plonky2/plonky2/src/fri/recursive_verifier.rs

use itertools::Itertools;
use plonky2_field::extension::Extendable;
use plonky2_util::{log2_strict, reverse_index_bits_in_place};

use crate::fri::proof::{
    FriChallengesTarget, FriInitialTreeProofTarget, FriProofTarget, FriQueryRoundTarget,
    FriQueryStepTarget,
};
use crate::fri::structure::{FriBatchInfoTarget, FriInstanceInfoTarget, FriOpeningsTarget};
use crate::fri::{FriConfig, FriParams};
use crate::gates::gate::Gate;
use crate::gates::high_degree_interpolation::HighDegreeInterpolationGate;
use crate::gates::interpolation::InterpolationGate;
use crate::gates::low_degree_interpolation::LowDegreeInterpolationGate;
use crate::gates::random_access::RandomAccessGate;
use crate::hash::hash_types::{MerkleCapTarget, RichField};
use crate::iop::ext_target::{flatten_target, ExtensionTarget};
use crate::iop::target::{BoolTarget, Target};
use crate::plonk::circuit_builder::CircuitBuilder;
use crate::plonk::config::{AlgebraicHasher, GenericConfig};
use crate::util::reducing::ReducingFactorTarget;
use crate::with_context;

impl<F: RichField + 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<C: GenericConfig<D, F = F>>(
        &mut self,
        x: Target,
        x_index_within_coset_bits: &[BoolTarget],
        arity_bits: usize,
        evals: &[ExtensionTarget<D>],
        beta: ExtensionTarget<D>,
    ) -> ExtensionTarget<D> {
        let arity = 1 << arity_bits;
        debug_assert_eq!(evals.len(), arity);

        let g = F::primitive_root_of_unity(arity_bits);
        let g_inv = g.exp_u64((arity as u64) - 1);

        // The evaluation vector needs to be reordered first.
        let mut evals = evals.to_vec();
        reverse_index_bits_in_place(&mut evals);
        // Want `g^(arity - rev_x_index_within_coset)` as in the out-of-circuit version. Compute it
        // as `(g^-1)^rev_x_index_within_coset`.
        let start = self.exp_from_bits_const_base(g_inv, x_index_within_coset_bits.iter().rev());
        let coset_start = self.mul(start, x);

        // The answer is gotten by interpolating {(x*g^i, P(x*g^i))} and evaluating at beta.
        // `HighDegreeInterpolationGate` has degree `arity`, so we use the low-degree gate if
        // the arity is too large.
        if arity > self.config.max_quotient_degree_factor {
            self.interpolate_coset::<LowDegreeInterpolationGate<F, D>>(
                arity_bits,
                coset_start,
                &evals,
                beta,
            )
        } else {
            self.interpolate_coset::<HighDegreeInterpolationGate<F, D>>(
                arity_bits,
                coset_start,
                &evals,
                beta,
            )
        }
    }

    /// Make sure we have enough wires and routed wires to do the FRI checks efficiently. This check
    /// isn't required -- without it we'd get errors elsewhere in the stack -- but just gives more
    /// helpful errors.
    fn check_recursion_config<C: GenericConfig<D, F = F>>(&self, max_fri_arity_bits: usize) {
        let random_access = RandomAccessGate::<F, D>::new_from_config(
            &self.config,
            max_fri_arity_bits.max(self.config.fri_config.cap_height),
        );
        let (interpolation_wires, interpolation_routed_wires) =
            if 1 << max_fri_arity_bits > self.config.max_quotient_degree_factor {
                let gate = LowDegreeInterpolationGate::<F, D>::new(max_fri_arity_bits);
                (gate.num_wires(), gate.num_routed_wires())
            } else {
                let gate = HighDegreeInterpolationGate::<F, D>::new(max_fri_arity_bits);
                (gate.num_wires(), gate.num_routed_wires())
            };

        let min_wires = random_access.num_wires().max(interpolation_wires);
        let min_routed_wires = random_access
            .num_routed_wires()
            .max(interpolation_routed_wires);

        assert!(
            self.config.num_wires >= min_wires,
            "To efficiently perform FRI checks with an arity of 2^{}, at least {} wires are needed. Consider reducing arity.",
            max_fri_arity_bits,
            min_wires
        );

        assert!(
            self.config.num_routed_wires >= min_routed_wires,
            "To efficiently perform FRI checks with an arity of 2^{}, at least {} routed wires are needed. Consider reducing arity.",
            max_fri_arity_bits,
            min_routed_wires
        );
    }

