Circom C Witness Generator

Circom C Witness Generator { I know a pretty long name }, allows you to compile your witness generators for Circom circuits for both x86_64 and arm64 machines!

Hence, it allows you to use C compiled witness generators directly in your M1,M2 macs { yes no need for x86_64 emulated docker containers }

Usage

Using it is simple:

1. Clone the Github Repo

git clone git@github.com:bajpai244/circom_c_witness_generator.git
cd circom_c_witness_generator

2. compile your Circom file with --c flag

Now, you need to compile your Circom file to c via using --c flag during Circom compilation.

We have an example multiplication circuit in the examples directory! It already has a c build made for it the Circom compiler.

3. Edit your circuit.cpp file

After compiling your Circom circuit for c, inside your circuit_cpp directory, there will be a circuit.cpp file.

We need to wrap the whole code inside the following namespace. { this step might be automated in future, feel free to push a PR }

#include <stdio.h>
#include <iostream>
#include <assert.h>
#include "circom.hpp"
#include "calcwit.hpp"
namespace CIRCUIT_NAME{
...
}

Do not subsitute CIRCUIT_NAME with your circuit's name

You can take a look examples/multiply_cpp/multiply.cpp for an example on how to do this.

4. Run ./witness_generator.sh shell script

This shell script's usage requires 3 inputs:

• architecture of the machine { arm64 or x86_64 }
• Circom C compiled witness generator cpp
• Circom C compiled witness generator .dat file

Check out ./example dir, which shows its usage.

./witness_generator.sh arm64 ./example/multiply_cpp/multiply.cpp ./example/multiply_cpp/multiply.dat

The above command will creates a wtns_build folder in the $PWD, which contains the final build of the witness generator.

cd wtns_build
# generate witness
./circuit ../example/input.json ./wtns.wtns

You can call witness_generator.sh from any directory, and it will do its job! You can setup an alias to it use like a normal shell command.

Shoutout to Polygon ID

Shoutout to the amazing Polygon ID team, the project wouldn’t have been possible if they had not cracked the missing arm64 assembly and the associated cpp files!

ps: They also helped me by pointing me to the repo for Polygon ID circuits, where this is being used in production!

Link to the Polygon ID Repo: https://github.com/0xPolygonID/witnesscalc