backend.md

On-chain contracts:

1. The issuer will deploy an NFT (SBT) contract "QuickKYC ZKCred V1 (QZK)"

   Deploy a non-transferable NFT smart contract (zkCredential issuer will be the owner) to EVM with following APIs

   • Use Openzeppelin ERC721Mintable (https://docs.openzeppelin.com/contracts/2.x/api/token/erc721#ERC721Mintable) as base and remove transfer-related methods.

   • constructor will take single parameter of zkCredential type (e.g., "https://ld.findora.org/creddential-kyc/v1").

     constructor(string credType)

   • mint function will take 2 parameters

     mint(address to, string(or bytes) commitment)

   • credType returns zk credential type

     credType() public view returns (string _credType)

   • commitment returns zkCredential's commitment

     commitment(uint256 tokenId) public view returns (string commitment)

2. The issuer also deploys 2 smart contracts to provide zk proof verification service.

   • https://github.com/FindoraNetwork/zkdid-circuits/blob/main/src/circuits/build/zkcredit/zkcredit_700_js/zkcredit_650_verifier.sol
   • https://github.com/FindoraNetwork/zkdid-circuits/blob/main/src/circuits/build/zkcredit/zkcredit_700_js/zkcredit_700_verifier.sol
   • verifyProof returns true only when proof is valid AND matching the input

       verifyProof(
            uint[2] memory a,
            uint[2][2] memory b,
            uint[2] memory c,
            uint[2] memory input
        ) public view returns (bool r)

3. The verifier deploys a smart contract "ZKVerifier" to EVM with following APIs

   • whiteList takes 3 parameters for whitelisting:

     • credType (e.g., "https://ld.findora.org/creddential-kyc/v1")
     • token (the SBT token address)
     • verifier (the one deployed by issuer)
     • whiteList(string credType, address token, address verifier)

     Note: A "ZKVerifier" can whitelist many verifiers to support different types of zk credentials (and different checks) issued by different credential issuers.

   • credTypes returns full list of whitelisted credential types.

     credTypes() public view returns (string[] memory)

   • verify checks token, proof passed by the user according to credType

       verify(
           string credType,
           uint256 tokenId,
           uint[2] memory a,
           uint[2][2] memory b,
           uint[2] memory c,
           uint[2] memory input) public view returns (bool r)

    check1>: check and see if credType is whitelisted or not
    check2>: check ownership(msg.sender) of the tokenId if already whitelisted
    check3>: get commitment by tokenId from the SBT
    check4>: get whitelisted verifier contract address and call `verifyProof` function with above commitment.
    return>: true/false

Off-chain server:

1. Host an credential issuer server SpruceID HTTP Server

2. Host an SIEW (if have time) to verify ownership of Metamask wallet.

3. A zkCredential issue server to issue SBT to user's Metamask address

   • address: user metamask
   • cred: the credential struct
   • return: transaction Hash

   fn (address: string, cred: Credential) -> string

Demo credential

transparent: https://github.com/FindoraNetwork/zkdid-circuits/blob/main/creds/credential-signed.jsonld#L22

zero-knoledge: https://github.com/FindoraNetwork/zkdid-circuits/blob/main/creds/zk-credential-signed.jsonld#L3

Demo credential encoding

{
  dob:    12546000  ==> HEX:   00000000000000000000000000BF6FD0
                    ==> Input: 12546000

  name:   Bob       ==> HEX:   00000000000000000000000000426F62
                    ==> Input: 4353890

  ssn:    433543937 ==> HEX:   00000000000000000000000019D75B01
                    ==> Input: 433543937

  credit: 702       ==> HEX:   000000000000000000000000000002BE
                    ==> Input: 702
}

DATA: 00000000000000000000000000BF6FD000000000000000000000000000426F6200000000000000000000000019D75B01000000000000000000000000000002BE
HASH: 8d9d6f7e9202abc8d7143a7562bb9e8299c75c454c9d79669328cca5615e0ae0

How to generate zkProof using CLI

1. Create a input file (e.g., input.json) and fill in credential data.

echo "{\"dob\": \"12546000\", \"name\": \"4353890\", \"ssn\": \"433543937\", \"credit\": \"702\"}" > input.json

2. Compute witness with WebAssembly (and input file) on circuit zkcredit_650.circom.

JS_DIR=src/circuits/build/zkcredit/zkcredit_650_js && node $JS_DIR/generate_witness.js $JS_DIR/zkcredit_650.wasm input.json witness.wtns

3. Generate zero-knowledge proof with witness and proving key

JS_DIR=src/circuits/build/zkcredit/zkcredit_650_js && snarkjs groth16 prove $JS_DIR/zkcredit_0001.zkey witness.wtns proof.json public.json

How to generate zkProof in browser