Proving

Proving with Pico CLI and SDK APIs

Overview

Pico provides CLI and SDK tools to recursively prove the program to the developers.

Pico CLI provides a complete toolchain for compiling the RISC-V program and using Pico to complete end-to-end proof. Refer to the installation page to install the CLI toolchain. CLI default use the KoalaBear field for the backend proving, if you want to switch to other fields, read more details in Proving Backends Page.

Like the CLI, the Pico-SDK includes lower-level APIs that can prove the program directly. The prover package of the template project repository provides an example of how to import and initialize the SDK and quickly generate a RISC-V proof using the Pico SDK. In the Proving Steps Section, you can read more about VM e2e proving and the Gnark EVM proof generation for On-chain verification

Let's quickly go through the Pico SDK usage and generate a Fibonacci RISC-V proof.

  1. Import pico-sdk

# Cargo.toml
pico-sdk = { git = "https://github.com/brevis-network/pico" }

  1. Execute the proving process and generate RISC-V proof.

// prover/src/main.rs
fn main() {
    // Initialize logger
    init_logger();

    // Load the ELF file
    let elf = load_elf("../elf/riscv32im-pico-zkvm-elf");

    // Initialize the prover client
    let client = DefaultProverClient::new(&elf);
    let stdin_builder = client.get_stdin_builder();

    // Set up input and generate proof
    let n = 100u32;
    stdin_builder.borrow_mut().write(&n);

    // Generate proof
    let proof = client.prove_fast().expect("Failed to generate proof");

    // Decodes public values from the proof's public value stream.
    let public_buffer = proof.pv_stream.unwrap();
    let public_values = PublicValuesStruct::abi_decode(&public_buffer, true).unwrap();

    // Verify the public values
    verify_public_values(n, &public_values);
}

/// Verifies that the computed Fibonacci values match the public values.
fn verify_public_values(n: u32, public_values: &PublicValuesStruct) {
    println!(
        "Public value n: {:?}, a: {:?}, b: {:?}",
        public_values.n, public_values.a, public_values.b
    );

    // Compute Fibonacci values locally
    let (result_a, result_b) = fibonacci(0, 1, n);

    // Assert that the computed values match the public values
    assert_eq!(result_a, public_values.a, "Mismatch in value 'a'");
    assert_eq!(result_b, public_values.b, "Mismatch in value 'b'");
}

Pico EmulatorStdin

Stdin Writer

  1. Pico SDK supports writing the serializable object and bytes to Pico.

/// Write a serializable struct to the buffer.
pub fn write<T: Serialize>(&mut self, data: &T);
/// Write a slice of bytes to the buffer.
pub fn write_slice(&mut self, slice: &[u8]);

Examples:

use std::vec;
use pico_sdk::client::SDKProverClient;
use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize)]
pub struct FibonacciInputs {
    pub a: u32,
    pub b: u32,
    pub n: u32,
}

fn main() {
    // Initialize the prover client
    let client = SDKProverClient::new(&elf, false);
    let stdin_builder = client.get_stdin_builder(); // Shared instance
    // example 1: write a u32 to the VM
    let n = 100u32;
    stdin_builder.borrow_mut().write(&n);
    
    // example 2: write a struct 
    let inputs = FibonacciInputs { a: 0, b: 1, n };
    stdin_builder.borrow_mut().write(&inputs);
    
    // example 3: write a byte array
    let bytes = vec![1, 2, 3, 4];
    stdin_builder.borrow_mut().write_slice(&bytes);
}

  1. CLI input option

The prove command --input option can take a hex string or a file path. the hex string must be match the length of the read type. For example, the input n = 100u32; the hex string should be 0x0A000000 in little-endian format.

RUST_LOG=info cargo pico prove --input "0x0A000000" --fast

Read

Corresponding to the writer functions, there are read_as and read_slice tools for io reading the serializable object or bytes into the program.

SDK examples:


use pico_sdk::io::{read_as, read_vec};

#[derive(Serialize, Deserialize)]
pub struct FibonacciInputs {
    pub a: u32,
    pub b: u32,
    pub n: u32,
}

fn main() {
    // example 1: read the u32 input `n`
    let n: u32 = read_as();
    
    // example 2: read FibonacciInputs struct  
    let inputs = read_as::<FibonacciInputs>();
    
    // example 3: read a byte array
    let bytes: Vec<u8> = read_vec();
}

End-to-end Proving

This section introduces more advanced CLI options and SDK APIs to complete the end-to-end proving process. The Proving process consists of multiple stages, including RISCV, RECURSION, and EVM Phases. Pico SDK includes various ProverClients in different proving backends. Here, we use the KoalaBearProverClient (based on STARK on KoalaBear) in the example code.

