halo2/src/circuit/gadget/sinsemilla/chip.rs

use super::{
    message::{Message, MessagePiece},
    HashDomains, SinsemillaInstructions,
};
use crate::{
    circuit::gadget::{
        ecc::chip::EccPoint,
        utilities::{CellValue, Var},
    },
    primitives::sinsemilla::{
        self, Q_COMMIT_IVK_M_GENERATOR, Q_MERKLE_CRH, Q_NOTE_COMMITMENT_M_GENERATOR,
    },
};

use ff::PrimeField;
use halo2::{
    arithmetic::{CurveAffine, FieldExt},
    circuit::{Chip, Layouter},
    plonk::{
        Advice, Column, ConstraintSystem, Error, Expression, Fixed, Permutation, Selector,
        VirtualCells,
    },
    poly::Rotation,
};
use pasta_curves::pallas;

use std::convert::TryInto;

mod generator_table;
pub use generator_table::get_s_by_idx;
use generator_table::GeneratorTableConfig;

mod hash_to_point;

/// Configuration for the Sinsemilla hash chip
#[derive(Eq, PartialEq, Clone, Debug)]
pub struct SinsemillaConfig {
    // Selector used in the lookup argument as well as Sinsemilla custom gates.
    q_sinsemilla1: Selector,
    // Fixed column used in Sinsemilla custom gates.
    q_sinsemilla2: Column<Fixed>,
    // Fixed column used to constrain hash initialization to be consistent with
    // the y-coordinate of the domain Q.
    fixed_y_q: Column<Fixed>,
    // Advice column used to store the x-coordinate of the accumulator at each
    // iteration of the hash.
    x_a: Column<Advice>,
    // Advice column used to store the x-coordinate of the generator corresponding
    // to the message word at each iteration of the hash. This is looked up in the
    // generator table.
    x_p: Column<Advice>,
    // Advice column used to load the message.
    bits: Column<Advice>,
    // Advice column used to store the lambda_1 intermediate value at each
    // iteration.
    lambda_1: Column<Advice>,
    // Advice column used to store the lambda_2 intermediate value at each
    // iteration.
    lambda_2: Column<Advice>,
    // The lookup table where (idx, x_p, y_p) are loaded for the 2^K generators
    // of the Sinsemilla hash.
    generator_table: GeneratorTableConfig,
    // Fixed column shared by the whole circuit. This is used to load the
    // x-coordinate of the domain Q, which is then constrained to equal the
    // initial x_a.
    constants: Column<Fixed>,
    // Permutation over all advice columns and the `constants` fixed column.
    perm: Permutation,
}

#[derive(Eq, PartialEq, Clone, Debug)]
pub struct SinsemillaChip {
    config: SinsemillaConfig,
}

impl Chip<pallas::Base> for SinsemillaChip {
    type Config = SinsemillaConfig;
    type Loaded = ();

    fn config(&self) -> &Self::Config {
        &self.config
    }

    fn loaded(&self) -> &Self::Loaded {
        &()
    }
}

impl SinsemillaChip {
    pub fn construct(config: <Self as Chip<pallas::Base>>::Config) -> Self {
        Self { config }
    }

    pub fn load(
        config: SinsemillaConfig,
        layouter: &mut impl Layouter<pallas::Base>,
    ) -> Result<<Self as Chip<pallas::Base>>::Loaded, Error> {
        // Load the lookup table.
        config.generator_table.load(layouter)
    }

    #[allow(clippy::too_many_arguments)]
    #[allow(non_snake_case)]
    pub fn configure(
        meta: &mut ConstraintSystem<pallas::Base>,
        advices: [Column<Advice>; 5],
        lookup: (Column<Fixed>, Column<Fixed>, Column<Fixed>),
        constants: Column<Fixed>,
        perm: Permutation,
    ) -> <Self as Chip<pallas::Base>>::Config {
        let config = SinsemillaConfig {
            q_sinsemilla1: meta.selector(),
            q_sinsemilla2: meta.fixed_column(),
            fixed_y_q: meta.fixed_column(),
            x_a: advices[0],
            x_p: advices[1],
            bits: advices[2],
            lambda_1: advices[3],
            lambda_2: advices[4],
            generator_table: GeneratorTableConfig {
                table_idx: lookup.0,
                table_x: lookup.1,
                table_y: lookup.2,
            },
            constants,
            perm,
        };

        // Set up lookup argument
        GeneratorTableConfig::configure(meta, config.clone());

        // Constant expressions
        let two = Expression::Constant(pallas::Base::from_u64(2));

