Chapter 1: Introduction to Zokrates

Zokrates is a toolbox for zkSNARK, hiding significant complexity inherent to zero-knowledge proofs (ZKP). It provides a python-like higher-level language for developers to code the computational problem they want to prove.

We extend it to generate and verify proofs on Bitcoin.

Install

From binary

curl -Ls https://scrypt.io/scripts/setup-zokrates.sh | sh -s -

From source

You can build our extended version from source with the following commands:

git clone https://github.com/sCrypt-Inc/zokrates
cd ZoKrates
cargo +nightly build -p zokrates_cli --release
cd target/release

Note Zokrates itself was written in Rust Language, you may need to set up the Rust environment first before trying to build it.

Workflow

The whole workflow is the same as the original ZoKrates, except that the verification step is done on Bitcoin. Generally speaking, it contains the following steps:

1. Design a circuit

Implement the circuit in the Zokrates language. For example, this simple circuit/program named factor.zok proves one knows the factorization of an integer n into two integers, without revealing the integers. The circuit has two private inputs named p and q and one public input named n. You can refer to https://zokrates.github.io/ for more information on how to use Zokrates.

// p and q are factorization of n
def main(private field p, private field q, field n) {
    assert(p * q == n);
    assert(p > 1);
    assert(q > 1);
    return;
}

2. Compile the circuit

Compile the circuit with the following command:

zokrates compile -i my_circuit.zok

3. Setup

This generates a proving key and a verification key for this circuit.

zokrates setup

4. Calculating a witness

A proof attests that a prover knows some secret/private information that satisfies the original program. This secret information is called a witness. In the following example, 7 and 13 are the witnesses, as they are factors of 91.

zokrates compute-witness -a 7 13 91

5. Create a proof

It produces a proof, using both the proving key and the witness.

zokrates generate-proof

6. Export an sCrypt verifier

This outputs a smart contract verifier project under verifier/, containing all the necessary code to verify a proof on chain.

zokrates export-verifier-scrypt

7. Verify the proof

You can verify it off-chain locally:

zokrates verify

Put it to the test

Complete the circuit on the right to ensure that the square of the private input x equals y .