Improve the design of circuit, config, and instances for CycleFold

folding-schemes/src/arith/ccs.rs

@@ -36,8 +36,7 @@ pub struct CCS<F: PrimeField> {
 }
 
 impl<F: PrimeField> Arith<F> for CCS<F> {
-    /// check that a CCS structure is satisfied by a z vector. Only for testing.
-    fn check_relation(&self, z: &[F]) -> Result<(), Error> {
+    fn eval_relation(&self, z: &[F]) -> Result<Vec<F>, Error> {
         let mut result = vec![F::zero(); self.m];
 
         for i in 0..self.q {
@@ -57,14 +56,7 @@ impl<F: PrimeField> Arith<F> for CCS<F> {
             result = vec_add(&result, &c_M_j_z)?;
         }
 
-        // make sure the final vector is all zeroes
-        for e in result {
-            if !e.is_zero() {
-                return Err(Error::NotSatisfied);
-            }
-        }
-
-        Ok(())
+        Ok(result)
     }
 
     fn params_to_le_bytes(&self) -> Vec<u8> {
@@ -113,7 +105,10 @@ impl<F: PrimeField> CCS<F> {
 #[cfg(test)]
 pub mod tests {
     use super::*;
-    use crate::arith::r1cs::tests::{get_test_r1cs, get_test_z as r1cs_get_test_z};
+    use crate::{
+        arith::r1cs::tests::{get_test_r1cs, get_test_z as r1cs_get_test_z},
+        utils::vec::is_zero_vec,
+    };
     use ark_pallas::Fr;
 
     pub fn get_test_ccs<F: PrimeField>() -> CCS<F> {
@@ -124,9 +119,22 @@ pub mod tests {
         r1cs_get_test_z(input)
     }
 
+    #[test]
+    fn test_eval_ccs_relation() {
+        let ccs = get_test_ccs::<Fr>();
+        let mut z = get_test_z(3);
+
+        let f_w = ccs.eval_relation(&z).unwrap();
+        assert!(is_zero_vec(&f_w));
+
+        z[1] = Fr::from(111);
+        let f_w = ccs.eval_relation(&z).unwrap();
+        assert!(!is_zero_vec(&f_w));
+    }
+
     /// Test that a basic CCS relation can be satisfied
     #[test]
-    fn test_ccs_relation() {
+    fn test_check_ccs_relation() {
         let ccs = get_test_ccs::<Fr>();
         let z = get_test_z(3);
 

folding-schemes/src/arith/mod.rs

@@ -6,8 +6,21 @@ pub mod ccs;
 pub mod r1cs;
 
 pub trait Arith<F: PrimeField> {
-    /// Checks that the given Arith structure is satisfied by a z vector. Used only for testing.
-    fn check_relation(&self, z: &[F]) -> Result<(), Error>;
+    /// Evaluate the given Arith structure at `z`, a vector of assignments, and
+    /// return the evaluation.
+    fn eval_relation(&self, z: &[F]) -> Result<Vec<F>, Error>;
+
+    /// Checks that the given Arith structure is satisfied by a z vector, i.e.,
+    /// if the evaluation is a zero vector
+    ///
+    /// Used only for testing.
+    fn check_relation(&self, z: &[F]) -> Result<(), Error> {
+        if self.eval_relation(z)?.iter().all(|f| f.is_zero()) {
+            Ok(())
+        } else {
+            Err(Error::NotSatisfied)
+        }
+    }
 
     /// Returns the bytes that represent the parameters, that is, the matrices sizes, the amount of
     /// public inputs, etc, without the matrices/polynomials values.

folding-schemes/src/arith/r1cs.rs

@@ -1,15 +1,13 @@
 use crate::commitment::CommitmentScheme;
-use crate::folding::nova::{CommittedInstance, Witness};
 use crate::RngCore;
-use ark_crypto_primitives::sponge::Absorb;
-use ark_ec::{CurveGroup, Group};
+use ark_ec::CurveGroup;
 use ark_ff::PrimeField;
 use ark_relations::r1cs::ConstraintSystem;
 use ark_serialize::{CanonicalDeserialize, CanonicalSerialize};
 use ark_std::rand::Rng;
 
 use super::Arith;
-use crate::utils::vec::{hadamard, mat_vec_mul, vec_add, vec_scalar_mul, vec_sub, SparseMatrix};
+use crate::utils::vec::{hadamard, mat_vec_mul, vec_scalar_mul, vec_sub, SparseMatrix};
 use crate::Error;
 
