📰How It Works

How ZKSAFE Password(ZKPass) works

ZKPass saves pwdhash hash of the password in the contract. ENS is to bind a name to an address, our ZKPass tries to bind pwdhash to an address.

We will sign User actions with Keccak256 to produce datahash. With expiration, chainId, an auto-incrementing nonce and the datahash, we will sign them again with Keccak256 to produce fullhash. nonce is used to avoid the multiple submission attempts.

In a ZK circuit，we use Poseidon algorithm to produce hash. The algorithm is chose as it requires a low gas fee.

The circuit is shown below.

pragma circom 2.0.0;

include "../../node_modules/circomlib/circuits/poseidon.circom";

template Main() {
    signal input in[3];
    signal output out[3];

    component poseidon1 = Poseidon(2);
    component poseidon2 = Poseidon(2);

    poseidon1.inputs[0] <== in[0];  //pwd
    poseidon1.inputs[1] <== in[1];  //address
    out[0] <== poseidon1.out; //pwdhash

    poseidon2.inputs[0] <== poseidon1.out;
    poseidon2.inputs[1] <== in[2]; //fullhash
    out[1] <== in[2]; //fullhash
    out[2] <== poseidon2.out; //allhash
}

component main = Main();

The chart below shows the logic.

password and address are first combined to produce pwdhash. With the use of address, it guarantees the pwdhash will be different even if the passwords are the same.

pwdhash and fullhash give us allhash. It covers all the user actions we want to verify.

At last, proof will show allhash, pwdhash and fullhash are all generated via the circuit. Without know what password is, we know that pwdhash is produced by password. As well as allhash and fullhash, we can be sure that only people knowing what the password is produce the hashes and give the proof for them.

fullhash as the output will prevent any unwanted modification.

Poseidon requires 254 bits input data, but Keccak256 consume 256 bits. We will fullhash right shift 3 bits.

Notes

ZKPass system only provide password change action for the user. We can verify the password with pwdhash off chain. On chain, ZKPass can be used as signature verification. For example, in ZKSAFE contract, ZKSAFE will use user actions to produce datahash. It will be passed to ZKPass for verification. After the user actions being verified, ZKSAFE knows whoever knows the password wants to perform these user actions.