Best plan of attack for digital certificates for proof of completion on EVM

Question

I am looking to explore the option of creating a digital certificate (as in proof) when someone has completed a portion of training, and for this to be issued on an EVM-compatible blockchain using Solidity.

I have prototyped using ERC721 NFTs to encode a "certificate" however, I'd like to prevent recipients from being able to transfer these certificates. To prevent transfer, I attempted to use the Pause.sol functionality from OpenZeppelin, however, this would result in the entire contract being paused, as opposed to a specific tokenId.

Does anyone have any recommendation on an approach? Am I overcomplicating it if I don't want recipients to be able to trade the certificates (i.e. for them to remain static)? Any pointers would be much appreciated!

Answer 1

The simplest and most raw solution is to just set a mapping value.

pragma solidity ^0.8;

contract TrainingResults {
    enum Stage {
        NONE,
        STAGE_1,
        STAGE_2,
        COMPLETED
    }

    mapping (address => Stage) public participantStage;

    function setParticipantStage(address _graduate, Stage _stage) external {
        require(msg.sender == address(0x123), "Not authorized");
        participantStage[_graduate] = _stage;
    }
}

---

Or if you want them to be able to see some kind of NFT in their wallet (that supports NFTs), you can modify the ERC-721 contract to disallow transfers.

For example the OpenZeppelin implementation uses a function named _beforeTokenTransfer() (GitHub link) that can be overwritten to disallow transfers altogether.

pragma solidity ^0.8;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract TrainingResults is ERC721 {
    constructor() ERC721("TrainingResults", "TR") {}

    function _beforeTokenTransfer(address from,address to, uint256 tokenId) override internal {
        // Allow only for the admin
        // as this function is called on token mint as well
        require(msg.sender == address(0x123), "Cannot transfer tokens");
    }
}