtime.CallingScriptHash;
    if (!Runtime.CheckWitness(operatorAddr)) throw new Exception("NEP11:NoAuth");
    var key = RedemptionKey(operatorAddr, tokenId, redemptionId);
    if (Storage.Get(Storage.CurrentContext, key) is not null)
        throw new Exception("NEP11:AlreadyRedeemed");
    Storage.Put(Storage.CurrentContext, key, 1);
    OnRedeem(operatorAddr, tokenId, OwnerOf(tokenId), redemptionId, memo);
}

public static bool IsRedeemed(UInt160 operatorAddr, ByteString tokenId, ByteString redemptionId)
    => Storage.Get(Storage.CurrentContext, RedemptionKey(operatorAddr, tokenId, redemptionId)) is not null;

ERC-6672 (Ethereum)	Neo Equivalent	Notes
redeem(tokenId, redemptionId, memo)	Redeem(tokenId, redemptionId, memo)	Operator-witness-checked
cancel(tokenId, redemptionId, memo)	Cancel(tokenId, redemptionId, memo)	Same operator only
isRedeemed(operator, tokenId, redemptionId)	IsRedeemed(operator, tokenId, redemptionId)	Direct port
getRedemptionIds(operator, tokenId)	GetRedemptionIds(operator, tokenId) enumerated via Storage.Find
Redeem / Cancel events	OnRedeem / OnCancel notifications

Comparison With ERC-2135 (Consumable)

Feature	ERC-2135	ERC-6672
Redemptions per NFT	One (boolean flag)	Many (per operator + redemptionId)
Cancel / undo	No (consumed = consumed)	Yes (operator can cancel)
Multi-operator	No	Yes (each operator has independent state)
Storage cost	O(1) per token	O(operators × redemptions)
Use case	Single-use ticket	Multi-perk membership

ERC-2135 is the lighter choice for one-shot vouchers; ERC-6672 is necessary for multi-event memberships. The Neo storage prefix design makes the cost difference negligible at small scales but grows linearly with redemption count.

Composition

• ERC-2135 — single-use consumable. Use ERC-6672 when multiple redemption events are needed.
• ERC-7432 — roles. The "redeemer" role can delegate redemption authority to a venue without giving away ownership.
• ERC-2981 — royalties. Each redemption can pay a royalty to the issuer.

Migration Notes

For loyalty / membership NFT systems:

1. Use keccak256("redemption-name") (or Sha256 on Neo) as the redemptionId — gives off-chain tools a deterministic key.
2. The operator address is the venue / merchant contract; multiple operators per NFT collection are supported by the per-operator storage prefix.
3. Renderers (wallet UIs) iterate the redemption prefix to show "Stamps redeemed: X / Y" against the NFT.

For multi-merchant loyalty (e.g. hotel rewards across chains), each brand operates as a separate operator — the NFT's redemption history is a unified view across all of them.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract ERC6672 {
    mapping(address => mapping(uint256 => mapping(bytes32 => bool))) public isRedeemed;

    event Redeem(address indexed _operator, uint256 indexed _tokenId,
                 address redeemer, bytes32 _redemptionId, string _memo);
    event Cancel(address indexed _operator, uint256 indexed _tokenId,
                 bytes32 _redemptionId, string _memo);

    function redeem(uint256 tokenId, bytes32 redemptionId, string calldata memo) external {
        require(!isRedeemed[msg.sender][tokenId][redemptionId], "already redeemed");
        isRedeemed[msg.sender][tokenId][redemptionId] = true;
        emit Redeem(msg.sender, tokenId, /* owner */ msg.sender, redemptionId, memo);
    }

    function cancel(uint256 tokenId, bytes32 redemptionId, string calldata memo) external {
        require(isRedeemed[msg.sender][tokenId][redemptionId], "not redeemed");
        isRedeemed[msg.sender][tokenId][redemptionId] = false;
        emit Cancel(msg.sender, tokenId, redemptionId, memo);
    }
}

using System;
using System.ComponentModel;
using Neo;
using Neo.SmartContract.Framework;
using Neo.SmartContract.Framework.Attributes;
using Neo.SmartContract.Framework.Native;
using Neo.SmartContract.Framework.Services;

namespace R3E.Examples;

[DisplayName("MultiRedeemNFT")]
[ContractPermission("*", "*")]
[SupportedStandards(NepStandard.Nep11)]
public class MultiRedeemNFT : Nep11Token<TokenState>
{
    public override string Symbol => "MLTR";

    private const byte Prefix_Redeemed = 0x70;

    [DisplayName("Redeem")]
    public static event Action<UInt160, ByteString, UInt160, ByteString, string> OnRedeem;
    [DisplayName("Cancel")]
    public static event Action<UInt160, ByteString, ByteString, string> OnCancel;

    public static void Redeem(ByteString tokenId, ByteString redemptionId, string memo)
    {
        var operatorAddr = Runtime.CallingScriptHash;
        if (!Runtime.CheckWitness(operatorAddr)) throw new Exception("NEP11:NoAuth");
        var key = RedemptionKey(operatorAddr, tokenId, redemptionId);
        if (Storage.Get(Storage.CurrentContext, key) is not null)
            throw new Exception("NEP11:AlreadyRedeemed");
        Storage.Put(Storage.CurrentContext, key, 1);
        OnRedeem(operatorAddr, tokenId, OwnerOf(tokenId), redemptionId, memo);
    }

    public static void Cancel(ByteString tokenId, ByteString redemptionId, string memo)
    {
        var operatorAddr = Runtime.CallingScriptHash;
        if (!Runtime.CheckWitness(operatorAddr)) throw new Exception("NEP11:NoAuth");
        var key = RedemptionKey(operatorAddr, tokenId, redemptionId);
        if (Storage.Get(Storage.CurrentContext, key) is null)
            throw new Exception("NEP11:NotRedeemed");
        Storage.Delete(Storage.CurrentContext, key);
        OnCancel(operatorAddr, tokenId, redemptionId, memo);
    }

    public static bool IsRedeemed(UInt160 operatorAddr, ByteString tokenId, ByteString redemptionId)
        => Storage.Get(Storage.CurrentContext, RedemptionKey(operatorAddr, tokenId, redemptionId)) is not null;

    public static ByteString[] GetRedemptionIds(UInt160 operatorAddr, ByteString tokenId)
    {
        var prefix = OperatorTokenPrefix(operatorAddr, tokenId);
        var iter = Storage.Find(Storage.CurrentContext, prefix, FindOptions.KeysOnly | FindOptions.RemovePrefix);
        var list = new System.Collections.Generic.List<ByteString>();
        while (iter.Next()) list.Add((ByteString)(byte[])iter.Value);
        return list.ToArray();
    }

    private static byte[] RedemptionKey(UInt160 operatorAddr, ByteString tokenId, ByteString redemptionId)
        => new byte[] { Prefix_Redeemed }.Concat(operatorAddr).Concat(tokenId).Concat(redemptionId);
    private static byte[] OperatorTokenPrefix(UInt160 operatorAddr, ByteString tokenId)
        => new byte[] { Prefix_Redeemed }.Concat(operatorAddr).Concat(tokenId);
}

Concert / Festival Wallet Integration

A festival wallet UI calls IsRedeemed(VENUE_OPERATOR, tokenId, DRINK_ID) for each NFT and renders "Drink ✓" / "Drink ⨯" badges. Backstage scanners run Redeem(tokenId, BACKSTAGE_ID, memo) when verifying entry. The NFT stays in the holder's wallet — proof of attendance for future drops.