2020-07-26 14:05:23 +02:00

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snarkjs

This is a JavaScript and Pure Web Assembly implementation of zkSNARK schemes. It uses the Groth16 Protocol (3 point only and 3 pairings).

This library includes all the tools required to perform trusted setup multi-party ceremonies: including the universal powers of tau ceremony, and the second phase circuit specific ceremonies.

Any zk-snark project can pick a round from the common phase 1 to start their circuit-specific phase 2 ceremony.

The formats used in this library for the multi-party computation are compatible with the ones used in Semaphore's Perpetual Powers of Tau and other implementations.

This library uses the compiled circuits generated by the circom compiler.

It works in node.js as well as directly in the browser.

It's an ES module, so it can be directly imported into bigger projects using Rollup or Webpack.

The low-level cryptography is performed directly in wasm, and uses worker threads to parallelize the computations. The result is a high performance library with benchmarks comparable to host implementations.

Preliminaries

Install node v14

First off, make sure you have a recent version of Node.js installed. While any version after v12 should work fine, we recommend you install v14 or later.

If you’re not sure which version of Node you have installed, you can run:

node -v

To download the latest version of Node, see here.

Install snarkjs and circom

To install circom and snarkjs, run:

npm install -g circom@latest
npm install -g snarkjs@latest

If you're seeing an error, try prefixing both commands with sudo.

Understand the `help` command

To see a list of all snarkjs commands, as well as descriptions about their inputs and outputs, run:

snarkjs --help

You can also use the --help option with specific commands:

snarkjs groth16 prove --help

Most of the commands have an alternative shorter alias (which you can discover using --help).

For example, the previous command can also be invoked with:

snarkjs g16p --help

Debugging tip

If you a feel a command is taking longer than it should, re-run it with a -v or --verbose option to see more details about how it's progressing and where it's getting blocked.

Guide

0. Create and move into a new directory

mkdir snarkjs_example
cd snarkjs_example

1. Start a new powers of tau ceremony

snarkjs powersoftau new bn128 12 pot12_0000.ptau -v

The new command is used to start a powers of tau ceremony.

The first parameter after new refers to the type of curve you wish to use. At the moment, we support both bn128 and bls12-381.

The second parameter, in this case 12, is the power of two of the maximum number of contraints that the ceremony can accept: in this case, the number of constraints is 2 ^ 12 = 4096. The maximum value supported here is 28, which means you can use snarkjs to securely generate zk-snark parameters for circuits with up to 2 ^ 28 (≈268 million) constraints.

2. Contribute to the ceremony

snarkjs powersoftau contribute pot12_0000.ptau pot12_0001.ptau --name="First contribution" -v

The contribute command creates a ptau file with a new contribution.

You'll be prompted to enter a random text as an extra source of entropy.

contribute takes as input the transcript of the protocol so far, in this case pot12_0000.ptau, and outputs a new transcript, in this case pot12_0001.ptau, which includes the computation carried out by the new contributor (ptau files contain a history of all the challenges and responses that have taken place so far).

name can be anything you want, and is just included for reference (it will be printed when you verify the file (step 5).

3. Provide a second contribution

snarkjs powersoftau contribute pot12_0001.ptau pot12_0002.ptau --name="Second contribution" -v -e="some random text"

By allowing you to write the random text as part of the command, the -e parameter allows contribute to be non-interactive.

4. Provide a third contribution using third party software

snarkjs powersoftau export challenge pot12_0002.ptau challenge_0003
snarkjs powersoftau challenge contribute bn128 challenge_0003 response_0003 -e="some random text"
snarkjs powersoftau import response pot12_0002.ptau response_0003 pot12_0003.ptau -n="Third contribution name"

The challenge and response files are compatible with this software.

This allows to use both softwares in a single ceremony.

5. Verify the protocol so far

snarkjs powersoftau verify pot12_0003.ptau

The verify command verifies a ptau (powers of tau) file. Which means it checks all the contributions to the multi-party computation (MPC) up to that point. It also prints the hashes of all the intermediary results to the console.

If everything checks out, you should see the following at the top of the output:

[INFO]  snarkJS: Powers Of tau file OK!

In sum, whenever a new zk-snark project needs to perform a trusted setup, you can just pick the latest ptau file, and run the verify command to verify the entire chain of challenges and responses so far.

6. Apply a random beacon

snarkjs powersoftau beacon pot12_0003.ptau pot12_beacon.ptau 0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f 10 -n="Final Beacon"

The beacon command creates a ptau file with a contribution applied in the form of a random beacon.

The next step is to apply a random beacon to it (we need to apply a random beacon in order to finalise phase 1 of the trusted setup).

To paraphrase Sean Bowe and Ariel Gabizon, a random beacon is a source of public randomness that is not available before a fixed time. The beacon itself can be a delayed hash function (e.g. 2^40 iterations of SHA256) evaluated on some high entropy and publicly available data. Possible sources of data include: the closing value of the stock market on a certain date in the future, the output of a selected set of national lotteries, or the value of a block at a particular height in one or more blockchains. E.g. the hash of the 11 millionth Ethereum block (which as of this writing is some 3 months in the future). See here for more on the importance of a random beacon.