 #[derive(Debug, Clone, Eq, PartialEq, CanonicalSerialize, CanonicalDeserialize)]
@@ -21,16 +19,24 @@ pub struct R1CS<F: PrimeField> {
 }
 
 impl<F: PrimeField> Arith<F> for R1CS<F> {
-    /// check that a R1CS structure is satisfied by a z vector. Only for testing.
-    fn check_relation(&self, z: &[F]) -> Result<(), Error> {
+    fn eval_relation(&self, z: &[F]) -> Result<Vec<F>, Error> {
+        if z.len() != self.A.n_cols {
+            return Err(Error::NotSameLength(
+                "z.len()".to_string(),
+                z.len(),
+                "number of variables in R1CS".to_string(),
+                self.A.n_cols,
+            ));
+        }
+
         let Az = mat_vec_mul(&self.A, z)?;
         let Bz = mat_vec_mul(&self.B, z)?;
         let Cz = mat_vec_mul(&self.C, z)?;
+        // Multiply Cz by z[0] (u) here, allowing this method to be reused for
+        // both relaxed and unrelaxed R1CS.
+        let uCz = vec_scalar_mul(&Cz, &z[0]);
         let AzBz = hadamard(&Az, &Bz)?;
-        if AzBz != Cz {
-            return Err(Error::NotSatisfied);
-        }
-        Ok(())
+        vec_sub(&AzBz, &uCz)
     }
 
     fn params_to_le_bytes(&self) -> Vec<u8> {
@@ -57,55 +63,50 @@ impl<F: PrimeField> R1CS<F> {
     pub fn split_z(&self, z: &[F]) -> (Vec<F>, Vec<F>) {
         (z[self.l + 1..].to_vec(), z[1..self.l + 1].to_vec())
     }
-
-    /// converts the R1CS instance into a RelaxedR1CS as described in
-    /// [Nova](https://eprint.iacr.org/2021/370.pdf) section 4.1.
-    pub fn relax(self) -> RelaxedR1CS<F> {
-        RelaxedR1CS::<F> {
-            l: self.l,
-            E: vec![F::zero(); self.A.n_rows],
-            A: self.A,
-            B: self.B,
-            C: self.C,
-            u: F::one(),
-        }
-    }
 }
 
-#[derive(Debug, Clone, Eq, PartialEq)]
-pub struct RelaxedR1CS<F: PrimeField> {
-    pub l: usize, // io len
-    pub A: SparseMatrix<F>,
-    pub B: SparseMatrix<F>,
-    pub C: SparseMatrix<F>,
-    pub u: F,
-    pub E: Vec<F>,
-}
+pub trait RelaxedR1CS<C: CurveGroup, W, U>: Arith<C::ScalarField> {
+    /// returns a dummy running instance (Witness and CommittedInstance) for the current R1CS structure
+    fn dummy_running_instance(&self) -> (W, U);
 
-impl<F: PrimeField> RelaxedR1CS<F> {
-    /// check that a RelaxedR1CS structure is satisfied by a z vector. Only for testing.
-    pub fn check_relation(&self, z: &[F]) -> Result<(), Error> {
-        let Az = mat_vec_mul(&self.A, z)?;
-        let Bz = mat_vec_mul(&self.B, z)?;
-        let Cz = mat_vec_mul(&self.C, z)?;
-        let uCz = vec_scalar_mul(&Cz, &self.u);
-        let uCzE = vec_add(&uCz, &self.E)?;
-        let AzBz = hadamard(&Az, &Bz)?;
-        if AzBz != uCzE {
-            return Err(Error::NotSatisfied);
+    /// returns a dummy incoming instance (Witness and CommittedInstance) for the current R1CS structure
+    fn dummy_incoming_instance(&self) -> (W, U);
+
+    /// checks if the given instance is relaxed
+    fn is_relaxed(w: &W, u: &U) -> bool;
+
+    /// extracts `z`, the vector of variables, from the given Witness and CommittedInstance
+    fn extract_z(w: &W, u: &U) -> Vec<C::ScalarField>;
+
+    /// checks if the computed error terms correspond to the actual one in `w`
+    /// or `u`
+    fn check_error_terms(w: &W, u: &U, e: Vec<C::ScalarField>) -> Result<(), Error>;
+
+    /// checks the tight (unrelaxed) R1CS relation
+    fn check_tight_relation(&self, w: &W, u: &U) -> Result<(), Error> {
+        if Self::is_relaxed(w, u) {
+            return Err(Error::R1CSUnrelaxedFail);
         }
 
-        Ok(())
+        let z = Self::extract_z(w, u);
+        self.check_relation(&z)
+    }
+
+    /// checks the relaxed R1CS relation
+    fn check_relaxed_relation(&self, w: &W, u: &U) -> Result<(), Error> {
+        let z = Self::extract_z(w, u);
+        let e = self.eval_relation(&z)?;
+        Self::check_error_terms(w, u, e)
     }
 
     // Computes the E term, given A, B, C, z, u
     fn compute_E(
-        A: &SparseMatrix<F>,
-        B: &SparseMatrix<F>,
-        C: &SparseMatrix<F>,
-        z: &[F],
-        u: &F,
-    ) -> Result<Vec<F>, Error> {
+        A: &SparseMatrix<C::ScalarField>,
+        B: &SparseMatrix<C::ScalarField>,
+        C: &SparseMatrix<C::ScalarField>,
+        z: &[C::ScalarField],
+        u: &C::ScalarField,
+    ) -> Result<Vec<C::ScalarField>, Error> {
         let Az = mat_vec_mul(A, z)?;
         let Bz = mat_vec_mul(B, z)?;
         let AzBz = hadamard(&Az, &Bz)?;
@@ -115,66 +116,9 @@ impl<F: PrimeField> RelaxedR1CS<F> {
         vec_sub(&AzBz, &uCz)
     }
 