RISCV-Phase

Prove RISC-V programs and generate an uncompressed proof with the --fast option. The command is mainly used to test and debug the program.

CLI:

RUST_LOG cargo pico prove --fast

For example, when executing the fast proving with inputs in the Fibonacci, the input n is a u32 data received through pico::sdk::read_as, and it must be in little-endian format and filled to 4 bytes.

RUST_LOG=info cargo pico prove --input "0x0A000000" --fast

SDK:

// Initialize the SDK.
let client = DefaultProverClient::new(&elf);

// get stdion builder and write the inputs by builder.
let stdin_builder = client.get_stdin_builder();
 
// Set up input
let n = 100u32;
stdin_builder.borrow_mut().write(&n);

let riscv_proof = client.prove_fast().expect("Failed to generate proof");

Fast proving is implemented by using only one FRI query which drastically reduces the theoretical security bits. DO NOT USE THIS OPTION IN PRODUCTION. ATTACKERS MAY BE ABLE TO COMMIT TO INVALID TRACES.

RECURSION-Phase

CLI:

RUST_LOG cargo pico prove --field kb # kb: koalabear (default), bb:babebear

Proving without the --fast argument will execute the prover up to and including the EMBED-Phase. The resulting proof can then be verified by the Gnark proof verification circuit, which can then be verified on-chain via contract.

options:

--field

Specify the field, When without this option, default to Koalabear field.

  • kb: Koalabear

  • bb: Babybear

--output

You can specify the output path to generate the files prepared for the Gnark verification and default is in the project root target/pico_out/

RUST_LOG cargo pico prove --output outputs

SDK:

// Initialize the SDK.
let client = DefaultProverClient::new(&elf);
let output_dir = PathBuf::from_str(&"./outputs").expect("the output dir is invalid");
let embed_proof = client.prove(output_dir);

Outputs

  • constraints.json: The schema of the stark proof constraints is used to transform to Gnark circuit constraints.

  • groth16_witness.json: input witnessness of Gnark circuit.

EVM-Phase

The Pico CLI provides an EVM option to generate the program Groth16 proof and verifier Contracts. You must ensure the Docker has been installed when using the evm option.

CLI:

# Setup groth16 PK/VK if its never generated or a new version update.
cargo pico prove --evm --setup 
# generate groth16 proof 
cargo pico prove --evm

SDK:

// Initialize the SDK.
let client = KoalaBearProveVKClient::new(&elf);
let output_dir = PathBuf::from_str(&"./outputs").expect("the output dir is invalid");
//first arg need_setup should be true when you haven't setup groth16 pk/vk yet.
prover_client.prove_evm(true, output_dir);

The outputs:

proof.data: Groth16 proof generated by the Gnark Verifier Circuit.

pv_file: The public values hex string; it's the input of Fibonacci Contract

When executing EVM proving, the Gnark Groth16 ProvingKey/VerificationKey is also generated at this step. The --setup only needs to be executed once to make sure the PK/VK is generated.

EVM Verification

The generated inputs.json format is as follows:

{
  "riscvVKey": "bytes32",
  "proof": "bytes32[]",
  "publicValues": "bytes"
}

After parsing the input data, you can call the PicoVerifier.sol as shown below:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

/// @title Pico Verifier Interface
/// @author Brevis Network
/// @notice This contract is the interface for the Pico Verifier.
interface IPicoVerifier {
    /// @notice Verifies a proof with given public values and riscv verification key.
    /// @param riscvVkey The verification key for the RISC-V program.
    /// @param publicValues The public values encoded as bytes.
    /// @param proof The proof of the riscv program execution in the Pico.
    function verifyPicoProof(
        bytes32 riscvVkey,
        bytes calldata publicValues,
        uint256[8] calldata proof
    ) external view;
}

The verifyPicoProof function in PicoVerifier.sol takes a RISC-V verification key, public values, and a Pico proof, using the Groth16 verifier to validate the proof and public inputs via the pairing algorithm. For the full implementation of the PicoVerifier, please refer to the repository here.

In production, you need to verify riscvVKey and parse the public values verified by PicoVerifier. You can refer to the Fibonacci.sol example in the repository here.

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