    fn fri_verify_proof_of_work<H: AlgebraicHasher<F>>(
        &mut self,
        fri_pow_response: Target,
        config: &FriConfig,
    ) {
        self.assert_leading_zeros(
            fri_pow_response,
            config.proof_of_work_bits + (64 - F::order().bits()) as u32,
        );
    }

    pub fn verify_fri_proof<C: GenericConfig<D, F = F>>(
        &mut self,
        instance: &FriInstanceInfoTarget<D>,
        openings: &FriOpeningsTarget<D>,
        challenges: &FriChallengesTarget<D>,
        initial_merkle_caps: &[MerkleCapTarget],
        proof: &FriProofTarget<D>,
        params: &FriParams,
    ) where
        C::Hasher: AlgebraicHasher<F>,
    {
        if let Some(max_arity_bits) = params.max_arity_bits() {
            self.check_recursion_config::<C>(max_arity_bits);
        }

        debug_assert_eq!(
            params.final_poly_len(),
            proof.final_poly.len(),
            "Final polynomial has wrong degree."
        );

        // Size of the LDE domain.
        let n = params.lde_size();

        with_context!(
            self,
            "check PoW",
            self.fri_verify_proof_of_work::<C::Hasher>(challenges.fri_pow_response, &params.config)
        );

        // Check that parameters are coherent.
        debug_assert_eq!(
            params.config.num_query_rounds,
            proof.query_round_proofs.len(),
            "Number of query rounds does not match config."
        );

        let precomputed_reduced_evals = with_context!(
            self,
            "precompute reduced evaluations",
            PrecomputedReducedOpeningsTarget::from_os_and_alpha(
                openings,
                challenges.fri_alpha,
                self
            )
        );

        for (i, round_proof) in proof.query_round_proofs.iter().enumerate() {
            // To minimize noise in our logs, we will only record a context for a single FRI query.
            // The very first query will have some extra gates due to constants being registered, so
            // the second query is a better representative.
            let level = if i == 1 {
                log::Level::Debug
            } else {
                log::Level::Trace
            };

            let num_queries = proof.query_round_proofs.len();
            with_context!(
                self,
                level,
                &format!("verify one (of {num_queries}) query rounds"),
                self.fri_verifier_query_round::<C>(
                    instance,
                    challenges,
                    &precomputed_reduced_evals,
                    initial_merkle_caps,
                    proof,
                    challenges.fri_query_indices[i],
                    n,
                    round_proof,
                    params,
                )
            );
        }
    }

    fn fri_verify_initial_proof<H: AlgebraicHasher<F>>(
        &mut self,
        x_index_bits: &[BoolTarget],
        proof: &FriInitialTreeProofTarget,
        initial_merkle_caps: &[MerkleCapTarget],
        cap_index: Target,
    ) {
        for (i, ((evals, merkle_proof), cap)) in proof
            .evals_proofs
            .iter()
            .zip(initial_merkle_caps)
            .enumerate()
        {
            with_context!(
                self,
                &format!("verify {i}'th initial Merkle proof"),
                self.verify_merkle_proof_to_cap_with_cap_index::<H>(
                    evals.clone(),
                    x_index_bits,
                    cap_index,
                    cap,
                    merkle_proof
                )
            );
        }
    }

    fn fri_combine_initial<C: GenericConfig<D, F = F>>(
        &mut self,
        instance: &FriInstanceInfoTarget<D>,
        proof: &FriInitialTreeProofTarget,
        alpha: ExtensionTarget<D>,
        subgroup_x: Target,
        precomputed_reduced_evals: &PrecomputedReducedOpeningsTarget<D>,
        params: &FriParams,
    ) -> ExtensionTarget<D> {
        assert!(D > 1, "Not implemented for D=1.");
        let degree_log = params.degree_bits;
        debug_assert_eq!(
            degree_log,
            params.config.cap_height + proof.evals_proofs[0].1.siblings.len()
                - params.config.rate_bits
        );
        let subgroup_x = self.convert_to_ext(subgroup_x);
        let mut alpha = ReducingFactorTarget::new(alpha);
        let mut sum = self.zero_extension();

        for (batch, reduced_openings) in instance
            .batches
            .iter()
            .zip(&precomputed_reduced_evals.reduced_openings_at_point)
        {
            let FriBatchInfoTarget { point, polynomials } = batch;
            let evals = polynomials
                .iter()
                .map(|p| {
                    let poly_blinding = instance.oracles[p.oracle_index].blinding;
                    let salted = params.hiding && poly_blinding;
                    proof.unsalted_eval(p.oracle_index, p.polynomial_index, salted)
                })
                .collect_vec();
            let reduced_evals = alpha.reduce_base(&evals, self);
            let numerator = self.sub_extension(reduced_evals, *reduced_openings);
            let denominator = self.sub_extension(subgroup_x, *point);
            sum = alpha.shift(sum, self);
            sum = self.div_add_extension(numerator, denominator, sum);
        }

