plonky2/src/fri/recursive_verifier.rs

use crate::circuit_builder::CircuitBuilder;
use crate::circuit_data::CommonCircuitData;
use crate::context;
use crate::field::extension_field::target::{flatten_target, ExtensionTarget};
use crate::field::extension_field::Extendable;
use crate::field::field::Field;
use crate::fri::FriConfig;
use crate::plonk_challenger::RecursiveChallenger;
use crate::plonk_common::PlonkPolynomials;
use crate::proof::{
    FriInitialTreeProofTarget, FriProofTarget, FriQueryRoundTarget, HashTarget, OpeningSetTarget,
};
use crate::target::Target;
use crate::util::scaling::ReducingFactorTarget;
use crate::util::{log2_strict, reverse_index_bits_in_place};

impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
    /// Computes P'(x^arity) from {P(x*g^i)}_(i=0..arity), where g is a `arity`-th root of unity
    /// and P' is the FRI reduced polynomial.
    fn compute_evaluation(
        &mut self,
        x: Target,
        old_x_index_bits: &[Target],
        arity_bits: usize,
        last_evals: &[ExtensionTarget<D>],
        beta: ExtensionTarget<D>,
    ) -> ExtensionTarget<D> {
        debug_assert_eq!(last_evals.len(), 1 << arity_bits);

        let g = F::primitive_root_of_unity(arity_bits);
        let gt = self.constant(g);

        // The evaluation vector needs to be reordered first.
        let mut evals = last_evals.to_vec();
        reverse_index_bits_in_place(&mut evals);
        // Want `g^(arity - rev_old_x_index)` as in the out-of-circuit version.
        // Compute it as `g^(arity-1-rev_old_x_index) * g`, where the first term is gotten using two's complement.
        let start = self.exp_from_complement_bits(gt, old_x_index_bits.iter().rev());
        let coset_start = self.mul_many(&[start, gt, x]);

        // The answer is gotten by interpolating {(x*g^i, P(x*g^i))} and evaluating at beta.
        let points = g
            .powers()
            .map(|y| {
                let yt = self.constant(y);
                self.mul(coset_start, yt)
            })
            .zip(evals)
            .collect::<Vec<_>>();

        self.interpolate(&points, beta)
    }

    fn fri_verify_proof_of_work(
        &mut self,
        proof: &FriProofTarget<D>,
        challenger: &mut RecursiveChallenger,
        config: &FriConfig,
    ) {
        let mut inputs = challenger.get_hash(self).elements.to_vec();
        inputs.push(proof.pow_witness);

        let hash = self.hash_n_to_m(inputs, 1, false)[0];
        self.assert_leading_zeros(hash, config.proof_of_work_bits + F::ORDER.leading_zeros());
    }

    pub fn verify_fri_proof(
        &mut self,
        purported_degree_log: usize,
        // Openings of the PLONK polynomials.
        os: &OpeningSetTarget<D>,
        // Point at which the PLONK polynomials are opened.
        zeta: ExtensionTarget<D>,
        // Scaling factor to combine polynomials.
        alpha: ExtensionTarget<D>,
        initial_merkle_roots: &[HashTarget],
        proof: &FriProofTarget<D>,
        challenger: &mut RecursiveChallenger,
        common_data: &CommonCircuitData<F, D>,
    ) {
        let config = &common_data.config;
        let total_arities = config.fri_config.reduction_arity_bits.iter().sum::<usize>();
        debug_assert_eq!(
            purported_degree_log,
            log2_strict(proof.final_poly.len()) + total_arities,
            "Final polynomial has wrong degree."
        );

        // Size of the LDE domain.
        let n = proof.final_poly.len() << (total_arities + config.rate_bits);

        let betas = context!(
            self,
            "recover the random betas used in the FRI reductions.",
            proof
                .commit_phase_merkle_roots
                .iter()
                .map(|root| {
                    challenger.observe_hash(root);
                    challenger.get_extension_challenge(self)
                })
                .collect::<Vec<_>>()
        );
        challenger.observe_extension_elements(&proof.final_poly.0);

        context!(
            self,
            "check PoW",
            self.fri_verify_proof_of_work(proof, challenger, &config.fri_config)
        );

