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Ahmed Castro
Ahmed Castro

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Escalando Ethereum con Halo2 [ZK ES Semana 5]

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Halo2 combina tecnologías de esquemas de commitment (commitment schemes), como FRI y KZG, con las últimas versiones de Plonk. Esto proporciona escalabilidad y expresividad, gracias a las nuevas funcionalidades que incluyen Custom Gates y Rotación. En la actualidad, Halo2 es utilizado por la PSE (exploraciones de escalabilidad y privacidad de Ethereum) y Scroll Layer 2, donde lo empleamos para crear pruebas ZK de transacciones de EVM a nivel de bytecode.

En esta sesión vamos a realizar dos versiones de un circuito para probar la computación del fibonacci. Una versión sin optimizar y otra versión donde con ayuda de la rotación de la Rotación logramos reducir el tamaño del circuito.

Antes de iniciar

Para ejecutar este proyecto necesitarás instalar Rust y CMake. Si estás usando linux lo puedes hacer con los comandos siguientes.

apt install cmake build-essential
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Preparativos del proyecto de rust

Podemos usar Halo2 desde un proyecto tradicional de Rust. Para iniciar creamos el archivo Cargo.toml donde indicamos el punto de entrada y las dependencias.

Cargo.toml

[package]
name = "fibonacci"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[lib]
name = "halo2_examples"
path = "src/lib.rs"

[features]
dev-graph = ["halo2_proofs/dev-graph", "plotters"]

[dependencies]
halo2_proofs = { git = "https://github.com/zcash/halo2.git", rev = "a898d65ae3ad3d41987666f6a03cfc15edae01c4"}
plotters = { version = "0.3.0", optional = true }
tabbycat = { version = "0.1", features = ["attributes"], optional = true }
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En este proyecto mostraremos crearemos dos versiones de fibonacci, una optimizada y otra sin optimizar. Para eso creamos un archivo intermediario que nos permitirá ejecutar ambas versiones.

src/lib.rs

mod fibonacci_unoptimized;
mod fibonacci_optimized;
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Fibonacci versión no optimizada

Ahora a continuación colocamos la versión sin optimizar del circuito de Fibonacci sin optimizar.

src/fibonacci_unoptimized.rs

use std::marker::PhantomData;
use halo2_proofs::{arithmetic::FieldExt, circuit::*, plonk::*, poly::Rotation};

#[derive(Debug, Clone)]
struct FibonacciConfig {
    pub col_a: Column<Advice>,
    pub col_b: Column<Advice>,
    pub col_c: Column<Advice>,
    pub selector: Selector,
    pub instance: Column<Instance>,
}

#[derive(Debug, Clone)]
struct FibonacciChip<F: FieldExt> {
    config: FibonacciConfig,
    _marker: PhantomData<F>,
}

impl<F: FieldExt> FibonacciChip<F> {
    pub fn construct(config: FibonacciConfig) -> Self {
        Self {
            config,
            _marker: PhantomData,
        }
    }

    pub fn configure(meta: &mut ConstraintSystem<F>) -> FibonacciConfig {
        let col_a = meta.advice_column();
        let col_b = meta.advice_column();
        let col_c = meta.advice_column();
        let selector = meta.selector();
        let instance = meta.instance_column();

        meta.enable_equality(col_a);
        meta.enable_equality(col_b);
        meta.enable_equality(col_c);
        meta.enable_equality(instance);

        meta.create_gate("add", |meta| {
            //
            // col_a | col_b | col_c | selector
            //   a      b        c       s
            //
            let s = meta.query_selector(selector);
            let a = meta.query_advice(col_a, Rotation::cur());
            let b = meta.query_advice(col_b, Rotation::cur());
            let c = meta.query_advice(col_c, Rotation::cur());
            vec![s * (a + b - c)]
        });

        FibonacciConfig {
            col_a,
            col_b,
            col_c,
            selector,
            instance,
        }
    }

    #[allow(clippy::type_complexity)]
    pub fn assign_first_row(
        &self,
        mut layouter: impl Layouter<F>,
    ) -> Result<(AssignedCell<F, F>, AssignedCell<F, F>, AssignedCell<F, F>), Error> {
        layouter.assign_region(
            || "first row",
            |mut region| {
                self.config.selector.enable(&mut region, 0)?;

                let a_cell = region.assign_advice_from_instance(
                    || "f(0)",
                    self.config.instance,
                    0,
                    self.config.col_a,
                    0)?;

                let b_cell = region.assign_advice_from_instance(
                    || "f(1)",
                    self.config.instance,
                    1,
                    self.config.col_b,
                    0)?;

                let c_cell = region.assign_advice(
                    || "a + b",
                    self.config.col_c,
                    0,
                    || a_cell.value().copied() + b_cell.value(),
                )?;