        // Multiply the final polynomial by `X`, so that `final_poly` has the maximum degree for
        // which the LDT will pass. See github.com/mir-protocol/plonky2/pull/436 for details.
        self.mul_extension(sum, subgroup_x)
    }

    fn fri_verifier_query_round<C: GenericConfig<D, F = F>>(
        &mut self,
        instance: &FriInstanceInfoTarget<D>,
        challenges: &FriChallengesTarget<D>,
        precomputed_reduced_evals: &PrecomputedReducedOpeningsTarget<D>,
        initial_merkle_caps: &[MerkleCapTarget],
        proof: &FriProofTarget<D>,
        x_index: Target,
        n: usize,
        round_proof: &FriQueryRoundTarget<D>,
        params: &FriParams,
    ) where
        C::Hasher: AlgebraicHasher<F>,
    {
        let n_log = log2_strict(n);

        // Note that this `low_bits` decomposition permits non-canonical binary encodings. Here we
        // verify that this has a negligible impact on soundness error.
        Self::assert_noncanonical_indices_ok(&params.config);
        let mut x_index_bits = self.low_bits(x_index, n_log, F::BITS);

        let cap_index =
            self.le_sum(x_index_bits[x_index_bits.len() - params.config.cap_height..].iter());
        with_context!(
            self,
            "check FRI initial proof",
            self.fri_verify_initial_proof::<C::Hasher>(
                &x_index_bits,
                &round_proof.initial_trees_proof,
                initial_merkle_caps,
                cap_index
            )
        );

        // `subgroup_x` is `subgroup[x_index]`, i.e., the actual field element in the domain.
        let mut subgroup_x = with_context!(self, "compute x from its index", {
            let g = self.constant(F::coset_shift());
            let phi = F::primitive_root_of_unity(n_log);
            let phi = self.exp_from_bits_const_base(phi, x_index_bits.iter().rev());
            // subgroup_x = g * phi
            self.mul(g, phi)
        });

        // old_eval is the last derived evaluation; it will be checked for consistency with its
        // committed "parent" value in the next iteration.
        let mut old_eval = with_context!(
            self,
            "combine initial oracles",
            self.fri_combine_initial::<C>(
                instance,
                &round_proof.initial_trees_proof,
                challenges.fri_alpha,
                subgroup_x,
                precomputed_reduced_evals,
                params,
            )
        );

        for (i, &arity_bits) in params.reduction_arity_bits.iter().enumerate() {
            let evals = &round_proof.steps[i].evals;

            // Split x_index into the index of the coset x is in, and the index of x within that coset.
            let coset_index_bits = x_index_bits[arity_bits..].to_vec();
            let x_index_within_coset_bits = &x_index_bits[..arity_bits];
            let x_index_within_coset = self.le_sum(x_index_within_coset_bits.iter());

            // Check consistency with our old evaluation from the previous round.
            let new_eval = self.random_access_extension(x_index_within_coset, evals.clone());
            self.connect_extension(new_eval, old_eval);

            // Infer P(y) from {P(x)}_{x^arity=y}.
            old_eval = with_context!(
                self,
                "infer evaluation using interpolation",
                self.compute_evaluation::<C>(
                    subgroup_x,
                    x_index_within_coset_bits,
                    arity_bits,
                    evals,
                    challenges.fri_betas[i],
                )
            );

            with_context!(
                self,
                "verify FRI round Merkle proof.",
                self.verify_merkle_proof_to_cap_with_cap_index::<C::Hasher>(
                    flatten_target(evals),
                    &coset_index_bits,
                    cap_index,
                    &proof.commit_phase_merkle_caps[i],
                    &round_proof.steps[i].merkle_proof,
                )
            );

            // Update the point x to x^arity.
            subgroup_x = self.exp_power_of_2(subgroup_x, arity_bits);

            x_index_bits = coset_index_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 = with_context!(
            self,
            &format!(
                "evaluate final polynomial of length {}",
                proof.final_poly.len()
            ),
            proof.final_poly.eval_scalar(self, subgroup_x)
        );
        self.connect_extension(eval, old_eval);
    }