        // Check that parameters are coherent.
        debug_assert_eq!(
            config.fri_config.num_query_rounds,
            proof.query_round_proofs.len(),
            "Number of query rounds does not match config."
        );
        debug_assert!(
            !config.fri_config.reduction_arity_bits.is_empty(),
            "Number of reductions should be non-zero."
        );

        let precomputed_reduced_evals =
            PrecomputedReducedEvalsTarget::from_os_and_alpha(os, alpha, self);
        for (i, round_proof) in proof.query_round_proofs.iter().enumerate() {
            context!(
                self,
                &format!("verify {}'th FRI query", i),
                self.fri_verifier_query_round(
                    zeta,
                    alpha,
                    precomputed_reduced_evals,
                    initial_merkle_roots,
                    proof,
                    challenger,
                    n,
                    &betas,
                    round_proof,
                    common_data,
                )
            );
        }
    }

    fn fri_verify_initial_proof(
        &mut self,
        x_index_bits: &[Target],
        proof: &FriInitialTreeProofTarget,
        initial_merkle_roots: &[HashTarget],
    ) {
        for (i, ((evals, merkle_proof), &root)) in proof
            .evals_proofs
            .iter()
            .zip(initial_merkle_roots)
            .enumerate()
        {
            context!(
                self,
                &format!("verify {}'th initial Merkle proof", i),
                self.verify_merkle_proof(evals.clone(), x_index_bits, root, merkle_proof)
            );
        }
    }

    fn fri_combine_initial(
        &mut self,
        proof: &FriInitialTreeProofTarget,
        alpha: ExtensionTarget<D>,
        zeta: ExtensionTarget<D>,
        subgroup_x: Target,
        precomputed_reduced_evals: PrecomputedReducedEvalsTarget<D>,
        common_data: &CommonCircuitData<F, D>,
    ) -> ExtensionTarget<D> {
        assert!(D > 1, "Not implemented for D=1.");
        let config = self.config.clone();
        let degree_log = proof.evals_proofs[0].1.siblings.len() - config.rate_bits;
        let subgroup_x = self.convert_to_ext(subgroup_x);
        let mut alpha = ReducingFactorTarget::new(alpha);
        let mut sum = self.zero_extension();

        // We will add three terms to `sum`:
        // - one for polynomials opened at `x` only
        // - one for polynomials opened at `x` and `g x`

        // Polynomials opened at `x`, i.e., the constants-sigmas, wires, quotient and partial products polynomials.
        let single_evals = [
            PlonkPolynomials::CONSTANTS_SIGMAS,
            PlonkPolynomials::WIRES,
            PlonkPolynomials::QUOTIENT,
        ]
        .iter()
        .flat_map(|&p| proof.unsalted_evals(p, config.zero_knowledge))
        .chain(
            &proof.unsalted_evals(PlonkPolynomials::ZS_PARTIAL_PRODUCTS, config.zero_knowledge)
                [common_data.partial_products_range()],
        )
        .map(|&e| self.convert_to_ext(e))
        .collect::<Vec<_>>();
        let single_composition_eval = alpha.reduce(&single_evals, self);
        let single_numerator =
            self.sub_extension(single_composition_eval, precomputed_reduced_evals.single);
        let single_denominator = self.sub_extension(subgroup_x, zeta);
        let quotient = self.div_unsafe_extension(single_numerator, single_denominator);
        sum = self.add_extension(sum, quotient);
        alpha.reset();

        // Polynomials opened at `x` and `g x`, i.e., the Zs polynomials.
        let zs_evals = proof
            .unsalted_evals(PlonkPolynomials::ZS_PARTIAL_PRODUCTS, config.zero_knowledge)
            .iter()
            .take(common_data.zs_range().end)
            .map(|&e| self.convert_to_ext(e))
            .collect::<Vec<_>>();
        let zs_composition_eval = alpha.reduce(&zs_evals, self);

        let g = self.constant_extension(F::Extension::primitive_root_of_unity(degree_log));
        let zeta_right = self.mul_extension(g, zeta);
        let interpol_val = self.interpolate2(
            [
                (zeta, precomputed_reduced_evals.zs),
                (zeta_right, precomputed_reduced_evals.zs_right),
            ],
            subgroup_x,
        );
        let zs_numerator = self.sub_extension(zs_composition_eval, interpol_val);
        let vanish_zeta = self.sub_extension(subgroup_x, zeta);
        let vanish_zeta_right = self.sub_extension(subgroup_x, zeta_right);
        let zs_denominator = self.mul_extension(vanish_zeta, vanish_zeta_right);
        let zs_quotient = self.div_unsafe_extension(zs_numerator, zs_denominator);
        sum = alpha.shift(sum, self);
        sum = self.add_extension(sum, zs_quotient);

        sum
    }

    fn fri_verifier_query_round(
        &mut self,
        zeta: ExtensionTarget<D>,
        alpha: ExtensionTarget<D>,
        precomputed_reduced_evals: PrecomputedReducedEvalsTarget<D>,
        initial_merkle_roots: &[HashTarget],
        proof: &FriProofTarget<D>,
        challenger: &mut RecursiveChallenger,
        n: usize,
        betas: &[ExtensionTarget<D>],
        round_proof: &FriQueryRoundTarget<D>,
        common_data: &CommonCircuitData<F, D>,
    ) {
        let config = &common_data.config.fri_config;
        let n_log = log2_strict(n);
        // TODO: Do we need to range check `x_index` to a target smaller than `p`?
        let x_index = challenger.get_challenge(self);
        let mut x_index_bits = self.low_bits(x_index, n_log, 64);
        let mut domain_size = n;
        context!(
            self,
            "check FRI initial proof",
            self.fri_verify_initial_proof(
                &x_index_bits,
                &round_proof.initial_trees_proof,
                initial_merkle_roots,
            )
        );
        let mut old_x_index_bits = Vec::new();

