Zokrates_Demo

A demo using Zokrates to build an encrypted transaction on Ethereum/Qtum based on zkSNARKs algorithm.

This is a proof-of-concept implementation. It has not been tested for production.

Zokrates Introduction

ZoKrates is a toolbox for zkSNARKs on Ethereum. It helps you use verifiable computation in your DApp, from the specification of your program in a high level language to generating proofs of computation to verifying those proofs in Solidity.

Using Zokrates, we can implement the zkSNARKs in the platform which supports Solidity.

The goal of zero-knowledge proofs is for a verifier to be able to convince herself that a prover possesses knowledge of a secret parameter, called a witness, satisfying some relation, without revealing the witness to the verifier or anyone else.

In order to let platform like Ethereum/Qutm support zkSNARKs, we use Zokrates to generate the proof.

Zokrates Installation

Zokrates official manual can be found in Here.

One-line install

We provide a one-line install for Linux, MacOS and FreeBSD:

curl -LSfs get.zokrat.es | sh

Docker

ZoKrates is available on Dockerhub.

docker run -ti zokrates/zokrates /bin/bash

From there on, you can use the zokrates CLI.

From source

You can build the container yourself from source with the following commands:

git clone https://github.com/ZoKrates/ZoKrates
cd ZoKrates
cargo +nightly build --release
cd target/release

Implementation

Design

In this chapter we aim to give an overview of zkSNARKs from a practical viewpoint. We will treat the actual math as a black box but will try to develop some intuitions around how we can use them.

In this Demo, we want to encrypt the balance to protect user's privacy. Usually, user's information like balance was storing in clear text, which will expose user's privacy. In this demo, we want to replace it with hashvalue.

Here is a simple example of how zkSNARKs can help with privacy on Ethereum/Qtum.

Suppose we have a simple token contract. Normally a token contract would have at its core a mapping from addresses to balances:

mapping (address => uint256) balances;

For example, Alice has 100 coins and Bob has 50 coins, Alice wants to send 10 coins to Bobs using a transaction. If all the information are stored by clear text, we can verify the balance easily. But if both the balance and the transfer value are stored using hash value, we can hardly verify the correctness because no one can knows the value behind the hash. So we must use zero-knowledge proof to verify the correctness.

We are going to retain the same basic core, except replace a balance with the hash of a balance:

mapping (address => bytes32) balanceHashes;

We are not going to hide the sender or receiver of transactions, but we’ll be able to hide the balances and sent amounts.

Remember: Two zk-SNARKs will be used to send tokens from one account to another, one proof created by the sender and one by the receiver.

Thus what our zkSNARKs would need to prove is that this holds as well as that the updated hashes matches the updated balances.

Program

The main idea is that the sender will use their starting balance and the transaction value as private inputs, and hashes of starting balance, ending balance and value as public inputs. Similarly the receiver will use starting balance and value as secret inputs and hashes of starting balance, ending balance and value as public inputs.

Below is the program we will use for the sender zkSNARKs, where private field represents private input and field represent public input:

import "hashes/sha256/512bitPacked.code" as sha256packed

def main(private field value, private field before, field valueHash, field beforeHash, field afterHash) -> (field):
	priBefore = sha256packed([0, 0, 0, before])
	priAfter = sha256packed([0, 0, 0, before-value])
    field result = if(\
    	value > before &&\
    	priBefore[0] == beforeHash &&\
    	priAfter[0] == afterHash \
    ) then 1 else 0 fi
    return result

The program used by the receiver is below:

import "hashes/sha256/512bitPacked.code" as sha256packed

def main(private field value, private field before, field valueHash, field beforeHash, field afterHash) -> (field):
	priBefore = sha256packed([0, 0, 0, before])
	priAfter = sha256packed([0, 0, 0, before+value])
    field result = if(\
    	priBefore[1] == beforeHash &&\
    	priAfter[1] == afterHash \
    ) then 1 else 0 fi
    return result

The programs check that the sending balance is larger than the value being sent, as well as checking that all hashes match. The most important difference between sender and receiver is that sender need to check the balance is larger than the value but receiver is no need.

Compile and Generate Proof

The sender and receiver are compiled in the same way. Take sender as an example:

Make sure you have finished installation Zokrates, and enter the environment.

Create a file sender.code to store the code.

mkdir sender
cd sender
vim sender.code #Paste the code for sender in this file

Then, compile the code and run the setup:

# compile
zokrates compile -i sender.code
# perform the setup phase
zokrates setup

After that, we can get a circuit used to execute a proof, in this program, we have 5 inputs, so we must input 5 variables.

