Snark

In order to use circom, it is recommended to use a linux system (I tried windows and some libraries do not work in windows).

Installation

1. Install linux system. It is fine to stay in root. Otherwise create a linux account and make sure to grant the user sudo access.
2. Install Nodejs
   • sudo apt install nodejs npm -y
   • check installation by using
     • npm --version
     • nodejs --version
3. Install rust with the following command. (Current location /home/username)
   • curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
   • source $HOME/.cargo/env
   • those two commands allow you to install rust and use 'cargo' terminal command
   • check installation by using
     • cargo --version
4. Create a new directory where you can put the circom files. cd to that directory (Current location /home/username/Snark)
5. Install circom with following commands
   • git clone https://github.com/iden3/circom.git
   • cd circom
   • cargo build --release
   • echo 'export PATH="$HOME/circom/target/release:$PATH"' >> ~/.bashrc
   • source ~/.bashrc
   • check installation by using
     • circom --version
6. Install Snarkjs (Current location /home/username)
   • npm install -g snarkjs
   • check installation by using
     • snarkjs --version
7. Install some libraries for C++ (Current location /home/username)
   • sudo apt install -y nlohmann-json3-dev libgmp-dev nasm
8. Some additional notes
   • if Snarkjs prints out error that says "Error: Scalar size does not match", you need to go change a file in snarkjs.
   • First, use command to open main.cjs
     • cd $(npm root -g)/snarkjs/node_modules/ffjavascript/build
     • nano main.cjs
   • Second, open the main.cjs by using 'nano main.cjs' (The file is super long, using vscode iss highly recommanded refer to this link: Vscode in wsl)
   • Next, search for the line (if you are using terminal nano, press control and w two keys at same time to search)
     • const nPoints = Math.floor(buffBases.byteLength / sGIn);
     • const sScalar = Math.floor(buffScalars.byteLength / nPoints);
   • Finally, replace them with (basically adding the bolded line in middle to check divide by 0 error)
     • const nPoints = Math.floor(buffBases.byteLength / sGIn);
     • if (nPoints == 0) return G.zero; // add new here
     • const sScalar = Math.floor(buffScalars.byteLength / nPoints);
   • save all your changes.
   • refer to this link if you have any problems

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Run an example

It is recommended to run an example file to make sure everything is install properly.

1. Download the example folder in this github repo, and use the multiand example.
2. cd example
3. cd MultiAnd
4. circom multiand.circom --r1cs --wasm --sym
   • This step will generate a folder called multiand_js. Open the folder, and drag all three files inside the folder out, so it will be in the same directory as multiand.circom.
5. Create a file called 'input.json' in the same directory as multiand.circom, put the following inside it: {"in": [1, 1, 0, 1]}
6. snarkjs powersoftau new bn128 12 pot12_0000.ptau -v
7. snarkjs powersoftau contribute pot12_0000.ptau pot12_0001.ptau --name="First contribution" -v
8. snarkjs powersoftau prepare phase2 pot12_0001.ptau pot12_final.ptau -v
9. snarkjs groth16 setup multiand.r1cs pot12_final.ptau multiand_0000.zkey
10. snarkjs zkey contribute multiand_0000.zkey multiand_0001.zkey --name="1st Contributor Name" -v
11. snarkjs zkey export verificationkey multiand_0001.zkey verification_key.json
12. node generate_witness.js multiand.wasm input.json witness.wtns
13. snarkjs groth16 prove ./multiand_0001.zkey witness.wtns proof.json public.json
14. snarkjs groth16 verify ./verification_key.json public.json proof.json
15. After all those steps, the terminal should print: snarkjs: OK. Which indicates that it is valid.
16. In order to make sure it will reject the invalid input, change the code in public.json from ["0"] to ["1"], and use command snarkjs groth16 verify ./verification_key.json public.json proof.json again, it will show: SnarkJS: Invalide Proof