ProofOfExistence.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.1;

contract ProofOfExistence {
    event ProofCreated(uint256 indexed id, bytes32 documentHash);

    address public owner;

    mapping(uint256 => bytes32) hashesById;

    modifier onlyOwner() {
        require(
            msg.sender == owner,
            "Only the owner is allowed to access this function."
        );
        _;
    }

    modifier noHashExistsYet(uint256 id) {
        require(hashesById[id] == "", "No hash exists for this id.");
        _;
    }

    constructor() {
        owner = msg.sender;
    }

    function notarizeHash(uint256 id, bytes32 documentHash)
        public
        onlyOwner
        noHashExistsYet(id)
    {
        hashesById[id] = documentHash;

        emit ProofCreated(id, documentHash);
    }

    function doesProofExist(uint256 id, bytes32 documentHash)
        public
        view
        returns (bool)
    {
        return hashesById[id] == documentHash;
    }
}