-    pub fn check_sampled_relaxed_r1cs(&self, u: F, E: &[F], z: &[F]) -> bool {
-        let sampled = RelaxedR1CS {
-            l: self.l,
-            A: self.A.clone(),
-            B: self.B.clone(),
-            C: self.C.clone(),
-            u,
-            E: E.to_vec(),
-        };
-        sampled.check_relation(z).is_ok()
-    }
-
-    // Implements sampling a (committed) RelaxedR1CS
-    // See construction 5 in https://eprint.iacr.org/2023/573.pdf
-    pub fn sample<C, CS>(
-        &self,
-        params: &CS::ProverParams,
-        mut rng: impl RngCore,
-    ) -> Result<(CommittedInstance<C>, Witness<C>), Error>
+    fn sample<CS>(&self, params: &CS::ProverParams, rng: impl RngCore) -> Result<(W, U), Error>
     where
-        C: CurveGroup,
-        C: CurveGroup<ScalarField = F>,
-        <C as Group>::ScalarField: Absorb,
-        CS: CommitmentScheme<C, true>,
-    {
-        let u = C::ScalarField::rand(&mut rng);
-        let rE = C::ScalarField::rand(&mut rng);
-        let rW = C::ScalarField::rand(&mut rng);
-
-        let W = (0..self.A.n_cols - self.l - 1)
-            .map(|_| F::rand(&mut rng))
-            .collect();
-        let x = (0..self.l).map(|_| F::rand(&mut rng)).collect::<Vec<F>>();
-        let mut z = vec![u];
-        z.extend(&x);
-        z.extend(&W);
-
-        let E = RelaxedR1CS::compute_E(&self.A, &self.B, &self.C, &z, &u)?;
-
-        debug_assert!(
-            z.len() == self.A.n_cols,
-            "Length of z is {}, while A has {} columns.",
-            z.len(),
-            self.A.n_cols
-        );
-
-        debug_assert!(
-            self.check_sampled_relaxed_r1cs(u, &E, &z),
-            "Sampled a non satisfiable relaxed R1CS, sampled u: {}, computed E: {:?}",
-            u,
-            E
-        );
-
-        let witness = Witness { E, rE, W, rW };
-        let mut cm_witness = witness.commit::<CS, true>(params, x)?;
-
-        // witness.commit() sets u to 1, we set it to the sampled u value
-        cm_witness.u = u;
-        Ok((cm_witness, witness))
-    }
+        CS: CommitmentScheme<C, true>;
 }
 
 /// extracts arkworks ConstraintSystem matrices into crate::utils::vec::SparseMatrix format as R1CS
@@ -221,9 +165,13 @@ pub fn extract_w_x<F: PrimeField>(cs: &ConstraintSystem<F>) -> (Vec<F>, Vec<F>)
 #[cfg(test)]
 pub mod tests {
     use super::*;
+    use crate::folding::nova::{CommittedInstance, Witness};
     use crate::{
         commitment::pedersen::Pedersen,
-        utils::vec::tests::{to_F_matrix, to_F_vec},
+        utils::vec::{
+            is_zero_vec,
+            tests::{to_F_matrix, to_F_vec},
+        },
     };
 
     use ark_pallas::{Fr, Projective};
@@ -234,9 +182,8 @@ pub mod tests {
         let r1cs = get_test_r1cs::<Fr>();
         let (prover_params, _) = Pedersen::<Projective>::setup(rng, r1cs.A.n_rows).unwrap();
 
-        let relaxed_r1cs = r1cs.relax();
-        let sampled =
-            relaxed_r1cs.sample::<Projective, Pedersen<Projective, true>>(&prover_params, rng);
+        let sampled: Result<(Witness<Projective>, CommittedInstance<Projective>), _> =
+            r1cs.sample::<Pedersen<Projective, true>>(&prover_params, rng);
         assert!(sampled.is_ok());
     }
 
@@ -294,10 +241,23 @@ pub mod tests {
     }
 
     #[test]
-    fn test_check_relation() {
+    fn test_eval_r1cs_relation() {
+        let mut rng = ark_std::test_rng();
+        let r1cs = get_test_r1cs::<Fr>();
+        let mut z = get_test_z::<Fr>(rng.gen::<u16>() as usize);
+
+        let f_w = r1cs.eval_relation(&z).unwrap();
+        assert!(is_zero_vec(&f_w));
+
+        z[1] = Fr::from(111);
+        let f_w = r1cs.eval_relation(&z).unwrap();
+        assert!(!is_zero_vec(&f_w));
+    }
+
+    #[test]
+    fn test_check_r1cs_relation() {
         let r1cs = get_test_r1cs::<Fr>();
         let z = get_test_z(5);
         r1cs.check_relation(&z).unwrap();
-        r1cs.relax().check_relation(&z).unwrap();
     }
 }