        // `subgroup_x` is `subgroup[x_index]`, i.e., the actual field element in the domain.
        let mut subgroup_x = context!(self, "compute x from its index", {
            let g = self.constant(F::MULTIPLICATIVE_GROUP_GENERATOR);
            let phi = self.constant(F::primitive_root_of_unity(n_log));

            let reversed_x = self.base_sum(x_index_bits.iter().rev());
            let phi = self.exp(phi, reversed_x, n_log);
            self.mul(g, phi)
        });

        let mut evaluations: Vec<Vec<ExtensionTarget<D>>> = Vec::new();
        for (i, &arity_bits) in config.reduction_arity_bits.iter().enumerate() {
            let next_domain_size = domain_size >> arity_bits;
            let e_x = if i == 0 {
                context!(
                    self,
                    "combine initial oracles",
                    self.fri_combine_initial(
                        &round_proof.initial_trees_proof,
                        alpha,
                        zeta,
                        subgroup_x,
                        precomputed_reduced_evals,
                        common_data,
                    )
                )
            } else {
                let last_evals = &evaluations[i - 1];
                // Infer P(y) from {P(x)}_{x^arity=y}.
                context!(
                    self,
                    "infer evaluation using interpolation",
                    self.compute_evaluation(
                        subgroup_x,
                        &old_x_index_bits,
                        config.reduction_arity_bits[i - 1],
                        last_evals,
                        betas[i - 1],
                    )
                )
            };
            let mut evals = round_proof.steps[i].evals.clone();
            // Insert P(y) into the evaluation vector, since it wasn't included by the prover.
            let high_x_index_bits = x_index_bits.split_off(arity_bits);
            old_x_index_bits = x_index_bits;
            let low_x_index = self.base_sum(old_x_index_bits.iter());
            evals = self.insert(low_x_index, e_x, evals);
            context!(
                self,
                "verify FRI round Merkle proof.",
                self.verify_merkle_proof(
                    flatten_target(&evals),
                    &high_x_index_bits,
                    proof.commit_phase_merkle_roots[i],
                    &round_proof.steps[i].merkle_proof,
                )
            );
            evaluations.push(evals);

            if i > 0 {
                // Update the point x to x^arity.
                subgroup_x = self.exp_power_of_2(subgroup_x, config.reduction_arity_bits[i - 1]);
            }
            domain_size = next_domain_size;
            x_index_bits = high_x_index_bits;
        }

        let last_evals = evaluations.last().unwrap();
        let final_arity_bits = *config.reduction_arity_bits.last().unwrap();
        let purported_eval = context!(
            self,
            "infer final evaluation using interpolation",
            self.compute_evaluation(
                subgroup_x,
                &old_x_index_bits,
                final_arity_bits,
                last_evals,
                *betas.last().unwrap(),
            )
        );
        subgroup_x = self.exp_power_of_2(subgroup_x, final_arity_bits);

        // Final check of FRI. After all the reductions, we check that the final polynomial is equal
        // to the one sent by the prover.
        let eval = context!(
            self,
            "evaluate final polynomial",
            proof.final_poly.eval_scalar(self, subgroup_x)
        );
        self.assert_equal_extension(eval, purported_eval);
    }
}

#[derive(Copy, Clone)]
struct PrecomputedReducedEvalsTarget<const D: usize> {
    pub single: ExtensionTarget<D>,
    pub zs: ExtensionTarget<D>,
    pub zs_right: ExtensionTarget<D>,
}

impl<const D: usize> PrecomputedReducedEvalsTarget<D> {
    fn from_os_and_alpha<F: Extendable<D>>(
        os: &OpeningSetTarget<D>,
        alpha: ExtensionTarget<D>,
        builder: &mut CircuitBuilder<F, D>,
    ) -> Self {
        let mut alpha = ReducingFactorTarget::new(alpha);
        let single = alpha.reduce(
            &os.constants
                .iter()
                .chain(&os.plonk_sigmas)
                .chain(&os.wires)
                .chain(&os.quotient_polys)
                .chain(&os.partial_products)
                .copied()
                .collect::<Vec<_>>(),
            builder,
        );
        let zs = alpha.reduce(&os.plonk_zs, builder);
        let zs_right = alpha.reduce(&os.plonk_zs_right, builder);

        Self {
            single,
            zs,
            zs_right,
        }
    }
}