Confidential NFTs: Implementing Private Ownership via FHE

Blockchains are public ledgers, meaning every account balance and asset transfer is visible to the entire world. While this transparency is key to decentralization, it presents major privacy roadblocks for high-value collectors and institutions tokenizing Real-World Assets (RWA) like art or collectibles.

By leveraging Fully Homomorphic Encryption (FHE), we can build smart contracts that encrypt ownership data, keeping beneficial owner identities private while maintaining full auditability and standard ERC721 features.

The Dual Ownership Model

To bridge the gap between traditional NFT marketplaces (which expect public owner addresses) and complete ownership privacy, we can implement a Dual Ownership Model. Each NFT maintains two distinct records:

1. Public Holder (ERC721 Owner): The public-facing wallet address holding the token (e.g., a custodian, DAO treasury, or escrow wallet).
2. Encrypted Beneficial Owner: The true owner’s wallet address, encrypted on-chain.

Here is a simplified implementation of a confidential RWA contract in Solidity using Zama’s fhEVM library:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "@fhevm/lib/TFHE.sol";
import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract ConfidentialRWA is ERC721 {
    // Encrypted beneficial owner address for each tokenId
    mapping(uint256 => eaddress) private _encryptedOwners;

    constructor() ERC721("Confidential Asset", "CRWA") {}

    // Mint a new asset with an encrypted beneficiary owner address
    func mintAsset(
        address publicHolder,
        bytes calldata encryptedBeneficiary,
        uint256 tokenId
    ) public {
        _safeMint(publicHolder, tokenId);
        
        // Cast input bytes to FHE encrypted address (eaddress)
        _encryptedOwners[tokenId] = TFHE.asEaddress(encryptedBeneficiary);
    }

    // Verify true ownership without exposing the owner's address
    func isBeneficiary(
        uint256 tokenId,
        bytes calldata encryptedAddress
    ) public view returns (ebool) {
        eaddress inputAddress = TFHE.asEaddress(encryptedAddress);
        eaddress trueOwner = _encryptedOwners[tokenId];
        
        // Perform homomorphic comparison
        return TFHE.eq(inputAddress, trueOwner);
    }
}

How It Works

• Encrypted Storage: The true owner address is stored on-chain as a eaddress type, which is encrypted.
• Confidential Operations: Homomorphic operations allow smart contracts to compute comparisons (like checking if a caller is the beneficial owner) on encrypted inputs without ever decrypting the actual values.
• Standard Integration: Because the public owner record remains standard ERC721, the token can still be listed on public marketplaces (e.g., OpenSea) or held in multi-sig vault contracts.

Conclusion

FHE unlocks a new paradigm for blockchain privacy. By mapping public custodial holders with encrypted beneficial owners, web3 developers can design robust institutional-grade RWA platforms that satisfy strict privacy regulations and protect collectors from public tracking.