For example, sender has 1000 coins, he wants to transfer to receiver 50 coins, after that sender still has 950 coins, so in this example, we must input 50, 1000, sha256(50), sha256(1000), sha256(950) :

# execute the program
zokrates compute-witness -a 50 1000 242738482787324818092317501628658271637 853498718274837825789312739748392789743 438758372489912996993285694393204086976

Then, we can get a witness file, we use this file to generate a proof:

# generate a proof of computation
zokrates generate-proof

Finally, we can get a json file which contains the proof, verified person can use this to proof:

{
        "proof": {
            "a": ["0x25eaee22d216321bfdb2106e4d82cb57cb68ff5b2bc2392f3af46136c32864a9", "0x1ae67dfbcc13ff83a3101107a22c01003026d497ec0e43412cfdcd3f71a9bff2"],
            "b": [["0x1e7c39359a8f77f3ef922d14d99421df80c1e482b84e440f0ffd7c04ba774ecb", "0x13905c2bc7e57fb43005f1418c8ed2fef5901199a2ddef3a10c83d408d241f08"], ["0x2d54fb29e5dca94e115f5e5638e1d0b76a82d3b2eaaaa65558947c9a4154bda6", "0x1b1822bf4fc6de9eadde6bea6cceb55584a20fa1601be543b2b93d85463ea2c8"]],
            "c": ["0x05f3c493eacb68b4349e5321fe45af6eca70dc3a7abd31b29c1902a69c3aa0ef", "0x09fe5c2631a47feea9c30d8f60348dcc86e5b3e18c980275528cdefc2454c8cf"]
        },
        "inputs": ["0x00000000000000000000000000000000b69dbb3437a3e859225943db8ef8c595", "0x000000000000000000000000000000028219dfb80d5c593e8837d46577ece6ef", "0x000000000000000000000000000000014a15c9ee54bfb820a51232819ea418c0", "0x0000000000000000000000000000000000000000000000000000000000000001"]
    }

Solidity Contract Generate

Using Zokrates, we can generate a verifier, which can be deployed in the Ethereum/Qtum.

zokrates export-verifier

After that, users can find a file named verifier.sol in the folder. Then we use the Remix to test the verifier contract.

Paste the code into the Remix

Compile it

Find Verifier Class and Delopy

Use VerifyTx to Execute Proof

Input the Parameters in Json and Verify

If the program execute successfully and return true, means the user is honest and has pass the proof, if user is dishonest, the program won't pass.

Publish on Qtum and Invoke it

In this Chapter, you must finish install the Qtum docker develop environment. You can find the installation manaul HERE.

In previous, we have finished test the contract, now, we want to deploy it in the Qtum and invoke it. Compile the contract and we can get the EVM bytecode:

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Use qcli createcontract command to create the contract, remember to replace the bytecode and address with your's bytecode and address:

qcli createcontract [bytecode] 500000 0.0000004 [address] true

After create contract, you can find a message return, don't forget to record the address and check out whether it deploy successful or not:

Use qcli getaccountinfo to check whether contract deploy or not:

qcli getaccountinfo [contract_address]

In order to execute the functions in the contract, you must use the JSON interface file to create the ABI data. There is a simple tool called ethabi that can help you achieve this. Make sure the JSON file is saved, assuming the name is interface.json.

First we need to generate parameters for verifyTx to invoke, we must use ethabi to generate params. For example, transfer the data array to string:

ethabi encode params -v uint[] [25eaee22d216321bfdb2106e4d82cb57cb68ff5b2bc2392f3af46136c32864a9,25eaee22d216321bfdb2106e4d82cb57cb68ff5b2bc2392f3af46136c32864a9]

Then use encode function to generate function abi invoke information:

ethabi encode function ./verifier.json verifyTx -p -l [params]

Use this function, we can get the return number like 3450bd6a:

In order to invoke the contract, we use callcontract to get the return number:

qcli callcontract [contract_address] 3450bd6a

The result of this command after running contains a lot of useful fields, but currently we only need the field output, its value represents the check result.

About Qtum

Qtum is compatible with the Bitcoin and Ethereum ecosystems and aims at producing a variation of Bitcoin with Ethereum Virtual Machine (EVM) compatibility.

Note that differently to Ethereum, the Qtum EVM is constantly backwards compatible. Pursuing a pragmatic design approach, Qtum employs industry use cases with a strategy comprising mobile devices.

The latter allows Qtum promoting blockchain technology to a wide array of Internet users and thereby, decentralizing PoS transaction validation.