Digest simplifications (#238)

* remove unused digest computations

* avoid a verifier having to recompute a digest

* update crate version

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "nova-snark"
-version = "0.24.0"
+version = "0.25.0"
 authors = ["Srinath Setty <srinath@microsoft.com>"]
 edition = "2021"
 description = "Recursive zkSNARKs without trusted setup"

src/r1cs/mod.rs

@@ -5,7 +5,6 @@ mod util;
 
 use crate::{
   constants::{BN_LIMB_WIDTH, BN_N_LIMBS},
-  digest::{DigestComputer, SimpleDigestible},
   errors::NovaError,
   gadgets::{
     nonnative::{bignat::nat_to_limbs, util::f_to_nat},
@@ -18,8 +17,6 @@ use crate::{
 };
 use core::{cmp::max, marker::PhantomData};
 use ff::Field;
-use once_cell::sync::OnceCell;
-
 use rayon::prelude::*;
 use serde::{Deserialize, Serialize};
 
@@ -41,12 +38,8 @@ pub struct R1CSShape<G: Group> {
   pub(crate) A: SparseMatrix<G::Scalar>,
   pub(crate) B: SparseMatrix<G::Scalar>,
   pub(crate) C: SparseMatrix<G::Scalar>,
-  #[serde(skip, default = "OnceCell::new")]
-  pub(crate) digest: OnceCell<G::Scalar>,
 }
 
-impl<G: Group> SimpleDigestible for R1CSShape<G> {}
-
 /// A type that holds a witness for a given R1CS instance
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub struct R1CSWitness<G: Group> {
@@ -141,19 +134,9 @@ impl<G: Group> R1CSShape<G> {
       A,
       B,
       C,
-      digest: OnceCell::new(),
     })
   }
 
-  /// returned the digest of the `R1CSShape`
-  pub fn digest(&self) -> G::Scalar {
-    self
-      .digest
-      .get_or_try_init(|| DigestComputer::new(self).digest())
-      .cloned()
-      .expect("Failure retrieving digest")
-  }
-
   // Checks regularity conditions on the R1CSShape, required in Spartan-class SNARKs
   // Panics if num_cons, num_vars, or num_io are not powers of two, or if num_io > num_vars
   #[inline]
@@ -321,7 +304,6 @@ impl<G: Group> R1CSShape<G> {
         A: self.A.clone(),
         B: self.B.clone(),
         C: self.C.clone(),
-        digest: OnceCell::new(),
       };
     }
 
@@ -357,7 +339,6 @@ impl<G: Group> R1CSShape<G> {
       A: A_padded,
       B: B_padded,
       C: C_padded,
-      digest: OnceCell::new(),
     }
   }
 }

src/spartan/ppsnark.rs

@@ -666,6 +666,16 @@ pub struct ProverKey<G: Group, EE: EvaluationEngineTrait<G>> {
 #[derive(Clone, Serialize, Deserialize)]
 #[serde(bound = "")]
 pub struct VerifierKey<G: Group, EE: EvaluationEngineTrait<G>> {
+  num_cons: usize,
+  num_vars: usize,
+  vk_ee: EE::VerifierKey,
+  S_comm: R1CSShapeSparkCommitment<G>,
+  digest: G::Scalar,
+}
+
+#[derive(Clone, Serialize, Deserialize)]
+#[serde(bound = "")]
+struct VerifierKeyInternal<G: Group, EE: EvaluationEngineTrait<G>> {
   num_cons: usize,
   num_vars: usize,
   vk_ee: EE::VerifierKey,
@@ -674,7 +684,7 @@ pub struct VerifierKey<G: Group, EE: EvaluationEngineTrait<G>> {
   digest: OnceCell<G::Scalar>,
 }
 
-impl<G: Group, EE: EvaluationEngineTrait<G>> SimpleDigestible for VerifierKey<G, EE> {}
+impl<G: Group, EE: EvaluationEngineTrait<G>> SimpleDigestible for VerifierKeyInternal<G, EE> {}
 
 /// A succinct proof of knowledge of a witness to a relaxed R1CS instance
 /// The proof is produced using Spartan's combination of the sum-check and
@@ -842,14 +852,14 @@ impl<G: Group, EE: EvaluationEngineTrait<G>> RelaxedR1CSSNARK<G, EE> {
   }
 }
 
-impl<G: Group, EE: EvaluationEngineTrait<G>> VerifierKey<G, EE> {
+impl<G: Group, EE: EvaluationEngineTrait<G>> VerifierKeyInternal<G, EE> {
   fn new(
     num_cons: usize,
     num_vars: usize,
     S_comm: R1CSShapeSparkCommitment<G>,
     vk_ee: EE::VerifierKey,
   ) -> Self {
-    VerifierKey {
+    VerifierKeyInternal {
       num_cons,
       num_vars,
       S_comm,
@@ -887,14 +897,23 @@ impl<G: Group, EE: EvaluationEngineTrait<G>> RelaxedR1CSSNARKTrait<G> for Relaxe
     let S_repr = R1CSShapeSparkRepr::new(&S);
     let S_comm = S_repr.commit(ck);
 
-    let vk = VerifierKey::new(S.num_cons, S.num_vars, S_comm.clone(), vk_ee);
+    let vk_internal: VerifierKeyInternal<G, EE> =
+      VerifierKeyInternal::new(S.num_cons, S.num_vars, S_comm.clone(), vk_ee.clone());
 
     let pk = ProverKey {
       pk_ee,
       S,
       S_repr,
+      S_comm: S_comm.clone(),
+      vk_digest: vk_internal.digest(),
+    };
+
+    let vk = VerifierKey {
+      num_cons: vk_internal.num_cons,
+      num_vars: vk_internal.num_vars,
+      vk_ee,
       S_comm,
-      vk_digest: vk.digest(),
+      digest: vk_internal.digest(),
     };
 
     Ok((pk, vk))
@@ -1516,7 +1535,7 @@ impl<G: Group, EE: EvaluationEngineTrait<G>> RelaxedR1CSSNARKTrait<G> for Relaxe
     let mut u_vec: Vec<PolyEvalInstance<G>> = Vec::new();
 
     // append the verifier key (including commitment to R1CS matrices) and the RelaxedR1CSInstance to the transcript
-    transcript.absorb(b"vk", &vk.digest());
+    transcript.absorb(b"vk", &vk.digest);
     transcript.absorb(b"U", U);
 
     let comm_Az = Commitment::<G>::decompress(&self.comm_Az)?;