        // Closures for expressions that are derived multiple times
        // x_r = lambda_1^2 - x_a - x_p
        let x_r = |meta: &mut VirtualCells<pallas::Base>, rotation| {
            let x_a = meta.query_advice(config.x_a, rotation);
            let x_p = meta.query_advice(config.x_p, rotation);
            let lambda_1 = meta.query_advice(config.lambda_1, rotation);
            lambda_1.square() - x_a - x_p
        };

        // Y_A = (lambda_1 + lambda_2) * (x_a - x_r)
        let Y_A = |meta: &mut VirtualCells<pallas::Base>, rotation| {
            let x_a = meta.query_advice(config.x_a, rotation);
            let lambda_1 = meta.query_advice(config.lambda_1, rotation);
            let lambda_2 = meta.query_advice(config.lambda_2, rotation);
            (lambda_1 + lambda_2) * (x_a - x_r(meta, rotation))
        };

        // Check that the initial x_A, x_P, lambda_1, lambda_2 are consistent with y_Q.
        meta.create_gate("Initial y_Q", |meta| {
            let fixed_y_q = meta.query_fixed(config.fixed_y_q, Rotation::cur());

            // Y_A = (lambda_1 + lambda_2) * (x_a - x_r)
            let Y_A = Y_A(meta, Rotation::cur());

            // fixed_y_q * (2 * fixed_y_q - Y_{A,0}) = 0
            vec![fixed_y_q.clone() * (two.clone() * fixed_y_q - Y_A)]
        });

        meta.create_gate("Sinsemilla gate", |meta| {
            let q_s1 = meta.query_selector(config.q_sinsemilla1);
            let q_s2 = meta.query_fixed(config.q_sinsemilla2, Rotation::cur());
            let q_s3 = {
                let one = Expression::Constant(pallas::Base::one());
                q_s2.clone() * (q_s2 - one)
            };

            let lambda_1_next = meta.query_advice(config.lambda_1, Rotation::next());
            let lambda_2_cur = meta.query_advice(config.lambda_2, Rotation::cur());
            let x_a_cur = meta.query_advice(config.x_a, Rotation::cur());
            let x_a_next = meta.query_advice(config.x_a, Rotation::next());

            // x_r = lambda_1^2 - x_a_cur - x_p
            let x_r = x_r(meta, Rotation::cur());

            // Y_A = (lambda_1 + lambda_2) * (x_a - x_r)
            let Y_A_cur = Y_A(meta, Rotation::cur());

            // Y_A = (lambda_1 + lambda_2) * (x_a - x_r)
            let Y_A_next = Y_A(meta, Rotation::next());

            // lambda2^2 - (x_a_next + x_r + x_a_cur) = 0
            let secant_line =
                lambda_2_cur.clone().square() - (x_a_next.clone() + x_r + x_a_cur.clone());

            // lhs - rhs = 0, where
            //    - lhs = 4 * lambda_2_cur * (x_a_cur - x_a_next)
            //    - rhs = (2 * Y_A_cur + (2 - q_s3) * Y_A_next + 2 * q_s3 * y_a_final)
            let expr = {
                // lhs = 4 * lambda_2_cur * (x_a_cur - x_a_next)
                let lhs = lambda_2_cur * pallas::Base::from_u64(4) * (x_a_cur - x_a_next);

                // rhs = 2 * Y_A_cur + (2 - q_s3) * Y_A_next + 2 * q_s3 * y_a_final
                let rhs = {
                    // y_a_final is assigned to the lambda1 column on the next offset.
                    let y_a_final = lambda_1_next;

                    two.clone() * Y_A_cur
                        + (two.clone() - q_s3.clone()) * Y_A_next
                        + two * q_s3 * y_a_final
                };
                lhs - rhs
            };

            vec![
                ("Secant line", q_s1.clone() * secant_line),
                ("Sinsemilla gate", q_s1 * expr),
            ]
        });

        config
    }
}

// Implement `SinsemillaInstructions` for `SinsemillaChip`
impl SinsemillaInstructions<pallas::Affine, { sinsemilla::K }, { sinsemilla::C }>
    for SinsemillaChip
{
    type CellValue = CellValue<pallas::Base>;

    type Message = Message<pallas::Base, { sinsemilla::K }, { sinsemilla::C }>;
    type MessagePiece = MessagePiece<pallas::Base, { sinsemilla::K }>;

    type X = CellValue<pallas::Base>;
    type Point = EccPoint;

    type HashDomains = SinsemillaHashDomains;

    #[allow(non_snake_case)]
    fn witness_message(
        &self,
        mut layouter: impl Layouter<pallas::Base>,
        message: Vec<Option<bool>>,
    ) -> Result<Self::Message, Error> {
        // Message must be composed of `K`-bit words.
        assert_eq!(message.len() % sinsemilla::K, 0);

        // Message must have at most `sinsemilla::C` words.
        assert!(message.len() / sinsemilla::K <= sinsemilla::C);