In the above case, the beacon is essentially a delayed hash function evaluated on 0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f (in practice, this will be some form of high entropy and publicly available data). The 10 just means perform 2 ^ 10 iterations of this hash function.

Note that security holds even if an adversary has limited influence on the beacon.

7. Prepare phase 2

snarkjs powersoftau prepare phase2 pot12_beacon.ptau pot12_final.ptau -v

We're now ready to prepare phase 2 of the setup (the circuit-specific phase).

Under the hood, the prepare phase2 command calculates the encrypted evaluation of the Lagrange polynomials at tau for tau, alpha*tau and beta*tau. It takes the beacon ptau file we generated in the previous step, and outputs a final ptau file which will be used to generate the circuit proving and verification keys.

8. Verify the final `ptau`

snarkjs powersoftau verify pot12_final.ptau

The verify command verifies a powers of tau file.

Before we go ahead and create the circuit, we perform a final check and verify the final protocol transcript.

You can see now that there is not a warning any more informing that you have not run the prepare phase2.

9. Create the circuit

cat <<EOT > circuit.circom
template Multiplier(n) {
    signal private input a;
    signal private input b;
    signal output c;

    signal int[n];

    int[0] <== a*a + b;
    for (var i=1; i<n; i++) {
    int[i] <== int[i-1]*int[i-1] + b;
    }

    c <== int[n-1];
}

component main = Multiplier(1000);
EOT

We create a circom file that allows us to easily test the system with a different number of contraints.

In this case, we've chosen 1000, but we can change this to anything we want (as long as it's below the number we defined in step 1).

10. Compile the circuit

circom circuit.circom --r1cs --wasm --sym -v

The circom command takes one input (the circuit to compile, in our case circuit.circom) and three options:

r1cs: generates circuit.r1cs (the r1cs constraint system of the circuit in binary format).
wasm: generates circuit.wasm (the wasm code to generate the witness – more on that later).
sym: generates circuit.sym (a symbols file required for debugging and printing the constraint system in an annotated mode).

11. View information about the circuit

snarkjs r1cs info circuit.r1cs

The info command is used to print circuit stats.

You should see the following output:

[INFO]  snarkJS: Curve: bn-128
[INFO]  snarkJS: # of Wires: 1003
[INFO]  snarkJS: # of Constraints: 1000
[INFO]  snarkJS: # of Private Inputs: 2
[INFO]  snarkJS: # of Public Inputs: 0
[INFO]  snarkJS: # of Outputs: 1

This information fits with our mental map of the circuit we created: we had two private inputs a and b, one output c, and a thousand constraints of the form a * b = c.

12. Print the constraints

snarkjs r1cs print circuit.r1cs circuit.sym

To double check, we print the constraints of the circuit.

You should see a thousand constraints of the form:

[ -main.int[i] ] * [ main.int[i] ] - [ main.b -main.int[i+1] ] = 0

13. Export r1cs to json

snarkjs r1cs export json circuit.r1cs circuit.r1cs.json
cat circuit.r1cs.json

We export r1cs to json format to make it human readable.

14. Generate the reference `zkey` without phase2 contributions

snarkjs zkey new circuit.r1cs pot12_final.ptau circuit_0000.zkey

The zkey new command creates an initial zkey file with zero contributions.

The zkey is a zero-knowledge key that includes both the prooving and verification keys as well as phase2 contributions.

Importantly, one can verify whether a zkey belongs to a specific circuit or not.

Note that circuit_0000.zkey (the output of the zkey command above) does not include any contributions yet, so it cannot be used in a final circuit.

The following steps (15-20) are similar to the equivalent phase1 steps, except we use zkey instead of powersoftau as the main command, and we generate zkey rather that ptau files.

15. Contribute to the phase2 ceremony

snarkjs zkey contribute circuit_0000.zkey circuit_0001.zkey --name="1st Contributor Name" -v

The zkey contribute command creates a zkey file with a new contribution.

As in phase 1, you'll be prompted to enter a random text as an extra source of entropy.

16. Provide a second phase2 contribution

snarkjs zkey contribute circuit_0001.zkey circuit_0002.zkey --name="Second contribution Name" -v -e="Another random entropy"

We provide a second contribution.

17. Provide a third contribution using third party software

snarkjs zkey export bellman circuit_0002.zkey  challenge_phase2_0003
snarkjs zkey bellman contribute bn128 challenge_phase2_0003 response_phase2_0003 -e="some random text"
snarkjs zkey import bellman circuit_0002.zkey response_phase2_0003 circuit_0003.zkey -n="Third contribution name"

And a third using third-party software.

18. Verify the latest `zkey`

snarkjs zkey verify circuit.r1cs pot12_final.ptau circuit_0003.zkey

The zkey verify command verifies a zkey file. It also prints the hashes of all the intermediary results to the console.

