User-mintable NFTs are a critical part of the Centrifuge ecosystem. Minting an NFT from a Centrifuge Document allows a user to draw loans against the NFT asset, or simply provides a way to easily transact in privacy-preserving way with a document asset. This flow can be seen in the image below:

By using precise proofs, we can supply privacy preserving proofs that certain fields are present in the NFT to be minted, without revealing the exact value of these fields. Leaves are created by hashing and encoding the property, value, and salts of the data fields which we want to prove:

1bytes32[] memory leaves = new bytes32[](5);2 leaves[0] = sha256(abi.encodePacked(GROSS_AMOUNT, values[0], salts[0]));3 leaves[1] = sha256(abi.encodePacked(CURRENCY, values[1], salts[1]));4 leaves[2] = sha256(abi.encodePacked(DUE_DATE, values[2], salts[2]));5 leaves[3] = sha256(abi.encodePacked(NEXT_VERSION, values[3], salts[3]));6 leaves[4] = sha256(abi.encodePacked(properties[4], values[4], salts[4]));

The resulting array of leaves can be verified against the array of supplied proofs which can be generated with our precise-proofs library:

1proofs = new bytes32[][](5);2 proofs[0] = new bytes32[](7);3 proofs[0][0] = 0xd42948fa37dd912117ac5966b55d4b364005e4dc366e3afb6caf38649dce7d20;4 proofs[0][1] = 0xccaad8761ce1a541483d2fb98d621a9a9c11d7b03d82fdbeb2cefdbb8405916e;5 proofs[0][2] = 0x598e8661d6c2a206e632f12c9031a3b50e02af430b144f71db2bbde83e8da2ec;6 proofs[0][3] = 0x7f8410d0adbc62a9b9933c8cb16a45c0cca73dec62e89b185f22a06073e7b960;7 proofs[0][4] = 0x6a37a214f9f3cb50f0cbdfd4183040781860acf7ccb1a4c8453cf31003fc99e7;8 proofs[0][5] = 0x41337de1d0f1a323f5fcca15144b7a1f37cbb442a053fee73f84f27afbc3d719;9 proofs[0][6] = 0x480d3bf285726b8ecf2199da06f35bb77830e07828e036bf9a8dc8c95129f45e;10 ...

The verification method can be found in our optimised Merkle Verifier contract.

An example of an NFT with one proof which can be minted using methods from the precise-proofs library is as follows:

1pragma solidity >=0.4.23;2pragma experimental ABIEncoderV2;34import "privacy-enabled-erc721/nft.sol";567contract DocumentationNFT is NFT {8 // compact property for 'loanAmount'9 bytes constant internal AMOUNT = hex"010000000000001cdb691b0c78e9e1432d354d52e26b3cd5054cd1261c4272bf8fce2bcf285908f300000005";1011 struct TokenData {12 uint document_version;13 uint amount;14 address borrower;15 }16 mapping (uint => TokenData) public data;1718 constructor (address anchors_) NFT("Documentation NFT", "DNFT", anchors_) public {19 }2021 // --- Utils ---22 function bytesToUint(bytes memory b) public returns (uint256){23 uint256 number;24 for (uint i = 0; i < b.length; i++){25 number = number + uint8(b[i]) * (2 ** (8 * (b.length - (i + 1))));26 }27 return number;28 }2930 // --- Mint Method ---31 function mint(address usr, uint tkn, uint anchor, bytes32 data_root, bytes32 signatures_root, bytes[ memory value, bytes32 memory salt, bytes32[][] memory proofs) public {3233 data[tkn] = TokenData(34 anchor,35 bytesToUint(values),36 usr37 );3839 bytes32 leaf = sha256(abi.encodePacked(AMOUNT, value, salt));4041 require(verify(proofs, data_root, leaves), "Validation of proofs failed.");42 require(_checkAnchor(anchor, data_root, signatures_root), "Validation against document anchor failed.");43 _mint(usr, tkn);44 }45}

The mint method on this NFT would expect one proof of "loanAmount", which has been hardcoded as a state variable. It would furthermore also check that the document from which the NFT should be minted has been properly anchored, before minting the NFT.

We recommend a certain anatomy for an NFT created with the Centrifuge privacy enabled ERC721 library.

*Please note that the proposed NFT structure will change pending the launch of the Centrifuge Chain.*

It is useful to store publicly accessible data about the NFT in a separate metadata structure, in order to access and render this data in user facing applications.

We store this metadata in a TokenData structure which typically contains at least the document version and the address of the minter, as well as the fields which are contained in the precise proofs.

For example, an Invoice Document could contain these metadata fields:

1struct TokenData {2 uint document_version;3 uint gross_amount;4 uint currency;5 uint due_date;6 address invoice_sender;7}

There are several checks that should be done prior to the minting of an NFT:

`_checkAnchor`

: Checking that the document has been properly anchored`_signed`

: Checking that the identity trying to mint the NFT is a valid Centrifuge identity, and that the signing key used for minting contains a signing purpose and is not revoked`_checkTokenData`

: If there is a proof present that there is no other token minted in this registry for the provided document, checking that the token data in the passed in token uniqueness proof matches the data on the token to be minted`verify`

: Verify the submitted proofs against the NFT to be minted. For more information, please read the information in Verification of Proof Fields in the subsequent section

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