        // Message piece must be at most `ceil(pallas::Base::NUM_BITS / sinsemilla::K)` bits
        let piece_num_words = pallas::Base::NUM_BITS as usize / sinsemilla::K;
        let pieces: Result<Vec<_>, _> = message
            .chunks(piece_num_words * sinsemilla::K)
            .enumerate()
            .map(|(i, piece)| -> Result<Self::MessagePiece, Error> {
                self.witness_message_piece_bitstring(
                    layouter.namespace(|| format!("message piece {}", i)),
                    piece,
                )
            })
            .collect();

        pieces.map(|pieces| pieces.into())
    }

    #[allow(non_snake_case)]
    fn witness_message_piece_bitstring(
        &self,
        layouter: impl Layouter<pallas::Base>,
        message_piece: &[Option<bool>],
    ) -> Result<Self::MessagePiece, Error> {
        // Message must be composed of `K`-bit words.
        assert_eq!(message_piece.len() % sinsemilla::K, 0);
        let num_words = message_piece.len() / sinsemilla::K;

        // Message piece must be at most `ceil(C::Base::NUM_BITS / sinsemilla::K)` bits
        let piece_max_num_words = pallas::Base::NUM_BITS as usize / sinsemilla::K;
        assert!(num_words <= piece_max_num_words as usize);

        // Closure to parse a bitstring (little-endian) into a base field element.
        let to_base_field = |bits: &[Option<bool>]| -> Option<pallas::Base> {
            assert!(bits.len() <= pallas::Base::NUM_BITS as usize);

            let bits: Option<Vec<bool>> = bits.iter().cloned().collect();
            let bytes: Option<Vec<u8>> = bits.map(|bits| {
                // Pad bits to 256 bits
                let pad_len = 256 - bits.len();
                let mut bits = bits;
                bits.extend_from_slice(&vec![false; pad_len]);

                bits.chunks_exact(8)
                    .map(|byte| byte.iter().rev().fold(0u8, |acc, bit| acc * 2 + *bit as u8))
                    .collect()
            });
            bytes.map(|bytes| pallas::Base::from_bytes(&bytes.try_into().unwrap()).unwrap())
        };

        let piece_value = to_base_field(message_piece);
        self.witness_message_piece_field(layouter, piece_value, num_words)
    }

    fn witness_message_piece_field(
        &self,
        mut layouter: impl Layouter<pallas::Base>,
        field_elem: Option<pallas::Base>,
        num_words: usize,
    ) -> Result<Self::MessagePiece, Error> {
        let config = self.config().clone();

        let cell = layouter.assign_region(
            || "witness message piece",
            |mut region| {
                region.assign_advice(
                    || "witness message piece",
                    config.bits,
                    0,
                    || field_elem.ok_or(Error::SynthesisError),
                )
            },
        )?;
        Ok(MessagePiece::new(cell, field_elem, num_words))
    }

    #[allow(non_snake_case)]
    #[allow(clippy::type_complexity)]
    fn hash_to_point(
        &self,
        mut layouter: impl Layouter<pallas::Base>,
        Q: pallas::Affine,
        message: Self::Message,
    ) -> Result<(Self::Point, Vec<Vec<Self::CellValue>>), Error> {
        layouter.assign_region(
            || "hash_to_point",
            |mut region| self.hash_message(&mut region, Q, &message),
        )
    }

    fn extract(point: &Self::Point) -> Self::X {
        point.x()
    }
}

#[derive(Clone, Debug)]
pub enum SinsemillaHashDomains {
    NoteCommit,
    CommitIvk,
    MerkleCrh,
}

#[allow(non_snake_case)]
impl HashDomains<pallas::Affine> for SinsemillaHashDomains {
    fn Q(&self) -> pallas::Affine {
        match self {
            SinsemillaHashDomains::CommitIvk => pallas::Affine::from_xy(
                pallas::Base::from_bytes(&Q_COMMIT_IVK_M_GENERATOR.0).unwrap(),
                pallas::Base::from_bytes(&Q_COMMIT_IVK_M_GENERATOR.1).unwrap(),
            )
            .unwrap(),
            SinsemillaHashDomains::NoteCommit => pallas::Affine::from_xy(
                pallas::Base::from_bytes(&Q_NOTE_COMMITMENT_M_GENERATOR.0).unwrap(),
                pallas::Base::from_bytes(&Q_NOTE_COMMITMENT_M_GENERATOR.1).unwrap(),
            )
            .unwrap(),
            SinsemillaHashDomains::MerkleCrh => pallas::Affine::from_xy(
                pallas::Base::from_bytes(&Q_MERKLE_CRH.0).unwrap(),
                pallas::Base::from_bytes(&Q_MERKLE_CRH.1).unwrap(),
            )
            .unwrap(),
        }
    }
}