We verify the zkey file we created in the previous step. Which means we check all the contributions to the second phase of the multi-party computation (MPC) up to that point.

This command also checksh that the zkey file matches the circuit.

If everything checks out, you should see the following:

[INFO]  snarkJS: ZKey Ok!

19. Apply a random beacon

snarkjs zkey beacon circuit_0003.zkey circuit_final.zkey 0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f 10 -n="Final Beacon phase2"

The zkey beacon command creates a zkey file with a contribution applied in the form of a random beacon.

We use it to apply a random beacon to the latest zkey after all the final contribution has been made (this is necessary in order to generate a final zkey file and finalise phase 2 of the trusted setup).

20. Verify the final `zkey`

snarkjs zkey verify circuit.r1cs pot12_final.ptau circuit_final.zkey

Before we go ahead and export the verification key as a json, we perform a final check and verify the final protocol transcript (zkey).

21. Export the verification key

snarkjs zkey export verificationkey circuit_final.zkey verification_key.json

We export the verification key from circuit_final.zkey into verification_key.json.

22. Calculate the witness

cat <<EOT > input.json
{"a": 3, "b": 11}
EOT
snarkjs wtns calculate circuit.wasm input.json witness.wtns

Calculate the witness (given the inputs a = 3 and b = 11).

23. Debug the final witness calculation

snarkjs wtns debug circuit.wasm input.json witness.wtns circuit.sym --trigger --get --set

And check for any errors in the witness calculation process (this is best practice).

The above command will log every time a new component starts/ends (--trigger), when a signal is set (--set) and when it's read (--get).

24. Create the proof

snarkjs groth16 prove circuit_final.zkey witness.wtns proof.json public.json

We create the proof. groth16 prove generates the files proof.json and public.json: proof.json contains the actual proof, whereas public.json contains the values of the public inputs and output.

Note that it's also possible to create the proof and calculate the witness in the same command by running:

snarkjs groth16 fullprove input.json circuit.wasm circuit_final.zkey proof.json public.json

25. Verify the proof

snarkjs groth16 verify verification_key.json public.json proof.json

We use the groth16 verify command to verify the proof, passing in the verification_key we exported earlier.

If all is well, you should see that OK has been outputted to your console. This signifies the proof is valid.

26. Turn the verifier into a smart contract

snarkjs zkey export solidityverifier circuit_final.zkey verifier.sol

Finally, we export the verifier as a Solidity smart-contract so that we can publish it on-chain using remix for example. For the details on how to do this, see section 4 of this tutorial.

27. Simulate a verification call

snarkjs zkey export soliditycalldata public.json proof.json

We use soliditycalldata to simulate a verification call, and cut and paste the result directly in the verifyProof field in the deployed smart contract in the remix envirotment.

And voila! That's all there is to it :)

Using Node

npm init
npm install snarkjs

const snarkjs = require("snarkjs");
const fs = require("fs");

async function run() {
    const { proof, publicSignals } = await snarkjs.groth16.fullProve({a: 10, b: 21}, "circuit.wasm", "circuit_final.zkey");

    console.log("Proof: ");
    console.log(JSON.stringify(proof, null, 1));

    const vKey = JSON.parse(fs.readFileSync("verification_key.json"));

    const res = await snarkjs.groth16.verify(vKey, publicSignals, proof);

    if (res === true) {
        console.log("Verification OK");
    } else {
        console.log("Invalid proof");
    }

}

run().then(() => {
    process.exit(0);
});

In the browser

Load snarkjs.min.js and start using it as usual.

cp node_modules/snarkjs/build/snarkjs.min.js .

<!doctype html>
<html>
<head>
  <title>Snarkjs client example</title>
</head>
<body>

  <h1>Snarkjs client example</h1>
  <button id="bGenProof"> Create proof </button>

  <!-- JS-generated output will be added here. -->
  <pre class="proof"> Proof: <code id="proof"></code></pre>

  <pre class="proof"> Result: <code id="result"></code></pre>


  <script src="snarkjs.min.js">   </script>


  <!-- This is the bundle generated by rollup.js -->
  <script>

const proofCompnent = document.getElementById('proof');
const resultComponent = document.getElementById('result');
const bGenProof = document.getElementById("bGenProof");

bGenProof.addEventListener("click", calculateProof);

async function calculateProof() {

    const { proof, publicSignals } =
      await snarkjs.groth16.fullProve( { a: 3, b: 11}, "circuit.wasm", "circuit_final.zkey");

    proofCompnent.innerHTML = JSON.stringify(proof, null, 1);


    const vkey = await fetch("verification_key.json").then( function(res) {
        return res.json();
    });

    const res = await snarkjs.groth16.verify(vkey, publicSignals, proof);

    resultComponent.innerHTML = res;
}

  </script>

</body>
</html>

Further resources

Final note

We hope you enjoyed this quick walk-through. Please address any questions you may have to our telegram group (it’s also a great way to join the community and stay up-to-date with the latest circom and snarkjs developments) 💙

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