NFT Contract (AIP-721)

Relevant source files

The following files were used as context for generating this wiki page:

• src/nft_contract/Nargo.toml
• src/nft_contract/README.md
• src/ts/test/nft.test.ts
• benchmarks/nft_contract.benchmark.ts
• src/ts/test/utils.ts
• README.md

The NFT contract is a standardized implementation of non-fungible tokens for the Aztec Network, closely following the logic of Ethereum's ERC-721 but enhanced with Aztec-specific privacy extensions. It enables unique digital assets to exist in either a private state (confidential ownership via UTXO-like notes) or a public state (transparent ownership on a global ledger), providing a robust foundation for privacy-preserving decentralized applications.

Sources: src/nft_contract/README.md:3-7, README.md:46-48

The contract architecture supports seamless transitions between these two states, allowing users to "shield" their assets for privacy or "unshield" them for public interactions. It includes built-in access controls for minting, burning, and administrative upgrades, while leveraging Aztec's Authorization Witness (AuthWit) pattern for secure third-party interactions.

Sources: src/nft_contract/README.md:10-15, src/ts/test/nft.test.ts:88-100

Architecture and Data Structures

The contract utilizes several storage mappings to manage NFT metadata and ownership across both private and public domains.

Storage Model

Field	Type	Description
name	FieldCompressedString	The collection name (e.g., "TestNFT").
symbol	FieldCompressedString	The collection symbol (e.g., "TNFT").
private_nfts	Map<AztecAddress, PrivateSet<NFTNote>>	Private ownership set using encrypted notes.
nft_exists	Map<Field, bool>	Tracks the existence of a token ID.
public_owners	Map<Field, AztecAddress>	Mapping of token ID to transparent owner address.
minter	AztecAddress	Authorized address for minting operations.
upgrade_authority	AztecAddress	Address with rights to upgrade contract class.

Sources: src/nft_contract/README.md:17-23, src/ts/test/nft.test.ts:49-54

Identity and Access Control

The contract is initialized with a designated minter and an upgrade_authority. If the upgrade_authority is set to the zero address, the contract becomes non-upgradeable.

Sources: src/nft_contract/README.md:21-23, src/nft_contract/Nargo.toml:10-14

Functional Logic and Operations

Deployment and Initialization

The contract is typically deployed using a specialized initializer that defines the identity of the collection and the administrative roles.

const contract = await deployer
  .deploy('TestNFT', 'TNFT', deployerWallet, deployerWallet)
  .send({ contractAddressSalt: salt, from: deployerWallet });

Sources: src/ts/test/nft.test.ts:56-60, src/nft_contract/README.md:27-38

State Transitions (Private vs Public)

The NFT contract allows for four primary transfer paths, represented in the following flow:

Interaction Summary Table

Function Category	Name	Execution Context	Description
Minting	mint_to_private	Private	Mints a new NFT directly into a private note.
	mint_to_public	Public	Mints a new NFT to a public address.
Transfer	transfer_private_to_public	Private	Removes private note and updates public state.
	transfer_public_to_private	Private	Clears public owner and emits a private note.
	transfer_public_to_public	Public	Standard transparent transfer of ownership.
Commitment	initialize_transfer_commitment	Private	Returns a partial note to facilitate private transfers.
Admin	upgrade_contract	Public	Updates the contract class ID.

Sources: src/nft_contract/README.md:43-156, benchmarks/nft_contract.benchmark.ts:45-78

Authorization Witness (AuthWit) Logic

The contract implements Aztec's authorization witness pattern to allow third parties to execute transfers on behalf of an owner without the owner needing to be the transaction caller. This is critical for interactions with marketplaces or escrow contracts.

Nonce Usage

Every transfer and burn function requires a _nonce. Non-zero nonces are validated against authorization witnesses stored in the user's account contract to prevent unauthorized spending or replay attacks.

Sources: src/ts/test/nft.test.ts:192-210, src/nft_contract/README.md:121-128

Benchmarking and Utilities

The project includes a benchmarking suite to track the gas and resource consumption of various NFT operations, ensuring the contract remains efficient as the Aztec protocol evolves.

Key Benchmark Methods

• Private Mint: Benchmarks mint_to_private (Private context).
• Public Mint: Benchmarks mint_to_public (Public context).
• Shielding: Benchmarks transfer_public_to_private.
• Unshielding: Benchmarks transfer_private_to_public.

Sources: benchmarks/nft_contract.benchmark.ts:45-78

Ownership Verification (Unconstrained)

For client-side applications, the contract provides unconstrained utility functions to fetch ownership data without triggering circuit execution.

• get_private_nfts: Paginates through private notes to find IDs owned by an address.
• public_owner_of: Directly queries the public storage map for a token's owner.

Sources: src/nft_contract/README.md:183-195, src/ts/test/utils.ts:143-162

Summary

The NFT Contract (AIP-721) provides a privacy-first implementation of non-fungible tokens on Aztec. By separating storage into private_nfts and public_owners, and providing standardized methods for moving assets between these layers, it allows developers to build sophisticated digital asset systems that respect user confidentiality while maintaining the composability of the public blockchain.

Sources: README.md:46-48, src/nft_contract/README.md:3-7