papers/hanzo-identity-nft/sections/04-smart-contract-architecture.tex

\section{Smart Contract Architecture}


\subsection{Upgradeability}

The registry uses UUPS (Universal Upgradeable Proxy Standard)~\cite{eip1822}:

\begin{lstlisting}[language=Solidity]
contract HanzoRegistry is
    Initializable,
    UUPSUpgradeable,
    Ownable2StepUpgradeable
{
    function _authorizeUpgrade(address newImplementation)
        internal override onlyOwner {}
}
\end{lstlisting}

UUPS places upgrade logic in the implementation contract, reducing proxy complexity and gas costs.

\subsection{Access Control}

Two-step ownership transfer prevents accidental lockout:

\begin{lstlisting}[language=Solidity]
// Ownable2StepUpgradeable provides:
function transferOwnership(address newOwner) public;
function acceptOwnership() public;
function pendingOwner() public view returns (address);
\end{lstlisting}

\subsection{NFT Contract}

\begin{lstlisting}[language=Solidity]
contract HanzoNft is ERC721, Ownable {
    address public minter;  // Registry contract

    function mint(address to) external returns (uint256) {
        require(msg.sender == minter, "Not minter");
        uint256 tokenId = _tokenIdCounter++;
        _safeMint(to, tokenId);
        return tokenId;
    }

    function burn(uint256 tokenId) external {
        require(_isApprovedOrOwner(msg.sender, tokenId),
                "Not owner");
        _burn(tokenId);
    }
}
\end{lstlisting}

The NFT contract is minimal; all identity logic resides in the registry.