                Ok((a_cell, b_cell, c_cell))
            },
        )
    }

    pub fn assign_row(
        &self,
        mut layouter: impl Layouter<F>,
        prev_b: &AssignedCell<F, F>,
        prev_c: &AssignedCell<F, F>,
    ) -> Result<AssignedCell<F, F>, Error> {
        layouter.assign_region(
            || "next row",
            |mut region| {
                self.config.selector.enable(&mut region, 0)?;

                // Copy the value from b & c in previous row to a & b in current row
                prev_b.copy_advice(
                    || "a",
                    &mut region,
                    self.config.col_a,
                    0,
                )?;
                prev_c.copy_advice(
                    || "b",
                    &mut region,
                    self.config.col_b,
                    0,
                )?;

                let c_cell = region.assign_advice(
                    || "c",
                    self.config.col_c,
                    0,
                    || prev_b.value().copied() + prev_c.value(),
                )?;

                Ok(c_cell)
            },
        )
    }

    pub fn expose_public(
        &self,
        mut layouter: impl Layouter<F>,
        cell: &AssignedCell<F, F>,
        row: usize,
    ) -> Result<(), Error> {
        layouter.constrain_instance(cell.cell(), self.config.instance, row)
    }
}

#[derive(Default)]
struct MyCircuit<F>(PhantomData<F>);

impl<F: FieldExt> Circuit<F> for MyCircuit<F> {
    type Config = FibonacciConfig;
    type FloorPlanner = SimpleFloorPlanner;

    fn without_witnesses(&self) -> Self {
        Self::default()
    }

    fn configure(meta: &mut ConstraintSystem<F>) -> Self::Config {
        FibonacciChip::configure(meta)
    }

    fn synthesize(
        &self,
        config: Self::Config,
        mut layouter: impl Layouter<F>,
    ) -> Result<(), Error> {
        let chip = FibonacciChip::construct(config);

        let (_, mut prev_b, mut prev_c) =
            chip.assign_first_row(layouter.namespace(|| "first row"))?;

        for _i in 3..10 {
            let c_cell = chip.assign_row(layouter.namespace(|| "next row"), &prev_b, &prev_c)?;
            prev_b = prev_c;
            prev_c = c_cell;
        }

        chip.expose_public(layouter.namespace(|| "out"), &prev_c, 2)?;

        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use std::marker::PhantomData;

    use super::MyCircuit;
    use halo2_proofs::{dev::MockProver, pasta::Fp};

    #[test]
    fn fibonacci_example1() {
        let k = 4;

        let a = Fp::from(1); // F[0]
        let b = Fp::from(1); // F[1]
        let out = Fp::from(55); // F[9]

        let circuit = MyCircuit(PhantomData);

        let mut public_input = vec![a, b, out];

        let prover = MockProver::run(k, &circuit, vec![public_input.clone()]).unwrap();
        prover.assert_satisfied();

        public_input[2] += Fp::one();
        let _prover = MockProver::run(k, &circuit, vec![public_input]).unwrap();
        // uncomment the following line and the assert will fail
        // _prover.assert_satisfied();
    }

    #[cfg(feature = "dev-graph")]
    #[test]
    fn plot_fibonacci_unoptimized() {
        use plotters::prelude::*;

        let root = BitMapBackend::new("fibonacci_unoptimized.png", (1024, 3096)).into_drawing_area();
        root.fill(&WHITE).unwrap();
        let root = root.titled("Fibonacci Unoptimized Layout", ("sans-serif", 60)).unwrap();

        let circuit = MyCircuit::<Fp>(PhantomData);
        halo2_proofs::dev::CircuitLayout::default()
            .render(4, &circuit, &root)
            .unwrap();
    }
}
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Versión optimizada del fibonacci

Seguidamente colocamos la versión optimizada donde nos servimos de la rotación para comprimir el circuito.

src/fibonacci_optimized.rs

use halo2_proofs::{arithmetic::FieldExt, circuit::*, plonk::*, poly::Rotation};
use std::marker::PhantomData;

#[derive(Debug, Clone)]
struct ACell<F: FieldExt>(AssignedCell<F, F>);

#[derive(Debug, Clone)]
struct FibonacciConfig {
    advice: Column<Advice>,
    selector: Selector,
    instance: Column<Instance>,
}

#[derive(Debug, Clone)]
struct FibonacciChip<F: FieldExt> {
    config: FibonacciConfig,
    _marker: PhantomData<F>,
}

impl<F: FieldExt> FibonacciChip<F> {
    pub fn construct(config: FibonacciConfig) -> Self {
        Self {
            config,
            _marker: PhantomData,
        }
    }

    pub fn configure(
        meta: &mut ConstraintSystem<F>,
        advice: Column<Advice>,
        instance: Column<Instance>,
    ) -> FibonacciConfig {
        let selector = meta.selector();

        meta.enable_equality(advice);
        meta.enable_equality(instance);

        meta.create_gate("add", |meta| {
            //
            // advice | selector
            //   a    |   s
            //   b    |
            //   c    |
            //
            let s = meta.query_selector(selector);
            let a = meta.query_advice(advice, Rotation::cur());
            let b = meta.query_advice(advice, Rotation::next());
            let c = meta.query_advice(advice, Rotation(2));
            vec![s * (a + b - c)]
        });