    /// We decompose FRI query indices into bits without verifying that the decomposition given by
    /// the prover is the canonical one. In particular, if `x_index < 2^field_bits - p`, then the
    /// prover could supply the binary encoding of either `x_index` or `x_index + p`, since the are
    /// congruent mod `p`. However, this only occurs with probability
    ///     p_ambiguous = (2^field_bits - p) / p
    /// which is small for the field that we use in practice.
    ///
    /// In particular, the soundness error of one FRI query is roughly the codeword rate, which
    /// is much larger than this ambiguous-element probability given any reasonable parameters.
    /// Thus ambiguous elements contribute a negligible amount to soundness error.
    ///
    /// Here we compare the probabilities as a sanity check, to verify the claim above.
    fn assert_noncanonical_indices_ok(config: &FriConfig) {
        let num_ambiguous_elems = u64::MAX - F::ORDER + 1;
        let query_error = config.rate();
        let p_ambiguous = (num_ambiguous_elems as f64) / (F::ORDER as f64);
        assert!(p_ambiguous < query_error * 1e-5,
                "A non-negligible portion of field elements are in the range that permits non-canonical encodings. Need to do more analysis or enforce canonical encodings.");
    }

    pub fn add_virtual_fri_proof(
        &mut self,
        num_leaves_per_oracle: &[usize],
        params: &FriParams,
    ) -> FriProofTarget<D> {
        let cap_height = params.config.cap_height;
        let num_queries = params.config.num_query_rounds;
        let commit_phase_merkle_caps = (0..params.reduction_arity_bits.len())
            .map(|_| self.add_virtual_cap(cap_height))
            .collect();
        let query_round_proofs = (0..num_queries)
            .map(|_| self.add_virtual_fri_query(num_leaves_per_oracle, params))
            .collect();
        let final_poly = self.add_virtual_poly_coeff_ext(params.final_poly_len());
        let pow_witness = self.add_virtual_target();
        FriProofTarget {
            commit_phase_merkle_caps,
            query_round_proofs,
            final_poly,
            pow_witness,
        }
    }

    fn add_virtual_fri_query(
        &mut self,
        num_leaves_per_oracle: &[usize],
        params: &FriParams,
    ) -> FriQueryRoundTarget<D> {
        let cap_height = params.config.cap_height;
        assert!(params.lde_bits() >= cap_height);
        let mut merkle_proof_len = params.lde_bits() - cap_height;

        let initial_trees_proof =
            self.add_virtual_fri_initial_trees_proof(num_leaves_per_oracle, merkle_proof_len);

        let mut steps = vec![];
        for &arity_bits in &params.reduction_arity_bits {
            assert!(merkle_proof_len >= arity_bits);
            merkle_proof_len -= arity_bits;
            steps.push(self.add_virtual_fri_query_step(arity_bits, merkle_proof_len));
        }

        FriQueryRoundTarget {
            initial_trees_proof,
            steps,
        }
    }

    fn add_virtual_fri_initial_trees_proof(
        &mut self,
        num_leaves_per_oracle: &[usize],
        initial_merkle_proof_len: usize,
    ) -> FriInitialTreeProofTarget {
        let evals_proofs = num_leaves_per_oracle
            .iter()
            .map(|&num_oracle_leaves| {
                let leaves = self.add_virtual_targets(num_oracle_leaves);
                let merkle_proof = self.add_virtual_merkle_proof(initial_merkle_proof_len);
                (leaves, merkle_proof)
            })
            .collect();
        FriInitialTreeProofTarget { evals_proofs }
    }

    fn add_virtual_fri_query_step(
        &mut self,
        arity_bits: usize,
        merkle_proof_len: usize,
    ) -> FriQueryStepTarget<D> {
        FriQueryStepTarget {
            evals: self.add_virtual_extension_targets(1 << arity_bits),
            merkle_proof: self.add_virtual_merkle_proof(merkle_proof_len),
        }
    }
}

/// For each opening point, holds the reduced (by `alpha`) evaluations of each polynomial that's
/// opened at that point.
#[derive(Clone)]
struct PrecomputedReducedOpeningsTarget<const D: usize> {
    reduced_openings_at_point: Vec<ExtensionTarget<D>>,
}

impl<const D: usize> PrecomputedReducedOpeningsTarget<D> {
    fn from_os_and_alpha<F: RichField + Extendable<D>>(
        openings: &FriOpeningsTarget<D>,
        alpha: ExtensionTarget<D>,
        builder: &mut CircuitBuilder<F, D>,
    ) -> Self {
        let reduced_openings_at_point = openings
            .batches
            .iter()
            .map(|batch| ReducingFactorTarget::new(alpha).reduce(&batch.values, builder))
            .collect();
        Self {
            reduced_openings_at_point,
        }
    }
}