        FibonacciConfig {
            advice,
            selector,
            instance,
        }
    }

    pub fn assign(
        &self,
        mut layouter: impl Layouter<F>,
        nrows: usize,
    ) -> Result<AssignedCell<F, F>, Error> {
        layouter.assign_region(
            || "entire fibonacci table",
            |mut region| {
                self.config.selector.enable(&mut region, 0)?;
                self.config.selector.enable(&mut region, 1)?;

                let mut a_cell = region.assign_advice_from_instance(
                    || "1",
                    self.config.instance,
                    0,
                    self.config.advice,
                    0,
                )?;
                let mut b_cell = region.assign_advice_from_instance(
                    || "1",
                    self.config.instance,
                    1,
                    self.config.advice,
                    1,
                )?;

                for row in 2..nrows {
                    if row < nrows - 2 {
                        self.config.selector.enable(&mut region, row)?;
                    }

                    let c_cell = region.assign_advice(
                        || "advice",
                        self.config.advice,
                        row,
                        || a_cell.value().copied() + b_cell.value(),
                    )?;

                    a_cell = b_cell;
                    b_cell = c_cell;
                }

                Ok(b_cell)
            },
        )
    }

    pub fn expose_public(
        &self,
        mut layouter: impl Layouter<F>,
        cell: AssignedCell<F, F>,
        row: usize,
    ) -> Result<(), Error> {
        layouter.constrain_instance(cell.cell(), self.config.instance, row)
    }
}

#[derive(Default)]
struct MyCircuit<F>(PhantomData<F>);

impl<F: FieldExt> Circuit<F> for MyCircuit<F> {
    type Config = FibonacciConfig;
    type FloorPlanner = SimpleFloorPlanner;

    fn without_witnesses(&self) -> Self {
        Self::default()
    }

    fn configure(meta: &mut ConstraintSystem<F>) -> Self::Config {
        let advice = meta.advice_column();
        let instance = meta.instance_column();
        FibonacciChip::configure(meta, advice, instance)
    }

    fn synthesize(
        &self,
        config: Self::Config,
        mut layouter: impl Layouter<F>,
    ) -> Result<(), Error> {
        let chip = FibonacciChip::construct(config);

        let out_cell = chip.assign(layouter.namespace(|| "entire table"), 10)?;

        chip.expose_public(layouter.namespace(|| "out"), out_cell, 2)?;

        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::MyCircuit;
    use std::marker::PhantomData;
    use halo2_proofs::{dev::MockProver, pasta::Fp};

    #[test]
    fn fibonacci_example2() {
        let k = 4;

        let a = Fp::from(1); // F[0]
        let b = Fp::from(1); // F[1]
        let out = Fp::from(55); // F[9]

        let circuit = MyCircuit(PhantomData);

        let mut public_input = vec![a, b, out];

        let prover = MockProver::run(k, &circuit, vec![public_input.clone()]).unwrap();
        prover.assert_satisfied();

        public_input[2] += Fp::one();
        let _prover = MockProver::run(k, &circuit, vec![public_input]).unwrap();
        // uncomment the following line and the assert will fail
        // _prover.assert_satisfied();
    }

    #[cfg(feature = "dev-graph")]
    #[test]
    fn plot_fibonacci_optimized() {
        use plotters::prelude::*;
        let root = BitMapBackend::new("fibonacci_optimized.png", (1024, 3096)).into_drawing_area();
        root.fill(&WHITE).unwrap();
        let root = root.titled("Fibonacci Optimized Layout", ("sans-serif", 60)).unwrap();

        let circuit = MyCircuit::<Fp>(PhantomData);
        halo2_proofs::dev::CircuitLayout::default()
            .render(4, &circuit, &root)
            .unwrap();
    }
}
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Compilar y generar el gráfico

Ahora estamos listos para compilar los circuitos y probarlo. Halo2 incluye librerías para visualizar gráficas de circuitos donde podremos comparar la compresión. Para hacerlo ejecuta los siguientes comandos.

cargo build
cargo test --all-features -- --nocapture plot_fibonacci_unoptimized
cargo test --all-features -- --nocapture plot_fibonacci_optimized
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Ahora podrás ver la diferencia en fibonacci_optimized.png y fibonacci_unoptimized.png. Como los que muestro a continuación.

Como puedes observar en el gráfico, la versión no optimizada del circuito de fibonacci usamos 4 columnas.

Image description

Luego de optimizar, usamos únicamente 2 columnas.

Image description

Links recomendados sobre Halo2

Video tutoriales sobre fibonacci en Plonk:

Pizarras sobre las bases de Halo2 y Plonk:

Tutoriales en blogposts:

¿Qué se viene en ZK?

¡Felicidades y gracias por completar esta serie de Workshops!

Asegúrate de seguir a @Layer2es y a Filosofía Código para mantenerte al tanto de los avances en Layer 2 y desarrollo en blockchain.

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